vacuumlazy.c

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/*-------------------------------------------------------------------------
 *
 * vacuumlazy.c
 *    Concurrent ("lazy") vacuuming.
 *
 * The major space usage for vacuuming is storage for the dead tuple IDs that
 * are to be removed from indexes.  We want to ensure we can vacuum even the
 * very largest relations with finite memory space usage.  To do that, we set
 * upper bounds on the memory that can be used for keeping track of dead TIDs
 * at once.
 *
 * We are willing to use at most maintenance_work_mem (or perhaps
 * autovacuum_work_mem) memory space to keep track of dead TIDs.  If the
 * TID store is full, we must call lazy_vacuum to vacuum indexes (and to vacuum
 * the pages that we've pruned). This frees up the memory space dedicated to
 * to store dead TIDs.
 *
 * In practice VACUUM will often complete its initial pass over the target
 * heap relation without ever running out of space to store TIDs.  This means
 * that there only needs to be one call to lazy_vacuum, after the initial pass
 * completes.
 *
 * Portions Copyright (c) 1996-2024, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/access/heap/vacuumlazy.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"
 
#include <math.h>
 
#include "access/genam.h"
#include "access/heapam.h"
#include "access/heapam_xlog.h"
#include "access/htup_details.h"
#include "access/multixact.h"
#include "access/tidstore.h"
#include "access/transam.h"
#include "access/visibilitymap.h"
#include "access/xloginsert.h"
#include "catalog/storage.h"
#include "commands/dbcommands.h"
#include "commands/progress.h"
#include "commands/vacuum.h"
#include "common/int.h"
#include "executor/instrument.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "portability/instr_time.h"
#include "postmaster/autovacuum.h"
#include "storage/bufmgr.h"
#include "storage/freespace.h"
#include "storage/lmgr.h"
#include "utils/lsyscache.h"
#include "utils/memutils.h"
#include "utils/pg_rusage.h"
#include "utils/timestamp.h"
 
 
/*
 * Space/time tradeoff parameters: do these need to be user-tunable?
 *
 * To consider truncating the relation, we want there to be at least
 * REL_TRUNCATE_MINIMUM or (relsize / REL_TRUNCATE_FRACTION) (whichever
 * is less) potentially-freeable pages.
 */
#define REL_TRUNCATE_MINIMUM    1000
#define REL_TRUNCATE_FRACTION   16
 
/*
 * Timing parameters for truncate locking heuristics.
 *
 * These were not exposed as user tunable GUC values because it didn't seem
 * that the potential for improvement was great enough to merit the cost of
 * supporting them.
 */
#define VACUUM_TRUNCATE_LOCK_CHECK_INTERVAL     20  /* ms */
#define VACUUM_TRUNCATE_LOCK_WAIT_INTERVAL      50  /* ms */
#define VACUUM_TRUNCATE_LOCK_TIMEOUT            5000    /* ms */
 
/*
 * Threshold that controls whether we bypass index vacuuming and heap
 * vacuuming as an optimization
 */
#define BYPASS_THRESHOLD_PAGES  0.02    /* i.e. 2% of rel_pages */
 
/*
 * Perform a failsafe check each time we scan another 4GB of pages.
 * (Note that this is deliberately kept to a power-of-two, usually 2^19.)
 */
#define FAILSAFE_EVERY_PAGES \
    ((BlockNumber) (((uint64) 4 * 1024 * 1024 * 1024) / BLCKSZ))
 
/*
 * When a table has no indexes, vacuum the FSM after every 8GB, approximately
 * (it won't be exact because we only vacuum FSM after processing a heap page
 * that has some removable tuples).  When there are indexes, this is ignored,
 * and we vacuum FSM after each index/heap cleaning pass.
 */
#define VACUUM_FSM_EVERY_PAGES \
    ((BlockNumber) (((uint64) 8 * 1024 * 1024 * 1024) / BLCKSZ))
 
/*
 * Before we consider skipping a page that's marked as clean in
 * visibility map, we must've seen at least this many clean pages.
 */
#define SKIP_PAGES_THRESHOLD    ((BlockNumber) 32)
 
/*
 * Size of the prefetch window for lazy vacuum backwards truncation scan.
 * Needs to be a power of 2.
 */
#define PREFETCH_SIZE           ((BlockNumber) 32)
 
/*
 * Macro to check if we are in a parallel vacuum.  If true, we are in the
 * parallel mode and the DSM segment is initialized.
 */
#define ParallelVacuumIsActive(vacrel) ((vacrel)->pvs != NULL)
 
/* Phases of vacuum during which we report error context. */
typedef enum
{
    VACUUM_ERRCB_PHASE_UNKNOWN,
    VACUUM_ERRCB_PHASE_SCAN_HEAP,
    VACUUM_ERRCB_PHASE_VACUUM_INDEX,
    VACUUM_ERRCB_PHASE_VACUUM_HEAP,
    VACUUM_ERRCB_PHASE_INDEX_CLEANUP,
    VACUUM_ERRCB_PHASE_TRUNCATE,
} VacErrPhase;
 
typedef struct LVRelState
{
    /* Target heap relation and its indexes */
    Relation    rel;
    Relation   *indrels;
    int         nindexes;
 
    /* Buffer access strategy and parallel vacuum state */
    BufferAccessStrategy bstrategy;
    ParallelVacuumState *pvs;
 
    /* Aggressive VACUUM? (must set relfrozenxid >= FreezeLimit) */
    bool        aggressive;
    /* Use visibility map to skip? (disabled by DISABLE_PAGE_SKIPPING) */
    bool        skipwithvm;
    /* Consider index vacuuming bypass optimization? */
    bool        consider_bypass_optimization;
 
    /* Doing index vacuuming, index cleanup, rel truncation? */
    bool        do_index_vacuuming;
    bool        do_index_cleanup;
    bool        do_rel_truncate;
 
    /* VACUUM operation's cutoffs for freezing and pruning */
    struct VacuumCutoffs cutoffs;
    GlobalVisState *vistest;
    /* Tracks oldest extant XID/MXID for setting relfrozenxid/relminmxid */
    TransactionId NewRelfrozenXid;
    MultiXactId NewRelminMxid;
    bool        skippedallvis;
 
    /* Error reporting state */
    char       *dbname;
    char       *relnamespace;
    char       *relname;
    char       *indname;        /* Current index name */
    BlockNumber blkno;          /* used only for heap operations */
    OffsetNumber offnum;        /* used only for heap operations */
    VacErrPhase phase;
    bool        verbose;        /* VACUUM VERBOSE? */
 
    /*
     * dead_items stores TIDs whose index tuples are deleted by index
     * vacuuming. Each TID points to an LP_DEAD line pointer from a heap page
     * that has been processed by lazy_scan_prune.  Also needed by
     * lazy_vacuum_heap_rel, which marks the same LP_DEAD line pointers as
     * LP_UNUSED during second heap pass.
     *
     * Both dead_items and dead_items_info are allocated in shared memory in
     * parallel vacuum cases.
     */
    TidStore   *dead_items;     /* TIDs whose index tuples we'll delete */
    VacDeadItemsInfo *dead_items_info;
 
    BlockNumber rel_pages;      /* total number of pages */
    BlockNumber scanned_pages;  /* # pages examined (not skipped via VM) */
    BlockNumber removed_pages;  /* # pages removed by relation truncation */
    BlockNumber frozen_pages;   /* # pages with newly frozen tuples */
    BlockNumber lpdead_item_pages;  /* # pages with LP_DEAD items */
    BlockNumber missed_dead_pages;  /* # pages with missed dead tuples */
    BlockNumber nonempty_pages; /* actually, last nonempty page + 1 */
 
    /* Statistics output by us, for table */
    double      new_rel_tuples; /* new estimated total # of tuples */
    double      new_live_tuples;    /* new estimated total # of live tuples */
    /* Statistics output by index AMs */
    IndexBulkDeleteResult **indstats;
 
    /* Instrumentation counters */
    int         num_index_scans;
    /* Counters that follow are only for scanned_pages */
    int64       tuples_deleted; /* # deleted from table */
    int64       tuples_frozen;  /* # newly frozen */
    int64       lpdead_items;   /* # deleted from indexes */
    int64       live_tuples;    /* # live tuples remaining */
    int64       recently_dead_tuples;   /* # dead, but not yet removable */
    int64       missed_dead_tuples; /* # removable, but not removed */
 
    /* State maintained by heap_vac_scan_next_block() */
    BlockNumber current_block;  /* last block returned */
    BlockNumber next_unskippable_block; /* next unskippable block */
    bool        next_unskippable_allvis;    /* its visibility status */
    Buffer      next_unskippable_vmbuffer;  /* buffer containing its VM bit */
} LVRelState;
 
/* Struct for saving and restoring vacuum error information. */
typedef struct LVSavedErrInfo
{
    BlockNumber blkno;
    OffsetNumber offnum;
    VacErrPhase phase;
} LVSavedErrInfo;
 
 
/* non-export function prototypes */
static void lazy_scan_heap(LVRelState *vacrel);
static bool heap_vac_scan_next_block(LVRelState *vacrel, BlockNumber *blkno,
                                     bool *all_visible_according_to_vm);
static void find_next_unskippable_block(LVRelState *vacrel, bool *skipsallvis);
static bool lazy_scan_new_or_empty(LVRelState *vacrel, Buffer buf,
                                   BlockNumber blkno, Page page,
                                   bool sharelock, Buffer vmbuffer);
static void lazy_scan_prune(LVRelState *vacrel, Buffer buf,
                            BlockNumber blkno, Page page,
                            Buffer vmbuffer, bool all_visible_according_to_vm,
                            bool *has_lpdead_items);
static bool lazy_scan_noprune(LVRelState *vacrel, Buffer buf,
                              BlockNumber blkno, Page page,
                              bool *has_lpdead_items);
static void lazy_vacuum(LVRelState *vacrel);
static bool lazy_vacuum_all_indexes(LVRelState *vacrel);
static void lazy_vacuum_heap_rel(LVRelState *vacrel);
static void lazy_vacuum_heap_page(LVRelState *vacrel, BlockNumber blkno,
                                  Buffer buffer, OffsetNumber *offsets,
                                  int num_offsets, Buffer vmbuffer);
static bool lazy_check_wraparound_failsafe(LVRelState *vacrel);
static void lazy_cleanup_all_indexes(LVRelState *vacrel);
static IndexBulkDeleteResult *lazy_vacuum_one_index(Relation indrel,
                                                    IndexBulkDeleteResult *istat,
                                                    double reltuples,
                                                    LVRelState *vacrel);
static IndexBulkDeleteResult *lazy_cleanup_one_index(Relation indrel,
                                                     IndexBulkDeleteResult *istat,
                                                     double reltuples,
                                                     bool estimated_count,
                                                     LVRelState *vacrel);
static bool should_attempt_truncation(LVRelState *vacrel);
static void lazy_truncate_heap(LVRelState *vacrel);
static BlockNumber count_nondeletable_pages(LVRelState *vacrel,
                                            bool *lock_waiter_detected);
static void dead_items_alloc(LVRelState *vacrel, int nworkers);
static void dead_items_add(LVRelState *vacrel, BlockNumber blkno, OffsetNumber *offsets,
                           int num_offsets);
static void dead_items_reset(LVRelState *vacrel);
static void dead_items_cleanup(LVRelState *vacrel);
static bool heap_page_is_all_visible(LVRelState *vacrel, Buffer buf,
                                     TransactionId *visibility_cutoff_xid, bool *all_frozen);
static void update_relstats_all_indexes(LVRelState *vacrel);
static void vacuum_error_callback(void *arg);
static void update_vacuum_error_info(LVRelState *vacrel,
                                     LVSavedErrInfo *saved_vacrel,
                                     int phase, BlockNumber blkno,
                                     OffsetNumber offnum);
static void restore_vacuum_error_info(LVRelState *vacrel,
                                      const LVSavedErrInfo *saved_vacrel);
 
 
/*
 *  heap_vacuum_rel() -- perform VACUUM for one heap relation
 *
 *      This routine sets things up for and then calls lazy_scan_heap, where
 *      almost all work actually takes place.  Finalizes everything after call
 *      returns by managing relation truncation and updating rel's pg_class
 *      entry. (Also updates pg_class entries for any indexes that need it.)
 *
 *      At entry, we have already established a transaction and opened
 *      and locked the relation.
 */
void
heap_vacuum_rel(Relation rel, VacuumParams *params,
                BufferAccessStrategy bstrategy)
{
    LVRelState *vacrel;
    bool        verbose,
                instrument,
                skipwithvm,
                frozenxid_updated,
                minmulti_updated;
    BlockNumber orig_rel_pages,
                new_rel_pages,
                new_rel_allvisible;
    PGRUsage    ru0;
    TimestampTz starttime = 0;
    PgStat_Counter startreadtime = 0,
                startwritetime = 0;
    WalUsage    startwalusage = pgWalUsage;
    int64       StartPageHit = VacuumPageHit,
                StartPageMiss = VacuumPageMiss,
                StartPageDirty = VacuumPageDirty;
    ErrorContextCallback errcallback;
    char      **indnames = NULL;
 
    verbose = (params->options & VACOPT_VERBOSE) != 0;
    instrument = (verbose || (AmAutoVacuumWorkerProcess() &&
                              params->log_min_duration >= 0));
    if (instrument)
    {
        pg_rusage_init(&ru0);
        starttime = GetCurrentTimestamp();
        if (track_io_timing)
        {
            startreadtime = pgStatBlockReadTime;
            startwritetime = pgStatBlockWriteTime;
        }
    }
 
    pgstat_progress_start_command(PROGRESS_COMMAND_VACUUM,
                                  RelationGetRelid(rel));
 
    /*
     * Setup error traceback support for ereport() first.  The idea is to set
     * up an error context callback to display additional information on any
     * error during a vacuum.  During different phases of vacuum, we update
     * the state so that the error context callback always display current
     * information.
     *
     * Copy the names of heap rel into local memory for error reporting
     * purposes, too.  It isn't always safe to assume that we can get the name
     * of each rel.  It's convenient for code in lazy_scan_heap to always use
     * these temp copies.
     */
    vacrel = (LVRelState *) palloc0(sizeof(LVRelState));
    vacrel->dbname = get_database_name(MyDatabaseId);
    vacrel->relnamespace = get_namespace_name(RelationGetNamespace(rel));
    vacrel->relname = pstrdup(RelationGetRelationName(rel));
    vacrel->indname = NULL;
    vacrel->phase = VACUUM_ERRCB_PHASE_UNKNOWN;
    vacrel->verbose = verbose;
    errcallback.callback = vacuum_error_callback;
    errcallback.arg = vacrel;
    errcallback.previous = error_context_stack;
    error_context_stack = &errcallback;
 
    /* Set up high level stuff about rel and its indexes */
    vacrel->rel = rel;
    vac_open_indexes(vacrel->rel, RowExclusiveLock, &vacrel->nindexes,
                     &vacrel->indrels);
    vacrel->bstrategy = bstrategy;
    if (instrument && vacrel->nindexes > 0)
    {
        /* Copy index names used by instrumentation (not error reporting) */
        indnames = palloc(sizeof(char *) * vacrel->nindexes);
        for (int i = 0; i < vacrel->nindexes; i++)
            indnames[i] = pstrdup(RelationGetRelationName(vacrel->indrels[i]));
    }
 
    /*
     * The index_cleanup param either disables index vacuuming and cleanup or
     * forces it to go ahead when we would otherwise apply the index bypass
     * optimization.  The default is 'auto', which leaves the final decision
     * up to lazy_vacuum().
     *
     * The truncate param allows user to avoid attempting relation truncation,
     * though it can't force truncation to happen.
     */
    Assert(params->index_cleanup != VACOPTVALUE_UNSPECIFIED);
    Assert(params->truncate != VACOPTVALUE_UNSPECIFIED &&
           params->truncate != VACOPTVALUE_AUTO);
 
    /*
     * While VacuumFailSafeActive is reset to false before calling this, we
     * still need to reset it here due to recursive calls.
     */
    VacuumFailsafeActive = false;
    vacrel->consider_bypass_optimization = true;
    vacrel->do_index_vacuuming = true;
    vacrel->do_index_cleanup = true;
    vacrel->do_rel_truncate = (params->truncate != VACOPTVALUE_DISABLED);
    if (params->index_cleanup == VACOPTVALUE_DISABLED)
    {
        /* Force disable index vacuuming up-front */
        vacrel->do_index_vacuuming = false;
        vacrel->do_index_cleanup = false;
    }
    else if (params->index_cleanup == VACOPTVALUE_ENABLED)
    {
        /* Force index vacuuming.  Note that failsafe can still bypass. */
        vacrel->consider_bypass_optimization = false;
    }
    else
    {
        /* Default/auto, make all decisions dynamically */
        Assert(params->index_cleanup == VACOPTVALUE_AUTO);
    }
 
    /* Initialize page counters explicitly (be tidy) */
    vacrel->scanned_pages = 0;
    vacrel->removed_pages = 0;
    vacrel->frozen_pages = 0;
    vacrel->lpdead_item_pages = 0;
    vacrel->missed_dead_pages = 0;
    vacrel->nonempty_pages = 0;
    /* dead_items_alloc allocates vacrel->dead_items later on */
 
    /* Allocate/initialize output statistics state */
    vacrel->new_rel_tuples = 0;
    vacrel->new_live_tuples = 0;
    vacrel->indstats = (IndexBulkDeleteResult **)
        palloc0(vacrel->nindexes * sizeof(IndexBulkDeleteResult *));
 
    /* Initialize remaining counters (be tidy) */
    vacrel->num_index_scans = 0;
    vacrel->tuples_deleted = 0;
    vacrel->tuples_frozen = 0;
    vacrel->lpdead_items = 0;
    vacrel->live_tuples = 0;
    vacrel->recently_dead_tuples = 0;
    vacrel->missed_dead_tuples = 0;
 
    /*
     * Get cutoffs that determine which deleted tuples are considered DEAD,
     * not just RECENTLY_DEAD, and which XIDs/MXIDs to freeze.  Then determine
     * the extent of the blocks that we'll scan in lazy_scan_heap.  It has to
     * happen in this order to ensure that the OldestXmin cutoff field works
     * as an upper bound on the XIDs stored in the pages we'll actually scan
     * (NewRelfrozenXid tracking must never be allowed to miss unfrozen XIDs).
     *
     * Next acquire vistest, a related cutoff that's used in pruning.  We
     * expect vistest will always make heap_page_prune_and_freeze() remove any
     * deleted tuple whose xmax is < OldestXmin.  lazy_scan_prune must never
     * become confused about whether a tuple should be frozen or removed.  (In
     * the future we might want to teach lazy_scan_prune to recompute vistest
     * from time to time, to increase the number of dead tuples it can prune
     * away.)
     */
    vacrel->aggressive = vacuum_get_cutoffs(rel, params, &vacrel->cutoffs);
    vacrel->rel_pages = orig_rel_pages = RelationGetNumberOfBlocks(rel);
    vacrel->vistest = GlobalVisTestFor(rel);
    /* Initialize state used to track oldest extant XID/MXID */
    vacrel->NewRelfrozenXid = vacrel->cutoffs.OldestXmin;
    vacrel->NewRelminMxid = vacrel->cutoffs.OldestMxact;
    vacrel->skippedallvis = false;
    skipwithvm = true;
    if (params->options & VACOPT_DISABLE_PAGE_SKIPPING)
    {
        /*
         * Force aggressive mode, and disable skipping blocks using the
         * visibility map (even those set all-frozen)
         */
        vacrel->aggressive = true;
        skipwithvm = false;
    }
 
    vacrel->skipwithvm = skipwithvm;
 
    if (verbose)
    {
        if (vacrel->aggressive)
            ereport(INFO,
                    (errmsg("aggressively vacuuming \"%s.%s.%s\"",
                            vacrel->dbname, vacrel->relnamespace,
                            vacrel->relname)));
        else
            ereport(INFO,
                    (errmsg("vacuuming \"%s.%s.%s\"",
                            vacrel->dbname, vacrel->relnamespace,
                            vacrel->relname)));
    }
 
    /*
     * Allocate dead_items memory using dead_items_alloc.  This handles
     * parallel VACUUM initialization as part of allocating shared memory
     * space used for dead_items.  (But do a failsafe precheck first, to
     * ensure that parallel VACUUM won't be attempted at all when relfrozenxid
     * is already dangerously old.)
     */
    lazy_check_wraparound_failsafe(vacrel);
    dead_items_alloc(vacrel, params->nworkers);
 
    /*
     * Call lazy_scan_heap to perform all required heap pruning, index
     * vacuuming, and heap vacuuming (plus related processing)
     */
    lazy_scan_heap(vacrel);
 
    /*
     * Free resources managed by dead_items_alloc.  This ends parallel mode in
     * passing when necessary.
     */
    dead_items_cleanup(vacrel);
    Assert(!IsInParallelMode());
 
    /*
     * Update pg_class entries for each of rel's indexes where appropriate.
     *
     * Unlike the later update to rel's pg_class entry, this is not critical.
     * Maintains relpages/reltuples statistics used by the planner only.
     */
    if (vacrel->do_index_cleanup)
        update_relstats_all_indexes(vacrel);
 
    /* Done with rel's indexes */
    vac_close_indexes(vacrel->nindexes, vacrel->indrels, NoLock);
 
    /* Optionally truncate rel */
    if (should_attempt_truncation(vacrel))
        lazy_truncate_heap(vacrel);
 
    /* Pop the error context stack */
    error_context_stack = errcallback.previous;
 
    /* Report that we are now doing final cleanup */
    pgstat_progress_update_param(PROGRESS_VACUUM_PHASE,
                                 PROGRESS_VACUUM_PHASE_FINAL_CLEANUP);
 
    /*
     * Prepare to update rel's pg_class entry.
     *
     * Aggressive VACUUMs must always be able to advance relfrozenxid to a
     * value >= FreezeLimit, and relminmxid to a value >= MultiXactCutoff.
     * Non-aggressive VACUUMs may advance them by any amount, or not at all.
     */
    Assert(vacrel->NewRelfrozenXid == vacrel->cutoffs.OldestXmin ||
           TransactionIdPrecedesOrEquals(vacrel->aggressive ? vacrel->cutoffs.FreezeLimit :
                                         vacrel->cutoffs.relfrozenxid,
                                         vacrel->NewRelfrozenXid));
    Assert(vacrel->NewRelminMxid == vacrel->cutoffs.OldestMxact ||
           MultiXactIdPrecedesOrEquals(vacrel->aggressive ? vacrel->cutoffs.MultiXactCutoff :
                                       vacrel->cutoffs.relminmxid,
                                       vacrel->NewRelminMxid));
    if (vacrel->skippedallvis)
    {
        /*
         * Must keep original relfrozenxid in a non-aggressive VACUUM that
         * chose to skip an all-visible page range.  The state that tracks new
         * values will have missed unfrozen XIDs from the pages we skipped.
         */
        Assert(!vacrel->aggressive);
        vacrel->NewRelfrozenXid = InvalidTransactionId;
        vacrel->NewRelminMxid = InvalidMultiXactId;
    }
 
    /*
     * For safety, clamp relallvisible to be not more than what we're setting
     * pg_class.relpages to
     */
    new_rel_pages = vacrel->rel_pages;  /* After possible rel truncation */
    visibilitymap_count(rel, &new_rel_allvisible, NULL);
    if (new_rel_allvisible > new_rel_pages)
        new_rel_allvisible = new_rel_pages;
 
    /*
     * Now actually update rel's pg_class entry.
     *
     * In principle new_live_tuples could be -1 indicating that we (still)
     * don't know the tuple count.  In practice that can't happen, since we
     * scan every page that isn't skipped using the visibility map.
     */
    vac_update_relstats(rel, new_rel_pages, vacrel->new_live_tuples,
                        new_rel_allvisible, vacrel->nindexes > 0,
                        vacrel->NewRelfrozenXid, vacrel->NewRelminMxid,
                        &frozenxid_updated, &minmulti_updated, false);
 
    /*
     * Report results to the cumulative stats system, too.
     *
     * Deliberately avoid telling the stats system about LP_DEAD items that
     * remain in the table due to VACUUM bypassing index and heap vacuuming.
     * ANALYZE will consider the remaining LP_DEAD items to be dead "tuples".
     * It seems like a good idea to err on the side of not vacuuming again too
     * soon in cases where the failsafe prevented significant amounts of heap
     * vacuuming.
     */
    pgstat_report_vacuum(RelationGetRelid(rel),
                         rel->rd_rel->relisshared,
                         Max(vacrel->new_live_tuples, 0),
                         vacrel->recently_dead_tuples +
                         vacrel->missed_dead_tuples);
    pgstat_progress_end_command();
 
    if (instrument)
    {
        TimestampTz endtime = GetCurrentTimestamp();
 
        if (verbose || params->log_min_duration == 0 ||
            TimestampDifferenceExceeds(starttime, endtime,
                                       params->log_min_duration))
        {
            long        secs_dur;
            int         usecs_dur;
            WalUsage    walusage;
            StringInfoData buf;
            char       *msgfmt;
            int32       diff;
            int64       PageHitOp = VacuumPageHit - StartPageHit,
                        PageMissOp = VacuumPageMiss - StartPageMiss,
                        PageDirtyOp = VacuumPageDirty - StartPageDirty;
            double      read_rate = 0,
                        write_rate = 0;
 
            TimestampDifference(starttime, endtime, &secs_dur, &usecs_dur);
            memset(&walusage, 0, sizeof(WalUsage));
            WalUsageAccumDiff(&walusage, &pgWalUsage, &startwalusage);
 
            initStringInfo(&buf);
            if (verbose)
            {
                /*
                 * Aggressiveness already reported earlier, in dedicated
                 * VACUUM VERBOSE ereport
                 */
                Assert(!params->is_wraparound);
                msgfmt = _("finished vacuuming \"%s.%s.%s\": index scans: %d\n");
            }
            else if (params->is_wraparound)
            {
                /*
                 * While it's possible for a VACUUM to be both is_wraparound
                 * and !aggressive, that's just a corner-case -- is_wraparound
                 * implies aggressive.  Produce distinct output for the corner
                 * case all the same, just in case.
                 */
                if (vacrel->aggressive)
                    msgfmt = _("automatic aggressive vacuum to prevent wraparound of table \"%s.%s.%s\": index scans: %d\n");
                else
                    msgfmt = _("automatic vacuum to prevent wraparound of table \"%s.%s.%s\": index scans: %d\n");
            }
            else
            {
                if (vacrel->aggressive)
                    msgfmt = _("automatic aggressive vacuum of table \"%s.%s.%s\": index scans: %d\n");
                else
                    msgfmt = _("automatic vacuum of table \"%s.%s.%s\": index scans: %d\n");
            }
            appendStringInfo(&buf, msgfmt,
                             vacrel->dbname,
                             vacrel->relnamespace,
                             vacrel->relname,
                             vacrel->num_index_scans);
            appendStringInfo(&buf, _("pages: %u removed, %u remain, %u scanned (%.2f%% of total)\n"),
                             vacrel->removed_pages,
                             new_rel_pages,
                             vacrel->scanned_pages,
                             orig_rel_pages == 0 ? 100.0 :
                             100.0 * vacrel->scanned_pages / orig_rel_pages);
            appendStringInfo(&buf,
                             _("tuples: %lld removed, %lld remain, %lld are dead but not yet removable\n"),
                             (long long) vacrel->tuples_deleted,
                             (long long) vacrel->new_rel_tuples,
                             (long long) vacrel->recently_dead_tuples);
            if (vacrel->missed_dead_tuples > 0)
                appendStringInfo(&buf,
                                 _("tuples missed: %lld dead from %u pages not removed due to cleanup lock contention\n"),
                                 (long long) vacrel->missed_dead_tuples,
                                 vacrel->missed_dead_pages);
            diff = (int32) (ReadNextTransactionId() -
                            vacrel->cutoffs.OldestXmin);
            appendStringInfo(&buf,
                             _("removable cutoff: %u, which was %d XIDs old when operation ended\n"),
                             vacrel->cutoffs.OldestXmin, diff);
            if (frozenxid_updated)
            {
                diff = (int32) (vacrel->NewRelfrozenXid -
                                vacrel->cutoffs.relfrozenxid);
                appendStringInfo(&buf,
                                 _("new relfrozenxid: %u, which is %d XIDs ahead of previous value\n"),
                                 vacrel->NewRelfrozenXid, diff);
            }
            if (minmulti_updated)
            {
                diff = (int32) (vacrel->NewRelminMxid -
                                vacrel->cutoffs.relminmxid);
                appendStringInfo(&buf,
                                 _("new relminmxid: %u, which is %d MXIDs ahead of previous value\n"),
                                 vacrel->NewRelminMxid, diff);
            }
            appendStringInfo(&buf, _("frozen: %u pages from table (%.2f%% of total) had %lld tuples frozen\n"),
                             vacrel->frozen_pages,
                             orig_rel_pages == 0 ? 100.0 :
                             100.0 * vacrel->frozen_pages / orig_rel_pages,
                             (long long) vacrel->tuples_frozen);
            if (vacrel->do_index_vacuuming)
            {
                if (vacrel->nindexes == 0 || vacrel->num_index_scans == 0)
                    appendStringInfoString(&buf, _("index scan not needed: "));
                else
                    appendStringInfoString(&buf, _("index scan needed: "));
 
                msgfmt = _("%u pages from table (%.2f%% of total) had %lld dead item identifiers removed\n");
            }
            else
            {
                if (!VacuumFailsafeActive)
                    appendStringInfoString(&buf, _("index scan bypassed: "));
                else
                    appendStringInfoString(&buf, _("index scan bypassed by failsafe: "));
 
                msgfmt = _("%u pages from table (%.2f%% of total) have %lld dead item identifiers\n");
            }
            appendStringInfo(&buf, msgfmt,
                             vacrel->lpdead_item_pages,
                             orig_rel_pages == 0 ? 100.0 :
                             100.0 * vacrel->lpdead_item_pages / orig_rel_pages,
                             (long long) vacrel->lpdead_items);
            for (int i = 0; i < vacrel->nindexes; i++)
            {
                IndexBulkDeleteResult *istat = vacrel->indstats[i];
 
                if (!istat)
                    continue;
 
                appendStringInfo(&buf,
                                 _("index \"%s\": pages: %u in total, %u newly deleted, %u currently deleted, %u reusable\n"),
                                 indnames[i],
                                 istat->num_pages,
                                 istat->pages_newly_deleted,
                                 istat->pages_deleted,
                                 istat->pages_free);
            }
            if (track_io_timing)
            {
                double      read_ms = (double) (pgStatBlockReadTime - startreadtime) / 1000;
                double      write_ms = (double) (pgStatBlockWriteTime - startwritetime) / 1000;
 
                appendStringInfo(&buf, _("I/O timings: read: %.3f ms, write: %.3f ms\n"),
                                 read_ms, write_ms);
            }
            if (secs_dur > 0 || usecs_dur > 0)
            {
                read_rate = (double) BLCKSZ * PageMissOp / (1024 * 1024) /
                    (secs_dur + usecs_dur / 1000000.0);
                write_rate = (double) BLCKSZ * PageDirtyOp / (1024 * 1024) /
                    (secs_dur + usecs_dur / 1000000.0);
            }
            appendStringInfo(&buf, _("avg read rate: %.3f MB/s, avg write rate: %.3f MB/s\n"),
                             read_rate, write_rate);
            appendStringInfo(&buf,
                             _("buffer usage: %lld hits, %lld misses, %lld dirtied\n"),
                             (long long) PageHitOp,
                             (long long) PageMissOp,
                             (long long) PageDirtyOp);
            appendStringInfo(&buf,
                             _("WAL usage: %lld records, %lld full page images, %llu bytes\n"),
                             (long long) walusage.wal_records,
                             (long long) walusage.wal_fpi,
                             (unsigned long long) walusage.wal_bytes);
            appendStringInfo(&buf, _("system usage: %s"), pg_rusage_show(&ru0));
 
            ereport(verbose ? INFO : LOG,
                    (errmsg_internal("%s", buf.data)));
            pfree(buf.data);
        }
    }
 
    /* Cleanup index statistics and index names */
    for (int i = 0; i < vacrel->nindexes; i++)
    {
        if (vacrel->indstats[i])
            pfree(vacrel->indstats[i]);
 
        if (instrument)
            pfree(indnames[i]);
    }
}
 
/*
 *  lazy_scan_heap() -- workhorse function for VACUUM
 *
 *      This routine prunes each page in the heap, and considers the need to
 *      freeze remaining tuples with storage (not including pages that can be
 *      skipped using the visibility map).  Also performs related maintenance
 *      of the FSM and visibility map.  These steps all take place during an
 *      initial pass over the target heap relation.
 *
 *      Also invokes lazy_vacuum_all_indexes to vacuum indexes, which largely
 *      consists of deleting index tuples that point to LP_DEAD items left in
 *      heap pages following pruning.  Earlier initial pass over the heap will
 *      have collected the TIDs whose index tuples need to be removed.
 *
 *      Finally, invokes lazy_vacuum_heap_rel to vacuum heap pages, which
 *      largely consists of marking LP_DEAD items (from vacrel->dead_items)
 *      as LP_UNUSED.  This has to happen in a second, final pass over the
 *      heap, to preserve a basic invariant that all index AMs rely on: no
 *      extant index tuple can ever be allowed to contain a TID that points to
 *      an LP_UNUSED line pointer in the heap.  We must disallow premature
 *      recycling of line pointers to avoid index scans that get confused
 *      about which TID points to which tuple immediately after recycling.
 *      (Actually, this isn't a concern when target heap relation happens to
 *      have no indexes, which allows us to safely apply the one-pass strategy
 *      as an optimization).
 *
 *      In practice we often have enough space to fit all TIDs, and so won't
 *      need to call lazy_vacuum more than once, after our initial pass over
 *      the heap has totally finished.  Otherwise things are slightly more
 *      complicated: our "initial pass" over the heap applies only to those
 *      pages that were pruned before we needed to call lazy_vacuum, and our
 *      "final pass" over the heap only vacuums these same heap pages.
 *      However, we process indexes in full every time lazy_vacuum is called,
 *      which makes index processing very inefficient when memory is in short
 *      supply.
 */
static void
lazy_scan_heap(LVRelState *vacrel)
{
    BlockNumber rel_pages = vacrel->rel_pages,
                blkno,
                next_fsm_block_to_vacuum = 0;
    bool        all_visible_according_to_vm;
 
    TidStore   *dead_items = vacrel->dead_items;
    VacDeadItemsInfo *dead_items_info = vacrel->dead_items_info;
    Buffer      vmbuffer = InvalidBuffer;
    const int   initprog_index[] = {
        PROGRESS_VACUUM_PHASE,
        PROGRESS_VACUUM_TOTAL_HEAP_BLKS,
        PROGRESS_VACUUM_MAX_DEAD_TUPLE_BYTES
    };
    int64       initprog_val[3];
 
    /* Report that we're scanning the heap, advertising total # of blocks */
    initprog_val[0] = PROGRESS_VACUUM_PHASE_SCAN_HEAP;
    initprog_val[1] = rel_pages;
    initprog_val[2] = dead_items_info->max_bytes;
    pgstat_progress_update_multi_param(3, initprog_index, initprog_val);
 
    /* Initialize for the first heap_vac_scan_next_block() call */
    vacrel->current_block = InvalidBlockNumber;
    vacrel->next_unskippable_block = InvalidBlockNumber;
    vacrel->next_unskippable_allvis = false;
    vacrel->next_unskippable_vmbuffer = InvalidBuffer;
 
    while (heap_vac_scan_next_block(vacrel, &blkno, &all_visible_according_to_vm))
    {
        Buffer      buf;
        Page        page;
        bool        has_lpdead_items;
        bool        got_cleanup_lock = false;
 
        vacrel->scanned_pages++;
 
        /* Report as block scanned, update error traceback information */
        pgstat_progress_update_param(PROGRESS_VACUUM_HEAP_BLKS_SCANNED, blkno);
        update_vacuum_error_info(vacrel, NULL, VACUUM_ERRCB_PHASE_SCAN_HEAP,
                                 blkno, InvalidOffsetNumber);
 
        vacuum_delay_point();
 
        /*
         * Regularly check if wraparound failsafe should trigger.
         *
         * There is a similar check inside lazy_vacuum_all_indexes(), but
         * relfrozenxid might start to look dangerously old before we reach
         * that point.  This check also provides failsafe coverage for the
         * one-pass strategy, and the two-pass strategy with the index_cleanup
         * param set to 'off'.
         */
        if (vacrel->scanned_pages % FAILSAFE_EVERY_PAGES == 0)
            lazy_check_wraparound_failsafe(vacrel);
 
        /*
         * Consider if we definitely have enough space to process TIDs on page
         * already.  If we are close to overrunning the available space for
         * dead_items TIDs, pause and do a cycle of vacuuming before we tackle
         * this page.
         */
        if (TidStoreMemoryUsage(dead_items) > dead_items_info->max_bytes)
        {
            /*
             * Before beginning index vacuuming, we release any pin we may
             * hold on the visibility map page.  This isn't necessary for
             * correctness, but we do it anyway to avoid holding the pin
             * across a lengthy, unrelated operation.
             */
            if (BufferIsValid(vmbuffer))
            {
                ReleaseBuffer(vmbuffer);
                vmbuffer = InvalidBuffer;
            }
 
            /* Perform a round of index and heap vacuuming */
            vacrel->consider_bypass_optimization = false;
            lazy_vacuum(vacrel);
 
            /*
             * Vacuum the Free Space Map to make newly-freed space visible on
             * upper-level FSM pages.  Note we have not yet processed blkno.
             */
            FreeSpaceMapVacuumRange(vacrel->rel, next_fsm_block_to_vacuum,
                                    blkno);
            next_fsm_block_to_vacuum = blkno;
 
            /* Report that we are once again scanning the heap */
            pgstat_progress_update_param(PROGRESS_VACUUM_PHASE,
                                         PROGRESS_VACUUM_PHASE_SCAN_HEAP);
        }
 
        /*
         * Pin the visibility map page in case we need to mark the page
         * all-visible.  In most cases this will be very cheap, because we'll
         * already have the correct page pinned anyway.
         */
        visibilitymap_pin(vacrel->rel, blkno, &vmbuffer);
 
        buf = ReadBufferExtended(vacrel->rel, MAIN_FORKNUM, blkno, RBM_NORMAL,
                                 vacrel->bstrategy);
        page = BufferGetPage(buf);
 
        /*
         * We need a buffer cleanup lock to prune HOT chains and defragment
         * the page in lazy_scan_prune.  But when it's not possible to acquire
         * a cleanup lock right away, we may be able to settle for reduced
         * processing using lazy_scan_noprune.
         */
        got_cleanup_lock = ConditionalLockBufferForCleanup(buf);
 
        if (!got_cleanup_lock)
            LockBuffer(buf, BUFFER_LOCK_SHARE);
 
        /* Check for new or empty pages before lazy_scan_[no]prune call */
        if (lazy_scan_new_or_empty(vacrel, buf, blkno, page, !got_cleanup_lock,
                                   vmbuffer))
        {
            /* Processed as new/empty page (lock and pin released) */
            continue;
        }
 
        /*
         * If we didn't get the cleanup lock, we can still collect LP_DEAD
         * items in the dead_items area for later vacuuming, count live and
         * recently dead tuples for vacuum logging, and determine if this
         * block could later be truncated. If we encounter any xid/mxids that
         * require advancing the relfrozenxid/relminxid, we'll have to wait
         * for a cleanup lock and call lazy_scan_prune().
         */
        if (!got_cleanup_lock &&
            !lazy_scan_noprune(vacrel, buf, blkno, page, &has_lpdead_items))
        {
            /*
             * lazy_scan_noprune could not do all required processing.  Wait
             * for a cleanup lock, and call lazy_scan_prune in the usual way.
             */
            Assert(vacrel->aggressive);
            LockBuffer(buf, BUFFER_LOCK_UNLOCK);
            LockBufferForCleanup(buf);
            got_cleanup_lock = true;
        }
 
        /*
         * If we have a cleanup lock, we must now prune, freeze, and count
         * tuples. We may have acquired the cleanup lock originally, or we may
         * have gone back and acquired it after lazy_scan_noprune() returned
         * false. Either way, the page hasn't been processed yet.
         *
         * Like lazy_scan_noprune(), lazy_scan_prune() will count
         * recently_dead_tuples and live tuples for vacuum logging, determine
         * if the block can later be truncated, and accumulate the details of
         * remaining LP_DEAD line pointers on the page into dead_items. These
         * dead items include those pruned by lazy_scan_prune() as well as
         * line pointers previously marked LP_DEAD.
         */
        if (got_cleanup_lock)
            lazy_scan_prune(vacrel, buf, blkno, page,
                            vmbuffer, all_visible_according_to_vm,
                            &has_lpdead_items);
 
        /*
         * Now drop the buffer lock and, potentially, update the FSM.
         *
         * Our goal is to update the freespace map the last time we touch the
         * page. If we'll process a block in the second pass, we may free up
         * additional space on the page, so it is better to update the FSM
         * after the second pass. If the relation has no indexes, or if index
         * vacuuming is disabled, there will be no second heap pass; if this
         * particular page has no dead items, the second heap pass will not
         * touch this page. So, in those cases, update the FSM now.
         *
         * Note: In corner cases, it's possible to miss updating the FSM
         * entirely. If index vacuuming is currently enabled, we'll skip the
         * FSM update now. But if failsafe mode is later activated, or there
         * are so few dead tuples that index vacuuming is bypassed, there will
         * also be no opportunity to update the FSM later, because we'll never
         * revisit this page. Since updating the FSM is desirable but not
         * absolutely required, that's OK.
         */
        if (vacrel->nindexes == 0
            || !vacrel->do_index_vacuuming
            || !has_lpdead_items)
        {
            Size        freespace = PageGetHeapFreeSpace(page);
 
            UnlockReleaseBuffer(buf);
            RecordPageWithFreeSpace(vacrel->rel, blkno, freespace);
 
            /*
             * Periodically perform FSM vacuuming to make newly-freed space
             * visible on upper FSM pages. This is done after vacuuming if the
             * table has indexes. There will only be newly-freed space if we
             * held the cleanup lock and lazy_scan_prune() was called.
             */
            if (got_cleanup_lock && vacrel->nindexes == 0 && has_lpdead_items &&
                blkno - next_fsm_block_to_vacuum >= VACUUM_FSM_EVERY_PAGES)
            {
                FreeSpaceMapVacuumRange(vacrel->rel, next_fsm_block_to_vacuum,
                                        blkno);
                next_fsm_block_to_vacuum = blkno;
            }
        }
        else
            UnlockReleaseBuffer(buf);
    }
 
    vacrel->blkno = InvalidBlockNumber;
    if (BufferIsValid(vmbuffer))
        ReleaseBuffer(vmbuffer);
 
    /* report that everything is now scanned */
    pgstat_progress_update_param(PROGRESS_VACUUM_HEAP_BLKS_SCANNED, blkno);
 
    /* now we can compute the new value for pg_class.reltuples */
    vacrel->new_live_tuples = vac_estimate_reltuples(vacrel->rel, rel_pages,
                                                     vacrel->scanned_pages,
                                                     vacrel->live_tuples);
 
    /*
     * Also compute the total number of surviving heap entries.  In the
     * (unlikely) scenario that new_live_tuples is -1, take it as zero.
     */
    vacrel->new_rel_tuples =
        Max(vacrel->new_live_tuples, 0) + vacrel->recently_dead_tuples +
        vacrel->missed_dead_tuples;
 
    /*
     * Do index vacuuming (call each index's ambulkdelete routine), then do
     * related heap vacuuming
     */
    if (dead_items_info->num_items > 0)
        lazy_vacuum(vacrel);
 
    /*
     * Vacuum the remainder of the Free Space Map.  We must do this whether or
     * not there were indexes, and whether or not we bypassed index vacuuming.
     */
    if (blkno > next_fsm_block_to_vacuum)
        FreeSpaceMapVacuumRange(vacrel->rel, next_fsm_block_to_vacuum, blkno);
 
    /* report all blocks vacuumed */
    pgstat_progress_update_param(PROGRESS_VACUUM_HEAP_BLKS_VACUUMED, blkno);
 
    /* Do final index cleanup (call each index's amvacuumcleanup routine) */
    if (vacrel->nindexes > 0 && vacrel->do_index_cleanup)
        lazy_cleanup_all_indexes(vacrel);
}
 
/*
 *  heap_vac_scan_next_block() -- get next block for vacuum to process
 *
 * lazy_scan_heap() calls here every time it needs to get the next block to
 * prune and vacuum.  The function uses the visibility map, vacuum options,
 * and various thresholds to skip blocks which do not need to be processed and
 * sets blkno to the next block to process.
 *
 * The block number and visibility status of the next block to process are set
 * in *blkno and *all_visible_according_to_vm.  The return value is false if
 * there are no further blocks to process.
 *
 * vacrel is an in/out parameter here.  Vacuum options and information about
 * the relation are read.  vacrel->skippedallvis is set if we skip a block
 * that's all-visible but not all-frozen, to ensure that we don't update
 * relfrozenxid in that case.  vacrel also holds information about the next
 * unskippable block, as bookkeeping for this function.
 */
static bool
heap_vac_scan_next_block(LVRelState *vacrel, BlockNumber *blkno,
                         bool *all_visible_according_to_vm)
{
    BlockNumber next_block;
 
    /* relies on InvalidBlockNumber + 1 overflowing to 0 on first call */
    next_block = vacrel->current_block + 1;
 
    /* Have we reached the end of the relation? */
    if (next_block >= vacrel->rel_pages)
    {
        if (BufferIsValid(vacrel->next_unskippable_vmbuffer))
        {
            ReleaseBuffer(vacrel->next_unskippable_vmbuffer);
            vacrel->next_unskippable_vmbuffer = InvalidBuffer;
        }
        *blkno = vacrel->rel_pages;
        return false;
    }
 
    /*
     * We must be in one of the three following states:
     */
    if (next_block > vacrel->next_unskippable_block ||
        vacrel->next_unskippable_block == InvalidBlockNumber)
    {
        /*
         * 1. We have just processed an unskippable block (or we're at the
         * beginning of the scan).  Find the next unskippable block using the
         * visibility map.
         */
        bool        skipsallvis;
 
        find_next_unskippable_block(vacrel, &skipsallvis);
 
        /*
         * We now know the next block that we must process.  It can be the
         * next block after the one we just processed, or something further
         * ahead.  If it's further ahead, we can jump to it, but we choose to
         * do so only if we can skip at least SKIP_PAGES_THRESHOLD consecutive
         * pages.  Since we're reading sequentially, the OS should be doing
         * readahead for us, so there's no gain in skipping a page now and
         * then.  Skipping such a range might even discourage sequential
         * detection.
         *
         * This test also enables more frequent relfrozenxid advancement
         * during non-aggressive VACUUMs.  If the range has any all-visible
         * pages then skipping makes updating relfrozenxid unsafe, which is a
         * real downside.
         */
        if (vacrel->next_unskippable_block - next_block >= SKIP_PAGES_THRESHOLD)
        {
            next_block = vacrel->next_unskippable_block;
            if (skipsallvis)
                vacrel->skippedallvis = true;
        }
    }
 
    /* Now we must be in one of the two remaining states: */
    if (next_block < vacrel->next_unskippable_block)
    {
        /*
         * 2. We are processing a range of blocks that we could have skipped
         * but chose not to.  We know that they are all-visible in the VM,
         * otherwise they would've been unskippable.
         */
        *blkno = vacrel->current_block = next_block;
        *all_visible_according_to_vm = true;
        return true;
    }
    else
    {
        /*
         * 3. We reached the next unskippable block.  Process it.  On next
         * iteration, we will be back in state 1.
         */
        Assert(next_block == vacrel->next_unskippable_block);
 
        *blkno = vacrel->current_block = next_block;
        *all_visible_according_to_vm = vacrel->next_unskippable_allvis;
        return true;
    }
}
 
/*
 * Find the next unskippable block in a vacuum scan using the visibility map.
 * The next unskippable block and its visibility information is updated in
 * vacrel.
 *
 * Note: our opinion of which blocks can be skipped can go stale immediately.
 * It's okay if caller "misses" a page whose all-visible or all-frozen marking
 * was concurrently cleared, though.  All that matters is that caller scan all
 * pages whose tuples might contain XIDs < OldestXmin, or MXIDs < OldestMxact.
 * (Actually, non-aggressive VACUUMs can choose to skip all-visible pages with
 * older XIDs/MXIDs.  The *skippedallvis flag will be set here when the choice
 * to skip such a range is actually made, making everything safe.)
 */
static void
find_next_unskippable_block(LVRelState *vacrel, bool *skipsallvis)
{
    BlockNumber rel_pages = vacrel->rel_pages;
    BlockNumber next_unskippable_block = vacrel->next_unskippable_block + 1;
    Buffer      next_unskippable_vmbuffer = vacrel->next_unskippable_vmbuffer;
    bool        next_unskippable_allvis;
 
    *skipsallvis = false;
 
    for (;;)
    {
        uint8       mapbits = visibilitymap_get_status(vacrel->rel,
                                                       next_unskippable_block,
                                                       &next_unskippable_vmbuffer);
 
        next_unskippable_allvis = (mapbits & VISIBILITYMAP_ALL_VISIBLE) != 0;
 
        /*
         * A block is unskippable if it is not all visible according to the
         * visibility map.
         */
        if (!next_unskippable_allvis)
        {
            Assert((mapbits & VISIBILITYMAP_ALL_FROZEN) == 0);
            break;
        }
 
        /*
         * Caller must scan the last page to determine whether it has tuples
         * (caller must have the opportunity to set vacrel->nonempty_pages).
         * This rule avoids having lazy_truncate_heap() take access-exclusive
         * lock on rel to attempt a truncation that fails anyway, just because
         * there are tuples on the last page (it is likely that there will be
         * tuples on other nearby pages as well, but those can be skipped).
         *
         * Implement this by always treating the last block as unsafe to skip.
         */
        if (next_unskippable_block == rel_pages - 1)
            break;
 
        /* DISABLE_PAGE_SKIPPING makes all skipping unsafe */
        if (!vacrel->skipwithvm)
            break;
 
        /*
         * Aggressive VACUUM caller can't skip pages just because they are
         * all-visible.  They may still skip all-frozen pages, which can't
         * contain XIDs < OldestXmin (XIDs that aren't already frozen by now).
         */
        if ((mapbits & VISIBILITYMAP_ALL_FROZEN) == 0)
        {
            if (vacrel->aggressive)
                break;
 
            /*
             * All-visible block is safe to skip in non-aggressive case.  But
             * remember that the final range contains such a block for later.
             */
            *skipsallvis = true;
        }
 
        next_unskippable_block++;
    }
 
    /* write the local variables back to vacrel */
    vacrel->next_unskippable_block = next_unskippable_block;
    vacrel->next_unskippable_allvis = next_unskippable_allvis;
    vacrel->next_unskippable_vmbuffer = next_unskippable_vmbuffer;
}
 
/*
 *  lazy_scan_new_or_empty() -- lazy_scan_heap() new/empty page handling.
 *
 * Must call here to handle both new and empty pages before calling
 * lazy_scan_prune or lazy_scan_noprune, since they're not prepared to deal
 * with new or empty pages.
 *
 * It's necessary to consider new pages as a special case, since the rules for
 * maintaining the visibility map and FSM with empty pages are a little
 * different (though new pages can be truncated away during rel truncation).
 *
 * Empty pages are not really a special case -- they're just heap pages that
 * have no allocated tuples (including even LP_UNUSED items).  You might
 * wonder why we need to handle them here all the same.  It's only necessary
 * because of a corner-case involving a hard crash during heap relation
 * extension.  If we ever make relation-extension crash safe, then it should
 * no longer be necessary to deal with empty pages here (or new pages, for
 * that matter).
 *
 * Caller must hold at least a shared lock.  We might need to escalate the
 * lock in that case, so the type of lock caller holds needs to be specified
 * using 'sharelock' argument.
 *
 * Returns false in common case where caller should go on to call
 * lazy_scan_prune (or lazy_scan_noprune).  Otherwise returns true, indicating
 * that lazy_scan_heap is done processing the page, releasing lock on caller's
 * behalf.
 */
static bool
lazy_scan_new_or_empty(LVRelState *vacrel, Buffer buf, BlockNumber blkno,
                       Page page, bool sharelock, Buffer vmbuffer)
{
    Size        freespace;
 
    if (PageIsNew(page))
    {
        /*
         * All-zeroes pages can be left over if either a backend extends the
         * relation by a single page, but crashes before the newly initialized
         * page has been written out, or when bulk-extending the relation
         * (which creates a number of empty pages at the tail end of the
         * relation), and then enters them into the FSM.
         *
         * Note we do not enter the page into the visibilitymap. That has the
         * downside that we repeatedly visit this page in subsequent vacuums,
         * but otherwise we'll never discover the space on a promoted standby.
         * The harm of repeated checking ought to normally not be too bad. The
         * space usually should be used at some point, otherwise there
         * wouldn't be any regular vacuums.
         *
         * Make sure these pages are in the FSM, to ensure they can be reused.
         * Do that by testing if there's any space recorded for the page. If
         * not, enter it. We do so after releasing the lock on the heap page,
         * the FSM is approximate, after all.
         */
        UnlockReleaseBuffer(buf);
 
        if (GetRecordedFreeSpace(vacrel->rel, blkno) == 0)
        {
            freespace = BLCKSZ - SizeOfPageHeaderData;
 
            RecordPageWithFreeSpace(vacrel->rel, blkno, freespace);
        }
 
        return true;
    }
 
    if (PageIsEmpty(page))
    {
        /*
         * It seems likely that caller will always be able to get a cleanup
         * lock on an empty page.  But don't take any chances -- escalate to
         * an exclusive lock (still don't need a cleanup lock, though).
         */
        if (sharelock)
        {
            LockBuffer(buf, BUFFER_LOCK_UNLOCK);
            LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE);
 
            if (!PageIsEmpty(page))
            {
                /* page isn't new or empty -- keep lock and pin for now */
                return false;
            }
        }
        else
        {
            /* Already have a full cleanup lock (which is more than enough) */
        }
 
        /*
         * Unlike new pages, empty pages are always set all-visible and
         * all-frozen.
         */
        if (!PageIsAllVisible(page))
        {
            START_CRIT_SECTION();
 
            /* mark buffer dirty before writing a WAL record */
            MarkBufferDirty(buf);
 
            /*
             * It's possible that another backend has extended the heap,
             * initialized the page, and then failed to WAL-log the page due
             * to an ERROR.  Since heap extension is not WAL-logged, recovery
             * might try to replay our record setting the page all-visible and
             * find that the page isn't initialized, which will cause a PANIC.
             * To prevent that, check whether the page has been previously
             * WAL-logged, and if not, do that now.
             */
            if (RelationNeedsWAL(vacrel->rel) &&
                PageGetLSN(page) == InvalidXLogRecPtr)
                log_newpage_buffer(buf, true);
 
            PageSetAllVisible(page);
            visibilitymap_set(vacrel->rel, blkno, buf, InvalidXLogRecPtr,
                              vmbuffer, InvalidTransactionId,
                              VISIBILITYMAP_ALL_VISIBLE | VISIBILITYMAP_ALL_FROZEN);
            END_CRIT_SECTION();
        }
 
        freespace = PageGetHeapFreeSpace(page);
        UnlockReleaseBuffer(buf);
        RecordPageWithFreeSpace(vacrel->rel, blkno, freespace);
        return true;
    }
 
    /* page isn't new or empty -- keep lock and pin */
    return false;
}
 
/* qsort comparator for sorting OffsetNumbers */
static int
cmpOffsetNumbers(const void *a, const void *b)
{
    return pg_cmp_u16(*(const OffsetNumber *) a, *(const OffsetNumber *) b);
}
 
/*
 *  lazy_scan_prune() -- lazy_scan_heap() pruning and freezing.
 *
 * Caller must hold pin and buffer cleanup lock on the buffer.
 *
 * vmbuffer is the buffer containing the VM block with visibility information
 * for the heap block, blkno. all_visible_according_to_vm is the saved
 * visibility status of the heap block looked up earlier by the caller. We
 * won't rely entirely on this status, as it may be out of date.
 *
 * *has_lpdead_items is set to true or false depending on whether, upon return
 * from this function, any LP_DEAD items are still present on the page.
 */
static void
lazy_scan_prune(LVRelState *vacrel,
                Buffer buf,
                BlockNumber blkno,
                Page page,
                Buffer vmbuffer,
                bool all_visible_according_to_vm,
                bool *has_lpdead_items)
{
    Relation    rel = vacrel->rel;
    PruneFreezeResult presult;
    int         prune_options = 0;
 
    Assert(BufferGetBlockNumber(buf) == blkno);
 
    /*
     * Prune all HOT-update chains and potentially freeze tuples on this page.
     *
     * If the relation has no indexes, we can immediately mark would-be dead
     * items LP_UNUSED.
     *
     * The number of tuples removed from the page is returned in
     * presult.ndeleted.  It should not be confused with presult.lpdead_items;
     * presult.lpdead_items's final value can be thought of as the number of
     * tuples that were deleted from indexes.
     *
     * We will update the VM after collecting LP_DEAD items and freezing
     * tuples. Pruning will have determined whether or not the page is
     * all-visible.
     */
    prune_options = HEAP_PAGE_PRUNE_FREEZE;
    if (vacrel->nindexes == 0)
        prune_options |= HEAP_PAGE_PRUNE_MARK_UNUSED_NOW;
 
    heap_page_prune_and_freeze(rel, buf, vacrel->vistest, prune_options,
                               &vacrel->cutoffs, &presult, PRUNE_VACUUM_SCAN,
                               &vacrel->offnum,
                               &vacrel->NewRelfrozenXid, &vacrel->NewRelminMxid);
 
    Assert(MultiXactIdIsValid(vacrel->NewRelminMxid));
    Assert(TransactionIdIsValid(vacrel->NewRelfrozenXid));
 
    if (presult.nfrozen > 0)
    {
        /*
         * We don't increment the frozen_pages instrumentation counter when
         * nfrozen == 0, since it only counts pages with newly frozen tuples
         * (don't confuse that with pages newly set all-frozen in VM).
         */
        vacrel->frozen_pages++;
    }
 
    /*
     * VACUUM will call heap_page_is_all_visible() during the second pass over
     * the heap to determine all_visible and all_frozen for the page -- this
     * is a specialized version of the logic from this function.  Now that
     * we've finished pruning and freezing, make sure that we're in total
     * agreement with heap_page_is_all_visible() using an assertion.
     */
#ifdef USE_ASSERT_CHECKING
    /* Note that all_frozen value does not matter when !all_visible */
    if (presult.all_visible)
    {
        TransactionId debug_cutoff;
        bool        debug_all_frozen;
 
        Assert(presult.lpdead_items == 0);
 
        if (!heap_page_is_all_visible(vacrel, buf,
                                      &debug_cutoff, &debug_all_frozen))
            Assert(false);
 
        Assert(presult.all_frozen == debug_all_frozen);
 
        Assert(!TransactionIdIsValid(debug_cutoff) ||
               debug_cutoff == presult.vm_conflict_horizon);
    }
#endif
 
    /*
     * Now save details of the LP_DEAD items from the page in vacrel
     */
    if (presult.lpdead_items > 0)
    {
        vacrel->lpdead_item_pages++;
 
        /*
         * deadoffsets are collected incrementally in
         * heap_page_prune_and_freeze() as each dead line pointer is recorded,
         * with an indeterminate order, but dead_items_add requires them to be
         * sorted.
         */
        qsort(presult.deadoffsets, presult.lpdead_items, sizeof(OffsetNumber),
              cmpOffsetNumbers);
 
        dead_items_add(vacrel, blkno, presult.deadoffsets, presult.lpdead_items);
    }
 
    /* Finally, add page-local counts to whole-VACUUM counts */
    vacrel->tuples_deleted += presult.ndeleted;
    vacrel->tuples_frozen += presult.nfrozen;
    vacrel->lpdead_items += presult.lpdead_items;
    vacrel->live_tuples += presult.live_tuples;
    vacrel->recently_dead_tuples += presult.recently_dead_tuples;
 
    /* Can't truncate this page */
    if (presult.hastup)
        vacrel->nonempty_pages = blkno + 1;
 
    /* Did we find LP_DEAD items? */
    *has_lpdead_items = (presult.lpdead_items > 0);
 
    Assert(!presult.all_visible || !(*has_lpdead_items));
 
    /*
     * Handle setting visibility map bit based on information from the VM (as
     * of last heap_vac_scan_next_block() call), and from all_visible and
     * all_frozen variables
     */
    if (!all_visible_according_to_vm && presult.all_visible)
    {
        uint8       flags = VISIBILITYMAP_ALL_VISIBLE;
 
        if (presult.all_frozen)
        {
            Assert(!TransactionIdIsValid(presult.vm_conflict_horizon));
            flags |= VISIBILITYMAP_ALL_FROZEN;
        }
 
        /*
         * It should never be the case that the visibility map page is set
         * while the page-level bit is clear, but the reverse is allowed (if
         * checksums are not enabled).  Regardless, set both bits so that we
         * get back in sync.
         *
         * NB: If the heap page is all-visible but the VM bit is not set, we
         * don't need to dirty the heap page.  However, if checksums are
         * enabled, we do need to make sure that the heap page is dirtied
         * before passing it to visibilitymap_set(), because it may be logged.
         * Given that this situation should only happen in rare cases after a
         * crash, it is not worth optimizing.
         */
        PageSetAllVisible(page);
        MarkBufferDirty(buf);
        visibilitymap_set(vacrel->rel, blkno, buf, InvalidXLogRecPtr,
                          vmbuffer, presult.vm_conflict_horizon,
                          flags);
    }
 
    /*
     * As of PostgreSQL 9.2, the visibility map bit should never be set if the
     * page-level bit is clear.  However, it's possible that the bit got
     * cleared after heap_vac_scan_next_block() was called, so we must recheck
     * with buffer lock before concluding that the VM is corrupt.
     */
    else if (all_visible_according_to_vm && !PageIsAllVisible(page) &&
             visibilitymap_get_status(vacrel->rel, blkno, &vmbuffer) != 0)
    {
        elog(WARNING, "page is not marked all-visible but visibility map bit is set in relation \"%s\" page %u",
             vacrel->relname, blkno);
        visibilitymap_clear(vacrel->rel, blkno, vmbuffer,
                            VISIBILITYMAP_VALID_BITS);
    }
 
    /*
     * It's possible for the value returned by
     * GetOldestNonRemovableTransactionId() to move backwards, so it's not
     * wrong for us to see tuples that appear to not be visible to everyone
     * yet, while PD_ALL_VISIBLE is already set. The real safe xmin value
     * never moves backwards, but GetOldestNonRemovableTransactionId() is
     * conservative and sometimes returns a value that's unnecessarily small,
     * so if we see that contradiction it just means that the tuples that we
     * think are not visible to everyone yet actually are, and the
     * PD_ALL_VISIBLE flag is correct.
     *
     * There should never be LP_DEAD items on a page with PD_ALL_VISIBLE set,
     * however.
     */
    else if (presult.lpdead_items > 0 && PageIsAllVisible(page))
    {
        elog(WARNING, "page containing LP_DEAD items is marked as all-visible in relation \"%s\" page %u",
             vacrel->relname, blkno);
        PageClearAllVisible(page);
        MarkBufferDirty(buf);
        visibilitymap_clear(vacrel->rel, blkno, vmbuffer,
                            VISIBILITYMAP_VALID_BITS);
    }
 
    /*
     * If the all-visible page is all-frozen but not marked as such yet, mark
     * it as all-frozen.  Note that all_frozen is only valid if all_visible is
     * true, so we must check both all_visible and all_frozen.
     */
    else if (all_visible_according_to_vm && presult.all_visible &&
             presult.all_frozen && !VM_ALL_FROZEN(vacrel->rel, blkno, &vmbuffer))
    {
        /*
         * Avoid relying on all_visible_according_to_vm as a proxy for the
         * page-level PD_ALL_VISIBLE bit being set, since it might have become
         * stale -- even when all_visible is set
         */
        if (!PageIsAllVisible(page))
        {
            PageSetAllVisible(page);
            MarkBufferDirty(buf);
        }
 
        /*
         * Set the page all-frozen (and all-visible) in the VM.
         *
         * We can pass InvalidTransactionId as our cutoff_xid, since a
         * snapshotConflictHorizon sufficient to make everything safe for REDO
         * was logged when the page's tuples were frozen.
         */
        Assert(!TransactionIdIsValid(presult.vm_conflict_horizon));
        visibilitymap_set(vacrel->rel, blkno, buf, InvalidXLogRecPtr,
                          vmbuffer, InvalidTransactionId,
                          VISIBILITYMAP_ALL_VISIBLE |
                          VISIBILITYMAP_ALL_FROZEN);
    }
}
 
/*
 *  lazy_scan_noprune() -- lazy_scan_prune() without pruning or freezing
 *
 * Caller need only hold a pin and share lock on the buffer, unlike
 * lazy_scan_prune, which requires a full cleanup lock.  While pruning isn't
 * performed here, it's quite possible that an earlier opportunistic pruning
 * operation left LP_DEAD items behind.  We'll at least collect any such items
 * in dead_items for removal from indexes.
 *
 * For aggressive VACUUM callers, we may return false to indicate that a full
 * cleanup lock is required for processing by lazy_scan_prune.  This is only
 * necessary when the aggressive VACUUM needs to freeze some tuple XIDs from
 * one or more tuples on the page.  We always return true for non-aggressive
 * callers.
 *
 * If this function returns true, *has_lpdead_items gets set to true or false
 * depending on whether, upon return from this function, any LP_DEAD items are
 * present on the page. If this function returns false, *has_lpdead_items
 * is not updated.
 */
static bool
lazy_scan_noprune(LVRelState *vacrel,
                  Buffer buf,
                  BlockNumber blkno,
                  Page page,
                  bool *has_lpdead_items)
{
    OffsetNumber offnum,
                maxoff;
    int         lpdead_items,
                live_tuples,
                recently_dead_tuples,
                missed_dead_tuples;
    bool        hastup;
    HeapTupleHeader tupleheader;
    TransactionId NoFreezePageRelfrozenXid = vacrel->NewRelfrozenXid;
    MultiXactId NoFreezePageRelminMxid = vacrel->NewRelminMxid;
    OffsetNumber deadoffsets[MaxHeapTuplesPerPage];
 
    Assert(BufferGetBlockNumber(buf) == blkno);
 
    hastup = false;             /* for now */
 
    lpdead_items = 0;
    live_tuples = 0;
    recently_dead_tuples = 0;
    missed_dead_tuples = 0;
 
    maxoff = PageGetMaxOffsetNumber(page);
    for (offnum = FirstOffsetNumber;
         offnum <= maxoff;
         offnum = OffsetNumberNext(offnum))
    {
        ItemId      itemid;
        HeapTupleData tuple;
 
        vacrel->offnum = offnum;
        itemid = PageGetItemId(page, offnum);
 
        if (!ItemIdIsUsed(itemid))
            continue;
 
        if (ItemIdIsRedirected(itemid))
        {
            hastup = true;
            continue;
        }
 
        if (ItemIdIsDead(itemid))
        {
            /*
             * Deliberately don't set hastup=true here.  See same point in
             * lazy_scan_prune for an explanation.
             */
            deadoffsets[lpdead_items++] = offnum;
            continue;
        }
 
        hastup = true;          /* page prevents rel truncation */
        tupleheader = (HeapTupleHeader) PageGetItem(page, itemid);
        if (heap_tuple_should_freeze(tupleheader, &vacrel->cutoffs,
                                     &NoFreezePageRelfrozenXid,
                                     &NoFreezePageRelminMxid))
        {
            /* Tuple with XID < FreezeLimit (or MXID < MultiXactCutoff) */
            if (vacrel->aggressive)
            {
                /*
                 * Aggressive VACUUMs must always be able to advance rel's
                 * relfrozenxid to a value >= FreezeLimit (and be able to
                 * advance rel's relminmxid to a value >= MultiXactCutoff).
                 * The ongoing aggressive VACUUM won't be able to do that
                 * unless it can freeze an XID (or MXID) from this tuple now.
                 *
                 * The only safe option is to have caller perform processing
                 * of this page using lazy_scan_prune.  Caller might have to
                 * wait a while for a cleanup lock, but it can't be helped.
                 */
                vacrel->offnum = InvalidOffsetNumber;
                return false;
            }
 
            /*
             * Non-aggressive VACUUMs are under no obligation to advance
             * relfrozenxid (even by one XID).  We can be much laxer here.
             *
             * Currently we always just accept an older final relfrozenxid
             * and/or relminmxid value.  We never make caller wait or work a
             * little harder, even when it likely makes sense to do so.
             */
        }
 
        ItemPointerSet(&(tuple.t_self), blkno, offnum);
        tuple.t_data = (HeapTupleHeader) PageGetItem(page, itemid);
        tuple.t_len = ItemIdGetLength(itemid);
        tuple.t_tableOid = RelationGetRelid(vacrel->rel);
 
        switch (HeapTupleSatisfiesVacuum(&tuple, vacrel->cutoffs.OldestXmin,
                                         buf))
        {
            case HEAPTUPLE_DELETE_IN_PROGRESS:
            case HEAPTUPLE_LIVE:
 
                /*
                 * Count both cases as live, just like lazy_scan_prune
                 */
                live_tuples++;
 
                break;
            case HEAPTUPLE_DEAD:
 
                /*
                 * There is some useful work for pruning to do, that won't be
                 * done due to failure to get a cleanup lock.
                 */
                missed_dead_tuples++;
                break;
            case HEAPTUPLE_RECENTLY_DEAD:
 
                /*
                 * Count in recently_dead_tuples, just like lazy_scan_prune
                 */
                recently_dead_tuples++;
                break;
            case HEAPTUPLE_INSERT_IN_PROGRESS:
 
                /*
                 * Do not count these rows as live, just like lazy_scan_prune
                 */
                break;
            default:
                elog(ERROR, "unexpected HeapTupleSatisfiesVacuum result");
                break;
        }
    }
 
    vacrel->offnum = InvalidOffsetNumber;
 
    /*
     * By here we know for sure that caller can put off freezing and pruning
     * this particular page until the next VACUUM.  Remember its details now.
     * (lazy_scan_prune expects a clean slate, so we have to do this last.)
     */
    vacrel->NewRelfrozenXid = NoFreezePageRelfrozenXid;
    vacrel->NewRelminMxid = NoFreezePageRelminMxid;
 
    /* Save any LP_DEAD items found on the page in dead_items */
    if (vacrel->nindexes == 0)
    {
        /* Using one-pass strategy (since table has no indexes) */
        if (lpdead_items > 0)
        {
            /*
             * Perfunctory handling for the corner case where a single pass
             * strategy VACUUM cannot get a cleanup lock, and it turns out
             * that there is one or more LP_DEAD items: just count the LP_DEAD
             * items as missed_dead_tuples instead. (This is a bit dishonest,
             * but it beats having to maintain specialized heap vacuuming code
             * forever, for vanishingly little benefit.)
             */
            hastup = true;
            missed_dead_tuples += lpdead_items;
        }
    }
    else if (lpdead_items > 0)
    {
        /*
         * Page has LP_DEAD items, and so any references/TIDs that remain in
         * indexes will be deleted during index vacuuming (and then marked
         * LP_UNUSED in the heap)
         */
        vacrel->lpdead_item_pages++;
 
        dead_items_add(vacrel, blkno, deadoffsets, lpdead_items);
 
        vacrel->lpdead_items += lpdead_items;
    }
 
    /*
     * Finally, add relevant page-local counts to whole-VACUUM counts
     */
    vacrel->live_tuples += live_tuples;
    vacrel->recently_dead_tuples += recently_dead_tuples;
    vacrel->missed_dead_tuples += missed_dead_tuples;
    if (missed_dead_tuples > 0)
        vacrel->missed_dead_pages++;
 
    /* Can't truncate this page */
    if (hastup)
        vacrel->nonempty_pages = blkno + 1;
 
    /* Did we find LP_DEAD items? */
    *has_lpdead_items = (lpdead_items > 0);
 
    /* Caller won't need to call lazy_scan_prune with same page */
    return true;
}
 
/*
 * Main entry point for index vacuuming and heap vacuuming.
 *
 * Removes items collected in dead_items from table's indexes, then marks the
 * same items LP_UNUSED in the heap.  See the comments above lazy_scan_heap
 * for full details.
 *
 * Also empties dead_items, freeing up space for later TIDs.
 *
 * We may choose to bypass index vacuuming at this point, though only when the
 * ongoing VACUUM operation will definitely only have one index scan/round of
 * index vacuuming.
 */
static void
lazy_vacuum(LVRelState *vacrel)
{
    bool        bypass;
 
    /* Should not end up here with no indexes */
    Assert(vacrel->nindexes > 0);
    Assert(vacrel->lpdead_item_pages > 0);
 
    if (!vacrel->do_index_vacuuming)
    {
        Assert(!vacrel->do_index_cleanup);
        dead_items_reset(vacrel);
        return;
    }
 
    /*
     * Consider bypassing index vacuuming (and heap vacuuming) entirely.
     *
     * We currently only do this in cases where the number of LP_DEAD items
     * for the entire VACUUM operation is close to zero.  This avoids sharp
     * discontinuities in the duration and overhead of successive VACUUM
     * operations that run against the same table with a fixed workload.
     * Ideally, successive VACUUM operations will behave as if there are
     * exactly zero LP_DEAD items in cases where there are close to zero.
     *
     * This is likely to be helpful with a table that is continually affected
     * by UPDATEs that can mostly apply the HOT optimization, but occasionally
     * have small aberrations that lead to just a few heap pages retaining
     * only one or two LP_DEAD items.  This is pretty common; even when the
     * DBA goes out of their way to make UPDATEs use HOT, it is practically
     * impossible to predict whether HOT will be applied in 100% of cases.
     * It's far easier to ensure that 99%+ of all UPDATEs against a table use
     * HOT through careful tuning.
     */
    bypass = false;
    if (vacrel->consider_bypass_optimization && vacrel->rel_pages > 0)
    {
        BlockNumber threshold;
 
        Assert(vacrel->num_index_scans == 0);
        Assert(vacrel->lpdead_items == vacrel->dead_items_info->num_items);
        Assert(vacrel->do_index_vacuuming);
        Assert(vacrel->do_index_cleanup);
 
        /*
         * This crossover point at which we'll start to do index vacuuming is
         * expressed as a percentage of the total number of heap pages in the
         * table that are known to have at least one LP_DEAD item.  This is
         * much more important than the total number of LP_DEAD items, since
         * it's a proxy for the number of heap pages whose visibility map bits
         * cannot be set on account of bypassing index and heap vacuuming.
         *
         * We apply one further precautionary test: the space currently used
         * to store the TIDs (TIDs that now all point to LP_DEAD items) must
         * not exceed 32MB.  This limits the risk that we will bypass index
         * vacuuming again and again until eventually there is a VACUUM whose
         * dead_items space is not CPU cache resident.
         *
         * We don't take any special steps to remember the LP_DEAD items (such
         * as counting them in our final update to the stats system) when the
         * optimization is applied.  Though the accounting used in analyze.c's
         * acquire_sample_rows() will recognize the same LP_DEAD items as dead
         * rows in its own stats report, that's okay. The discrepancy should
         * be negligible.  If this optimization is ever expanded to cover more
         * cases then this may need to be reconsidered.
         */
        threshold = (double) vacrel->rel_pages * BYPASS_THRESHOLD_PAGES;
        bypass = (vacrel->lpdead_item_pages < threshold &&
                  (TidStoreMemoryUsage(vacrel->dead_items) < (32L * 1024L * 1024L)));
    }
 
    if (bypass)
    {
        /*
         * There are almost zero TIDs.  Behave as if there were precisely
         * zero: bypass index vacuuming, but do index cleanup.
         *
         * We expect that the ongoing VACUUM operation will finish very
         * quickly, so there is no point in considering speeding up as a
         * failsafe against wraparound failure. (Index cleanup is expected to
         * finish very quickly in cases where there were no ambulkdelete()
         * calls.)
         */
        vacrel->do_index_vacuuming = false;
    }
    else if (lazy_vacuum_all_indexes(vacrel))
    {
        /*
         * We successfully completed a round of index vacuuming.  Do related
         * heap vacuuming now.
         */
        lazy_vacuum_heap_rel(vacrel);
    }
    else
    {
        /*
         * Failsafe case.
         *
         * We attempted index vacuuming, but didn't finish a full round/full
         * index scan.  This happens when relfrozenxid or relminmxid is too
         * far in the past.
         *
         * From this point on the VACUUM operation will do no further index
         * vacuuming or heap vacuuming.  This VACUUM operation won't end up
         * back here again.
         */
        Assert(VacuumFailsafeActive);
    }
 
    /*
     * Forget the LP_DEAD items that we just vacuumed (or just decided to not
     * vacuum)
     */
    dead_items_reset(vacrel);
}
 
/*
 *  lazy_vacuum_all_indexes() -- Main entry for index vacuuming
 *
 * Returns true in the common case when all indexes were successfully
 * vacuumed.  Returns false in rare cases where we determined that the ongoing
 * VACUUM operation is at risk of taking too long to finish, leading to
 * wraparound failure.
 */
static bool
lazy_vacuum_all_indexes(LVRelState *vacrel)
{
    bool        allindexes = true;
    double      old_live_tuples = vacrel->rel->rd_rel->reltuples;
    const int   progress_start_index[] = {
        PROGRESS_VACUUM_PHASE,
        PROGRESS_VACUUM_INDEXES_TOTAL
    };
    const int   progress_end_index[] = {
        PROGRESS_VACUUM_INDEXES_TOTAL,
        PROGRESS_VACUUM_INDEXES_PROCESSED,
        PROGRESS_VACUUM_NUM_INDEX_VACUUMS
    };
    int64       progress_start_val[2];
    int64       progress_end_val[3];
 
    Assert(vacrel->nindexes > 0);
    Assert(vacrel->do_index_vacuuming);
    Assert(vacrel->do_index_cleanup);
 
    /* Precheck for XID wraparound emergencies */
    if (lazy_check_wraparound_failsafe(vacrel))
    {
        /* Wraparound emergency -- don't even start an index scan */
        return false;
    }
 
    /*
     * Report that we are now vacuuming indexes and the number of indexes to
     * vacuum.
     */
    progress_start_val[0] = PROGRESS_VACUUM_PHASE_VACUUM_INDEX;
    progress_start_val[1] = vacrel->nindexes;
    pgstat_progress_update_multi_param(2, progress_start_index, progress_start_val);
 
    if (!ParallelVacuumIsActive(vacrel))
    {
        for (int idx = 0; idx < vacrel->nindexes; idx++)
        {
            Relation    indrel = vacrel->indrels[idx];
            IndexBulkDeleteResult *istat = vacrel->indstats[idx];
 
            vacrel->indstats[idx] = lazy_vacuum_one_index(indrel, istat,
                                                          old_live_tuples,
                                                          vacrel);
 
            /* Report the number of indexes vacuumed */
            pgstat_progress_update_param(PROGRESS_VACUUM_INDEXES_PROCESSED,
                                         idx + 1);
 
            if (lazy_check_wraparound_failsafe(vacrel))
            {
                /* Wraparound emergency -- end current index scan */
                allindexes = false;
                break;
            }
        }
    }
    else
    {
        /* Outsource everything to parallel variant */
        parallel_vacuum_bulkdel_all_indexes(vacrel->pvs, old_live_tuples,
                                            vacrel->num_index_scans);
 
        /*
         * Do a postcheck to consider applying wraparound failsafe now.  Note
         * that parallel VACUUM only gets the precheck and this postcheck.
         */
        if (lazy_check_wraparound_failsafe(vacrel))
            allindexes = false;
    }
 
    /*
     * We delete all LP_DEAD items from the first heap pass in all indexes on
     * each call here (except calls where we choose to do the failsafe). This
     * makes the next call to lazy_vacuum_heap_rel() safe (except in the event
     * of the failsafe triggering, which prevents the next call from taking
     * place).
     */
    Assert(vacrel->num_index_scans > 0 ||
           vacrel->dead_items_info->num_items == vacrel->lpdead_items);
    Assert(allindexes || VacuumFailsafeActive);
 
    /*
     * Increase and report the number of index scans.  Also, we reset
     * PROGRESS_VACUUM_INDEXES_TOTAL and PROGRESS_VACUUM_INDEXES_PROCESSED.
     *
     * We deliberately include the case where we started a round of bulk
     * deletes that we weren't able to finish due to the failsafe triggering.
     */
    vacrel->num_index_scans++;
    progress_end_val[0] = 0;
    progress_end_val[1] = 0;
    progress_end_val[2] = vacrel->num_index_scans;
    pgstat_progress_update_multi_param(3, progress_end_index, progress_end_val);
 
    return allindexes;
}
 
/*
 *  lazy_vacuum_heap_rel() -- second pass over the heap for two pass strategy
 *
 * This routine marks LP_DEAD items in vacrel->dead_items as LP_UNUSED. Pages
 * that never had lazy_scan_prune record LP_DEAD items are not visited at all.
 *
 * We may also be able to truncate the line pointer array of the heap pages we
 * visit.  If there is a contiguous group of LP_UNUSED items at the end of the
 * array, it can be reclaimed as free space.  These LP_UNUSED items usually
 * start out as LP_DEAD items recorded by lazy_scan_prune (we set items from
 * each page to LP_UNUSED, and then consider if it's possible to truncate the
 * page's line pointer array).
 *
 * Note: the reason for doing this as a second pass is we cannot remove the
 * tuples until we've removed their index entries, and we want to process
 * index entry removal in batches as large as possible.
 */
static void
lazy_vacuum_heap_rel(LVRelState *vacrel)
{
    BlockNumber vacuumed_pages = 0;
    Buffer      vmbuffer = InvalidBuffer;
    LVSavedErrInfo saved_err_info;
    TidStoreIter *iter;
    TidStoreIterResult *iter_result;
 
    Assert(vacrel->do_index_vacuuming);
    Assert(vacrel->do_index_cleanup);
    Assert(vacrel->num_index_scans > 0);
 
    /* Report that we are now vacuuming the heap */
    pgstat_progress_update_param(PROGRESS_VACUUM_PHASE,
                                 PROGRESS_VACUUM_PHASE_VACUUM_HEAP);
 
    /* Update error traceback information */
    update_vacuum_error_info(vacrel, &saved_err_info,
                             VACUUM_ERRCB_PHASE_VACUUM_HEAP,
                             InvalidBlockNumber, InvalidOffsetNumber);
 
    iter = TidStoreBeginIterate(vacrel->dead_items);
    while ((iter_result = TidStoreIterateNext(iter)) != NULL)
    {
        BlockNumber blkno;
        Buffer      buf;
        Page        page;
        Size        freespace;
 
        vacuum_delay_point();
 
        blkno = iter_result->blkno;
        vacrel->blkno = blkno;
 
        /*
         * Pin the visibility map page in case we need to mark the page
         * all-visible.  In most cases this will be very cheap, because we'll
         * already have the correct page pinned anyway.
         */
        visibilitymap_pin(vacrel->rel, blkno, &vmbuffer);
 
        /* We need a non-cleanup exclusive lock to mark dead_items unused */
        buf = ReadBufferExtended(vacrel->rel, MAIN_FORKNUM, blkno, RBM_NORMAL,
                                 vacrel->bstrategy);
        LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE);
        lazy_vacuum_heap_page(vacrel, blkno, buf, iter_result->offsets,
                              iter_result->num_offsets, vmbuffer);
 
        /* Now that we've vacuumed the page, record its available space */
        page = BufferGetPage(buf);
        freespace = PageGetHeapFreeSpace(page);
 
        UnlockReleaseBuffer(buf);
        RecordPageWithFreeSpace(vacrel->rel, blkno, freespace);
        vacuumed_pages++;
    }
    TidStoreEndIterate(iter);
 
    vacrel->blkno = InvalidBlockNumber;
    if (BufferIsValid(vmbuffer))
        ReleaseBuffer(vmbuffer);
 
    /*
     * We set all LP_DEAD items from the first heap pass to LP_UNUSED during
     * the second heap pass.  No more, no less.
     */
    Assert(vacrel->num_index_scans > 1 ||
           (vacrel->dead_items_info->num_items == vacrel->lpdead_items &&
            vacuumed_pages == vacrel->lpdead_item_pages));
 
    ereport(DEBUG2,
            (errmsg("table \"%s\": removed %lld dead item identifiers in %u pages",
                    vacrel->relname, (long long) vacrel->dead_items_info->num_items,
                    vacuumed_pages)));
 
    /* Revert to the previous phase information for error traceback */
    restore_vacuum_error_info(vacrel, &saved_err_info);
}
 
/*
 *  lazy_vacuum_heap_page() -- free page's LP_DEAD items listed in the
 *                        vacrel->dead_items store.
 *
 * Caller must have an exclusive buffer lock on the buffer (though a full
 * cleanup lock is also acceptable).  vmbuffer must be valid and already have
 * a pin on blkno's visibility map page.
 */
static void
lazy_vacuum_heap_page(LVRelState *vacrel, BlockNumber blkno, Buffer buffer,
                      OffsetNumber *deadoffsets, int num_offsets,
                      Buffer vmbuffer)
{
    Page        page = BufferGetPage(buffer);
    OffsetNumber unused[MaxHeapTuplesPerPage];
    int         nunused = 0;
    TransactionId visibility_cutoff_xid;
    bool        all_frozen;
    LVSavedErrInfo saved_err_info;
 
    Assert(vacrel->do_index_vacuuming);
 
    pgstat_progress_update_param(PROGRESS_VACUUM_HEAP_BLKS_VACUUMED, blkno);
 
    /* Update error traceback information */
    update_vacuum_error_info(vacrel, &saved_err_info,
                             VACUUM_ERRCB_PHASE_VACUUM_HEAP, blkno,
                             InvalidOffsetNumber);
 
    START_CRIT_SECTION();
 
    for (int i = 0; i < num_offsets; i++)
    {
        ItemId      itemid;
        OffsetNumber toff = deadoffsets[i];
 
        itemid = PageGetItemId(page, toff);
 
        Assert(ItemIdIsDead(itemid) && !ItemIdHasStorage(itemid));
        ItemIdSetUnused(itemid);
        unused[nunused++] = toff;
    }
 
    Assert(nunused > 0);
 
    /* Attempt to truncate line pointer array now */
    PageTruncateLinePointerArray(page);
 
    /*
     * Mark buffer dirty before we write WAL.
     */
    MarkBufferDirty(buffer);
 
    /* XLOG stuff */
    if (RelationNeedsWAL(vacrel->rel))
    {
        log_heap_prune_and_freeze(vacrel->rel, buffer,
                                  InvalidTransactionId,
                                  false,    /* no cleanup lock required */
                                  PRUNE_VACUUM_CLEANUP,
                                  NULL, 0,  /* frozen */
                                  NULL, 0,  /* redirected */
                                  NULL, 0,  /* dead */
                                  unused, nunused);
    }
 
    /*
     * End critical section, so we safely can do visibility tests (which
     * possibly need to perform IO and allocate memory!). If we crash now the
     * page (including the corresponding vm bit) might not be marked all
     * visible, but that's fine. A later vacuum will fix that.
     */
    END_CRIT_SECTION();
 
    /*
     * Now that we have removed the LP_DEAD items from the page, once again
     * check if the page has become all-visible.  The page is already marked
     * dirty, exclusively locked, and, if needed, a full page image has been
     * emitted.
     */
    Assert(!PageIsAllVisible(page));
    if (heap_page_is_all_visible(vacrel, buffer, &visibility_cutoff_xid,
                                 &all_frozen))
    {
        uint8       flags = VISIBILITYMAP_ALL_VISIBLE;
 
        if (all_frozen)
        {
            Assert(!TransactionIdIsValid(visibility_cutoff_xid));
            flags |= VISIBILITYMAP_ALL_FROZEN;
        }
 
        PageSetAllVisible(page);
        visibilitymap_set(vacrel->rel, blkno, buffer, InvalidXLogRecPtr,
                          vmbuffer, visibility_cutoff_xid, flags);
    }
 
    /* Revert to the previous phase information for error traceback */
    restore_vacuum_error_info(vacrel, &saved_err_info);
}
 
/*
 * Trigger the failsafe to avoid wraparound failure when vacrel table has a
 * relfrozenxid and/or relminmxid that is dangerously far in the past.
 * Triggering the failsafe makes the ongoing VACUUM bypass any further index
 * vacuuming and heap vacuuming.  Truncating the heap is also bypassed.
 *
 * Any remaining work (work that VACUUM cannot just bypass) is typically sped
 * up when the failsafe triggers.  VACUUM stops applying any cost-based delay
 * that it started out with.
 *
 * Returns true when failsafe has been triggered.
 */
static bool
lazy_check_wraparound_failsafe(LVRelState *vacrel)
{
    /* Don't warn more than once per VACUUM */
    if (VacuumFailsafeActive)
        return true;
 
    if (unlikely(vacuum_xid_failsafe_check(&vacrel->cutoffs)))
    {
        const int   progress_index[] = {
            PROGRESS_VACUUM_INDEXES_TOTAL,
            PROGRESS_VACUUM_INDEXES_PROCESSED
        };
        int64       progress_val[2] = {0, 0};
 
        VacuumFailsafeActive = true;
 
        /*
         * Abandon use of a buffer access strategy to allow use of all of
         * shared buffers.  We assume the caller who allocated the memory for
         * the BufferAccessStrategy will free it.
         */
        vacrel->bstrategy = NULL;
 
        /* Disable index vacuuming, index cleanup, and heap rel truncation */
        vacrel->do_index_vacuuming = false;
        vacrel->do_index_cleanup = false;
        vacrel->do_rel_truncate = false;
 
        /* Reset the progress counters */
        pgstat_progress_update_multi_param(2, progress_index, progress_val);
 
        ereport(WARNING,
                (errmsg("bypassing nonessential maintenance of table \"%s.%s.%s\" as a failsafe after %d index scans",
                        vacrel->dbname, vacrel->relnamespace, vacrel->relname,
                        vacrel->num_index_scans),
                 errdetail("The table's relfrozenxid or relminmxid is too far in the past."),
                 errhint("Consider increasing configuration parameter maintenance_work_mem or autovacuum_work_mem.\n"
                         "You might also need to consider other ways for VACUUM to keep up with the allocation of transaction IDs.")));
 
        /* Stop applying cost limits from this point on */
        VacuumCostActive = false;
        VacuumCostBalance = 0;
 
        return true;
    }
 
    return false;
}
 
/*
 *  lazy_cleanup_all_indexes() -- cleanup all indexes of relation.
 */
static void
lazy_cleanup_all_indexes(LVRelState *vacrel)
{
    double      reltuples = vacrel->new_rel_tuples;
    bool        estimated_count = vacrel->scanned_pages < vacrel->rel_pages;
    const int   progress_start_index[] = {
        PROGRESS_VACUUM_PHASE,
        PROGRESS_VACUUM_INDEXES_TOTAL
    };
    const int   progress_end_index[] = {
        PROGRESS_VACUUM_INDEXES_TOTAL,
        PROGRESS_VACUUM_INDEXES_PROCESSED
    };
    int64       progress_start_val[2];
    int64       progress_end_val[2] = {0, 0};
 
    Assert(vacrel->do_index_cleanup);
    Assert(vacrel->nindexes > 0);
 
    /*
     * Report that we are now cleaning up indexes and the number of indexes to
     * cleanup.
     */
    progress_start_val[0] = PROGRESS_VACUUM_PHASE_INDEX_CLEANUP;
    progress_start_val[1] = vacrel->nindexes;
    pgstat_progress_update_multi_param(2, progress_start_index, progress_start_val);
 
    if (!ParallelVacuumIsActive(vacrel))
    {
        for (int idx = 0; idx < vacrel->nindexes; idx++)
        {
            Relation    indrel = vacrel->indrels[idx];
            IndexBulkDeleteResult *istat = vacrel->indstats[idx];
 
            vacrel->indstats[idx] =
                lazy_cleanup_one_index(indrel, istat, reltuples,
                                       estimated_count, vacrel);
 
            /* Report the number of indexes cleaned up */
            pgstat_progress_update_param(PROGRESS_VACUUM_INDEXES_PROCESSED,
                                         idx + 1);
        }
    }
    else
    {
        /* Outsource everything to parallel variant */
        parallel_vacuum_cleanup_all_indexes(vacrel->pvs, reltuples,
                                            vacrel->num_index_scans,
                                            estimated_count);
    }
 
    /* Reset the progress counters */
    pgstat_progress_update_multi_param(2, progress_end_index, progress_end_val);
}
 
/*
 *  lazy_vacuum_one_index() -- vacuum index relation.
 *
 *      Delete all the index tuples containing a TID collected in
 *      vacrel->dead_items.  Also update running statistics. Exact
 *      details depend on index AM's ambulkdelete routine.
 *
 *      reltuples is the number of heap tuples to be passed to the
 *      bulkdelete callback.  It's always assumed to be estimated.
 *      See indexam.sgml for more info.
 *
 * Returns bulk delete stats derived from input stats
 */
static IndexBulkDeleteResult *
lazy_vacuum_one_index(Relation indrel, IndexBulkDeleteResult *istat,
                      double reltuples, LVRelState *vacrel)
{
    IndexVacuumInfo ivinfo;
    LVSavedErrInfo saved_err_info;
 
    ivinfo.index = indrel;
    ivinfo.heaprel = vacrel->rel;
    ivinfo.analyze_only = false;
    ivinfo.report_progress = false;
    ivinfo.estimated_count = true;
    ivinfo.message_level = DEBUG2;
    ivinfo.num_heap_tuples = reltuples;
    ivinfo.strategy = vacrel->bstrategy;
 
    /*
     * Update error traceback information.
     *
     * The index name is saved during this phase and restored immediately
     * after this phase.  See vacuum_error_callback.
     */
    Assert(vacrel->indname == NULL);
    vacrel->indname = pstrdup(RelationGetRelationName(indrel));
    update_vacuum_error_info(vacrel, &saved_err_info,
                             VACUUM_ERRCB_PHASE_VACUUM_INDEX,
                             InvalidBlockNumber, InvalidOffsetNumber);
 
    /* Do bulk deletion */
    istat = vac_bulkdel_one_index(&ivinfo, istat, (void *) vacrel->dead_items,
                                  vacrel->dead_items_info);
 
    /* Revert to the previous phase information for error traceback */
    restore_vacuum_error_info(vacrel, &saved_err_info);
    pfree(vacrel->indname);
    vacrel->indname = NULL;
 
    return istat;
}
 
/*
 *  lazy_cleanup_one_index() -- do post-vacuum cleanup for index relation.
 *
 *      Calls index AM's amvacuumcleanup routine.  reltuples is the number
 *      of heap tuples and estimated_count is true if reltuples is an
 *      estimated value.  See indexam.sgml for more info.
 *
 * Returns bulk delete stats derived from input stats
 */
static IndexBulkDeleteResult *
lazy_cleanup_one_index(Relation indrel, IndexBulkDeleteResult *istat,
                       double reltuples, bool estimated_count,
                       LVRelState *vacrel)
{
    IndexVacuumInfo ivinfo;
    LVSavedErrInfo saved_err_info;
 
    ivinfo.index = indrel;
    ivinfo.heaprel = vacrel->rel;
    ivinfo.analyze_only = false;
    ivinfo.report_progress = false;
    ivinfo.estimated_count = estimated_count;
    ivinfo.message_level = DEBUG2;
 
    ivinfo.num_heap_tuples = reltuples;
    ivinfo.strategy = vacrel->bstrategy;
 
    /*
     * Update error traceback information.
     *
     * The index name is saved during this phase and restored immediately
     * after this phase.  See vacuum_error_callback.
     */
    Assert(vacrel->indname == NULL);
    vacrel->indname = pstrdup(RelationGetRelationName(indrel));
    update_vacuum_error_info(vacrel, &saved_err_info,
                             VACUUM_ERRCB_PHASE_INDEX_CLEANUP,
                             InvalidBlockNumber, InvalidOffsetNumber);
 
    istat = vac_cleanup_one_index(&ivinfo, istat);
 
    /* Revert to the previous phase information for error traceback */
    restore_vacuum_error_info(vacrel, &saved_err_info);
    pfree(vacrel->indname);
    vacrel->indname = NULL;
 
    return istat;
}
 
/*
 * should_attempt_truncation - should we attempt to truncate the heap?
 *
 * Don't even think about it unless we have a shot at releasing a goodly
 * number of pages.  Otherwise, the time taken isn't worth it, mainly because
 * an AccessExclusive lock must be replayed on any hot standby, where it can
 * be particularly disruptive.
 *
 * Also don't attempt it if wraparound failsafe is in effect.  The entire
 * system might be refusing to allocate new XIDs at this point.  The system
 * definitely won't return to normal unless and until VACUUM actually advances
 * the oldest relfrozenxid -- which hasn't happened for target rel just yet.
 * If lazy_truncate_heap attempted to acquire an AccessExclusiveLock to
 * truncate the table under these circumstances, an XID exhaustion error might
 * make it impossible for VACUUM to fix the underlying XID exhaustion problem.
 * There is very little chance of truncation working out when the failsafe is
 * in effect in any case.  lazy_scan_prune makes the optimistic assumption
 * that any LP_DEAD items it encounters will always be LP_UNUSED by the time
 * we're called.
 */
static bool
should_attempt_truncation(LVRelState *vacrel)
{
    BlockNumber possibly_freeable;
 
    if (!vacrel->do_rel_truncate || VacuumFailsafeActive)
        return false;
 
    possibly_freeable = vacrel->rel_pages - vacrel->nonempty_pages;
    if (possibly_freeable > 0 &&
        (possibly_freeable >= REL_TRUNCATE_MINIMUM ||
         possibly_freeable >= vacrel->rel_pages / REL_TRUNCATE_FRACTION))
        return true;
 
    return false;
}
 
/*
 * lazy_truncate_heap - try to truncate off any empty pages at the end
 */
static void
lazy_truncate_heap(LVRelState *vacrel)
{
    BlockNumber orig_rel_pages = vacrel->rel_pages;
    BlockNumber new_rel_pages;
    bool        lock_waiter_detected;
    int         lock_retry;
 
    /* Report that we are now truncating */
    pgstat_progress_update_param(PROGRESS_VACUUM_PHASE,
                                 PROGRESS_VACUUM_PHASE_TRUNCATE);
 
    /* Update error traceback information one last time */
    update_vacuum_error_info(vacrel, NULL, VACUUM_ERRCB_PHASE_TRUNCATE,
                             vacrel->nonempty_pages, InvalidOffsetNumber);
 
    /*
     * Loop until no more truncating can be done.
     */
    do
    {
        /*
         * We need full exclusive lock on the relation in order to do
         * truncation. If we can't get it, give up rather than waiting --- we
         * don't want to block other backends, and we don't want to deadlock
         * (which is quite possible considering we already hold a lower-grade
         * lock).
         */
        lock_waiter_detected = false;
        lock_retry = 0;
        while (true)
        {
            if (ConditionalLockRelation(vacrel->rel, AccessExclusiveLock))
                break;
 
            /*
             * Check for interrupts while trying to (re-)acquire the exclusive
             * lock.
             */
            CHECK_FOR_INTERRUPTS();
 
            if (++lock_retry > (VACUUM_TRUNCATE_LOCK_TIMEOUT /
                                VACUUM_TRUNCATE_LOCK_WAIT_INTERVAL))
            {
                /*
                 * We failed to establish the lock in the specified number of
                 * retries. This means we give up truncating.
                 */
                ereport(vacrel->verbose ? INFO : DEBUG2,
                        (errmsg("\"%s\": stopping truncate due to conflicting lock request",
                                vacrel->relname)));
                return;
            }
 
            (void) WaitLatch(MyLatch,
                             WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH,
                             VACUUM_TRUNCATE_LOCK_WAIT_INTERVAL,
                             WAIT_EVENT_VACUUM_TRUNCATE);
            ResetLatch(MyLatch);
        }
 
        /*
         * Now that we have exclusive lock, look to see if the rel has grown
         * whilst we were vacuuming with non-exclusive lock.  If so, give up;
         * the newly added pages presumably contain non-deletable tuples.
         */
        new_rel_pages = RelationGetNumberOfBlocks(vacrel->rel);
        if (new_rel_pages != orig_rel_pages)
        {
            /*
             * Note: we intentionally don't update vacrel->rel_pages with the
             * new rel size here.  If we did, it would amount to assuming that
             * the new pages are empty, which is unlikely. Leaving the numbers
             * alone amounts to assuming that the new pages have the same
             * tuple density as existing ones, which is less unlikely.
             */
            UnlockRelation(vacrel->rel, AccessExclusiveLock);
            return;
        }
 
        /*
         * Scan backwards from the end to verify that the end pages actually
         * contain no tuples.  This is *necessary*, not optional, because
         * other backends could have added tuples to these pages whilst we
         * were vacuuming.
         */
        new_rel_pages = count_nondeletable_pages(vacrel, &lock_waiter_detected);
        vacrel->blkno = new_rel_pages;
 
        if (new_rel_pages >= orig_rel_pages)
        {
            /* can't do anything after all */
            UnlockRelation(vacrel->rel, AccessExclusiveLock);
            return;
        }
 
        /*
         * Okay to truncate.
         */
        RelationTruncate(vacrel->rel, new_rel_pages);
 
        /*
         * We can release the exclusive lock as soon as we have truncated.
         * Other backends can't safely access the relation until they have
         * processed the smgr invalidation that smgrtruncate sent out ... but
         * that should happen as part of standard invalidation processing once
         * they acquire lock on the relation.
         */
        UnlockRelation(vacrel->rel, AccessExclusiveLock);
 
        /*
         * Update statistics.  Here, it *is* correct to adjust rel_pages
         * without also touching reltuples, since the tuple count wasn't
         * changed by the truncation.
         */
        vacrel->removed_pages += orig_rel_pages - new_rel_pages;
        vacrel->rel_pages = new_rel_pages;
 
        ereport(vacrel->verbose ? INFO : DEBUG2,
                (errmsg("table \"%s\": truncated %u to %u pages",
                        vacrel->relname,
                        orig_rel_pages, new_rel_pages)));
        orig_rel_pages = new_rel_pages;
    } while (new_rel_pages > vacrel->nonempty_pages && lock_waiter_detected);
}
 
/*
 * Rescan end pages to verify that they are (still) empty of tuples.
 *
 * Returns number of nondeletable pages (last nonempty page + 1).
 */
static BlockNumber
count_nondeletable_pages(LVRelState *vacrel, bool *lock_waiter_detected)
{
    BlockNumber blkno;
    BlockNumber prefetchedUntil;
    instr_time  starttime;
 
    /* Initialize the starttime if we check for conflicting lock requests */
    INSTR_TIME_SET_CURRENT(starttime);
 
    /*
     * Start checking blocks at what we believe relation end to be and move
     * backwards.  (Strange coding of loop control is needed because blkno is
     * unsigned.)  To make the scan faster, we prefetch a few blocks at a time
     * in forward direction, so that OS-level readahead can kick in.
     */
    blkno = vacrel->rel_pages;
    StaticAssertStmt((PREFETCH_SIZE & (PREFETCH_SIZE - 1)) == 0,
                     "prefetch size must be power of 2");
    prefetchedUntil = InvalidBlockNumber;
    while (blkno > vacrel->nonempty_pages)
    {
        Buffer      buf;
        Page        page;
        OffsetNumber offnum,
                    maxoff;
        bool        hastup;
 
        /*
         * Check if another process requests a lock on our relation. We are
         * holding an AccessExclusiveLock here, so they will be waiting. We
         * only do this once per VACUUM_TRUNCATE_LOCK_CHECK_INTERVAL, and we
         * only check if that interval has elapsed once every 32 blocks to
         * keep the number of system calls and actual shared lock table
         * lookups to a minimum.
         */
        if ((blkno % 32) == 0)
        {
            instr_time  currenttime;
            instr_time  elapsed;
 
            INSTR_TIME_SET_CURRENT(currenttime);
            elapsed = currenttime;
            INSTR_TIME_SUBTRACT(elapsed, starttime);
            if ((INSTR_TIME_GET_MICROSEC(elapsed) / 1000)
                >= VACUUM_TRUNCATE_LOCK_CHECK_INTERVAL)
            {
                if (LockHasWaitersRelation(vacrel->rel, AccessExclusiveLock))
                {
                    ereport(vacrel->verbose ? INFO : DEBUG2,
                            (errmsg("table \"%s\": suspending truncate due to conflicting lock request",
                                    vacrel->relname)));
 
                    *lock_waiter_detected = true;
                    return blkno;
                }
                starttime = currenttime;
            }
        }
 
        /*
         * We don't insert a vacuum delay point here, because we have an
         * exclusive lock on the table which we want to hold for as short a
         * time as possible.  We still need to check for interrupts however.
         */
        CHECK_FOR_INTERRUPTS();
 
        blkno--;
 
        /* If we haven't prefetched this lot yet, do so now. */
        if (prefetchedUntil > blkno)
        {
            BlockNumber prefetchStart;
            BlockNumber pblkno;
 
            prefetchStart = blkno & ~(PREFETCH_SIZE - 1);
            for (pblkno = prefetchStart; pblkno <= blkno; pblkno++)
            {
                PrefetchBuffer(vacrel->rel, MAIN_FORKNUM, pblkno);
                CHECK_FOR_INTERRUPTS();
            }
            prefetchedUntil = prefetchStart;
        }
 
        buf = ReadBufferExtended(vacrel->rel, MAIN_FORKNUM, blkno, RBM_NORMAL,
                                 vacrel->bstrategy);
 
        /* In this phase we only need shared access to the buffer */
        LockBuffer(buf, BUFFER_LOCK_SHARE);
 
        page = BufferGetPage(buf);
 
        if (PageIsNew(page) || PageIsEmpty(page))
        {
            UnlockReleaseBuffer(buf);
            continue;
        }
 
        hastup = false;
        maxoff = PageGetMaxOffsetNumber(page);
        for (offnum = FirstOffsetNumber;
             offnum <= maxoff;
             offnum = OffsetNumberNext(offnum))
        {
            ItemId      itemid;
 
            itemid = PageGetItemId(page, offnum);
 
            /*
             * Note: any non-unused item should be taken as a reason to keep
             * this page.  Even an LP_DEAD item makes truncation unsafe, since
             * we must not have cleaned out its index entries.
             */
            if (ItemIdIsUsed(itemid))
            {
                hastup = true;
                break;          /* can stop scanning */
            }
        }                       /* scan along page */
 
        UnlockReleaseBuffer(buf);
 
        /* Done scanning if we found a tuple here */
        if (hastup)
            return blkno + 1;
    }
 
    /*
     * If we fall out of the loop, all the previously-thought-to-be-empty
     * pages still are; we need not bother to look at the last known-nonempty
     * page.
     */
    return vacrel->nonempty_pages;
}
 
/*
 * Allocate dead_items and dead_items_info (either using palloc, or in dynamic
 * shared memory). Sets both in vacrel for caller.
 *
 * Also handles parallel initialization as part of allocating dead_items in
 * DSM when required.
 */
static void
dead_items_alloc(LVRelState *vacrel, int nworkers)
{
    VacDeadItemsInfo *dead_items_info;
    int         vac_work_mem = AmAutoVacuumWorkerProcess() &&
        autovacuum_work_mem != -1 ?
        autovacuum_work_mem : maintenance_work_mem;
 
    /*
     * Initialize state for a parallel vacuum.  As of now, only one worker can
     * be used for an index, so we invoke parallelism only if there are at
     * least two indexes on a table.
     */
    if (nworkers >= 0 && vacrel->nindexes > 1 && vacrel->do_index_vacuuming)
    {
        /*
         * Since parallel workers cannot access data in temporary tables, we
         * can't perform parallel vacuum on them.
         */
        if (RelationUsesLocalBuffers(vacrel->rel))
        {
            /*
             * Give warning only if the user explicitly tries to perform a
             * parallel vacuum on the temporary table.
             */
            if (nworkers > 0)
                ereport(WARNING,
                        (errmsg("disabling parallel option of vacuum on \"%s\" --- cannot vacuum temporary tables in parallel",
                                vacrel->relname)));
        }
        else
            vacrel->pvs = parallel_vacuum_init(vacrel->rel, vacrel->indrels,
                                               vacrel->nindexes, nworkers,
                                               vac_work_mem,
                                               vacrel->verbose ? INFO : DEBUG2,
                                               vacrel->bstrategy);
 
        /*
         * If parallel mode started, dead_items and dead_items_info spaces are
         * allocated in DSM.
         */
        if (ParallelVacuumIsActive(vacrel))
        {
            vacrel->dead_items = parallel_vacuum_get_dead_items(vacrel->pvs,
                                                                &vacrel->dead_items_info);
            return;
        }
    }
 
    /*
     * Serial VACUUM case. Allocate both dead_items and dead_items_info
     * locally.
     */
 
    dead_items_info = (VacDeadItemsInfo *) palloc(sizeof(VacDeadItemsInfo));
    dead_items_info->max_bytes = vac_work_mem * 1024L;
    dead_items_info->num_items = 0;
    vacrel->dead_items_info = dead_items_info;
 
    vacrel->dead_items = TidStoreCreateLocal(dead_items_info->max_bytes, true);
}
 
/*
 * Add the given block number and offset numbers to dead_items.
 */
static void
dead_items_add(LVRelState *vacrel, BlockNumber blkno, OffsetNumber *offsets,
               int num_offsets)
{
    TidStore   *dead_items = vacrel->dead_items;
 
    TidStoreSetBlockOffsets(dead_items, blkno, offsets, num_offsets);
    vacrel->dead_items_info->num_items += num_offsets;
 
    /* update the memory usage report */
    pgstat_progress_update_param(PROGRESS_VACUUM_DEAD_TUPLE_BYTES,
                                 TidStoreMemoryUsage(dead_items));
}
 
/*
 * Forget all collected dead items.
 */
static void
dead_items_reset(LVRelState *vacrel)
{
    TidStore   *dead_items = vacrel->dead_items;
 
    if (ParallelVacuumIsActive(vacrel))
    {
        parallel_vacuum_reset_dead_items(vacrel->pvs);
        return;
    }
 
    /* Recreate the tidstore with the same max_bytes limitation */
    TidStoreDestroy(dead_items);
    vacrel->dead_items = TidStoreCreateLocal(vacrel->dead_items_info->max_bytes, true);
 
    /* Reset the counter */
    vacrel->dead_items_info->num_items = 0;
}
 
/*
 * Perform cleanup for resources allocated in dead_items_alloc
 */
static void
dead_items_cleanup(LVRelState *vacrel)
{
    if (!ParallelVacuumIsActive(vacrel))
    {
        /* Don't bother with pfree here */
        return;
    }
 
    /* End parallel mode */
    parallel_vacuum_end(vacrel->pvs, vacrel->indstats);
    vacrel->pvs = NULL;
}
 
/*
 * Check if every tuple in the given page is visible to all current and future
 * transactions. Also return the visibility_cutoff_xid which is the highest
 * xmin amongst the visible tuples.  Set *all_frozen to true if every tuple
 * on this page is frozen.
 *
 * This is a stripped down version of lazy_scan_prune().  If you change
 * anything here, make sure that everything stays in sync.  Note that an
 * assertion calls us to verify that everybody still agrees.  Be sure to avoid
 * introducing new side-effects here.
 */
static bool
heap_page_is_all_visible(LVRelState *vacrel, Buffer buf,
                         TransactionId *visibility_cutoff_xid,
                         bool *all_frozen)
{
    Page        page = BufferGetPage(buf);
    BlockNumber blockno = BufferGetBlockNumber(buf);
    OffsetNumber offnum,
                maxoff;
    bool        all_visible = true;
 
    *visibility_cutoff_xid = InvalidTransactionId;
    *all_frozen = true;
 
    maxoff = PageGetMaxOffsetNumber(page);
    for (offnum = FirstOffsetNumber;
         offnum <= maxoff && all_visible;
         offnum = OffsetNumberNext(offnum))
    {
        ItemId      itemid;
        HeapTupleData tuple;
 
        /*
         * Set the offset number so that we can display it along with any
         * error that occurred while processing this tuple.
         */
        vacrel->offnum = offnum;
        itemid = PageGetItemId(page, offnum);
 
        /* Unused or redirect line pointers are of no interest */
        if (!ItemIdIsUsed(itemid) || ItemIdIsRedirected(itemid))
            continue;
 
        ItemPointerSet(&(tuple.t_self), blockno, offnum);
 
        /*
         * Dead line pointers can have index pointers pointing to them. So
         * they can't be treated as visible
         */
        if (ItemIdIsDead(itemid))
        {
            all_visible = false;
            *all_frozen = false;
            break;
        }
 
        Assert(ItemIdIsNormal(itemid));
 
        tuple.t_data = (HeapTupleHeader) PageGetItem(page, itemid);
        tuple.t_len = ItemIdGetLength(itemid);
        tuple.t_tableOid = RelationGetRelid(vacrel->rel);
 
        switch (HeapTupleSatisfiesVacuum(&tuple, vacrel->cutoffs.OldestXmin,
                                         buf))
        {
            case HEAPTUPLE_LIVE:
                {
                    TransactionId xmin;
 
                    /* Check comments in lazy_scan_prune. */
                    if (!HeapTupleHeaderXminCommitted(tuple.t_data))
                    {
                        all_visible = false;
                        *all_frozen = false;
                        break;
                    }
 
                    /*
                     * The inserter definitely committed. But is it old enough
                     * that everyone sees it as committed?
                     */
                    xmin = HeapTupleHeaderGetXmin(tuple.t_data);
                    if (!TransactionIdPrecedes(xmin,
                                               vacrel->cutoffs.OldestXmin))
                    {
                        all_visible = false;
                        *all_frozen = false;
                        break;
                    }
 
                    /* Track newest xmin on page. */
                    if (TransactionIdFollows(xmin, *visibility_cutoff_xid) &&
                        TransactionIdIsNormal(xmin))
                        *visibility_cutoff_xid = xmin;
 
                    /* Check whether this tuple is already frozen or not */
                    if (all_visible && *all_frozen &&
                        heap_tuple_needs_eventual_freeze(tuple.t_data))
                        *all_frozen = false;
                }
                break;
 
            case HEAPTUPLE_DEAD:
            case HEAPTUPLE_RECENTLY_DEAD:
            case HEAPTUPLE_INSERT_IN_PROGRESS:
            case HEAPTUPLE_DELETE_IN_PROGRESS:
                {
                    all_visible = false;
                    *all_frozen = false;
                    break;
                }
            default:
                elog(ERROR, "unexpected HeapTupleSatisfiesVacuum result");
                break;
        }
    }                           /* scan along page */
 
    /* Clear the offset information once we have processed the given page. */
    vacrel->offnum = InvalidOffsetNumber;
 
    return all_visible;
}
 
/*
 * Update index statistics in pg_class if the statistics are accurate.
 */
static void
update_relstats_all_indexes(LVRelState *vacrel)
{
    Relation   *indrels = vacrel->indrels;
    int         nindexes = vacrel->nindexes;
    IndexBulkDeleteResult **indstats = vacrel->indstats;
 
    Assert(vacrel->do_index_cleanup);
 
    for (int idx = 0; idx < nindexes; idx++)
    {
        Relation    indrel = indrels[idx];
        IndexBulkDeleteResult *istat = indstats[idx];
 
        if (istat == NULL || istat->estimated_count)
            continue;
 
        /* Update index statistics */
        vac_update_relstats(indrel,
                            istat->num_pages,
                            istat->num_index_tuples,
                            0,
                            false,
                            InvalidTransactionId,
                            InvalidMultiXactId,
                            NULL, NULL, false);
    }
}
 
/*
 * Error context callback for errors occurring during vacuum.  The error
 * context messages for index phases should match the messages set in parallel
 * vacuum.  If you change this function for those phases, change
 * parallel_vacuum_error_callback() as well.
 */
static void
vacuum_error_callback(void *arg)
{
    LVRelState *errinfo = arg;
 
    switch (errinfo->phase)
    {
        case VACUUM_ERRCB_PHASE_SCAN_HEAP:
            if (BlockNumberIsValid(errinfo->blkno))
            {
                if (OffsetNumberIsValid(errinfo->offnum))
                    errcontext("while scanning block %u offset %u of relation \"%s.%s\"",
                               errinfo->blkno, errinfo->offnum, errinfo->relnamespace, errinfo->relname);
                else
                    errcontext("while scanning block %u of relation \"%s.%s\"",
                               errinfo->blkno, errinfo->relnamespace, errinfo->relname);
            }
            else
                errcontext("while scanning relation \"%s.%s\"",
                           errinfo->relnamespace, errinfo->relname);
            break;
 
        case VACUUM_ERRCB_PHASE_VACUUM_HEAP:
            if (BlockNumberIsValid(errinfo->blkno))
            {
                if (OffsetNumberIsValid(errinfo->offnum))
                    errcontext("while vacuuming block %u offset %u of relation \"%s.%s\"",
                               errinfo->blkno, errinfo->offnum, errinfo->relnamespace, errinfo->relname);
                else
                    errcontext("while vacuuming block %u of relation \"%s.%s\"",
                               errinfo->blkno, errinfo->relnamespace, errinfo->relname);
            }
            else
                errcontext("while vacuuming relation \"%s.%s\"",
                           errinfo->relnamespace, errinfo->relname);
            break;
 
        case VACUUM_ERRCB_PHASE_VACUUM_INDEX:
            errcontext("while vacuuming index \"%s\" of relation \"%s.%s\"",
                       errinfo->indname, errinfo->relnamespace, errinfo->relname);
            break;
 
        case VACUUM_ERRCB_PHASE_INDEX_CLEANUP:
            errcontext("while cleaning up index \"%s\" of relation \"%s.%s\"",
                       errinfo->indname, errinfo->relnamespace, errinfo->relname);
            break;
 
        case VACUUM_ERRCB_PHASE_TRUNCATE:
            if (BlockNumberIsValid(errinfo->blkno))
                errcontext("while truncating relation \"%s.%s\" to %u blocks",
                           errinfo->relnamespace, errinfo->relname, errinfo->blkno);
            break;
 
        case VACUUM_ERRCB_PHASE_UNKNOWN:
        default:
            return;             /* do nothing; the errinfo may not be
                                 * initialized */
    }
}
 
/*
 * Updates the information required for vacuum error callback.  This also saves
 * the current information which can be later restored via restore_vacuum_error_info.
 */
static void
update_vacuum_error_info(LVRelState *vacrel, LVSavedErrInfo *saved_vacrel,
                         int phase, BlockNumber blkno, OffsetNumber offnum)
{
    if (saved_vacrel)
    {
        saved_vacrel->offnum = vacrel->offnum;
        saved_vacrel->blkno = vacrel->blkno;
        saved_vacrel->phase = vacrel->phase;
    }
 
    vacrel->blkno = blkno;
    vacrel->offnum = offnum;
    vacrel->phase = phase;
}
 
/*
 * Restores the vacuum information saved via a prior call to update_vacuum_error_info.
 */
static void
restore_vacuum_error_info(LVRelState *vacrel,
                          const LVSavedErrInfo *saved_vacrel)
{
    vacrel->blkno = saved_vacrel->blkno;
    vacrel->offnum = saved_vacrel->offnum;
    vacrel->phase = saved_vacrel->phase;
}