pruneheap.c

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/*-------------------------------------------------------------------------
 *
 * pruneheap.c
 *    heap page pruning and HOT-chain management code
 *
 * Portions Copyright (c) 1996-2026, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/access/heap/pruneheap.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"
 
#include "access/heapam.h"
#include "access/heapam_xlog.h"
#include "access/htup_details.h"
#include "access/multixact.h"
#include "access/transam.h"
#include "access/visibilitymap.h"
#include "access/xlog.h"
#include "access/xloginsert.h"
#include "commands/vacuum.h"
#include "executor/instrument.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "storage/bufmgr.h"
#include "storage/freespace.h"
#include "utils/rel.h"
#include "utils/snapmgr.h"
 
/* Working data for heap_page_prune_and_freeze() and subroutines */
typedef struct
{
    /*-------------------------------------------------------
     * Arguments passed to heap_page_prune_and_freeze()
     *-------------------------------------------------------
     */
 
    /* tuple visibility test, initialized for the relation */
    GlobalVisState *vistest;
    /* whether or not dead items can be set LP_UNUSED during pruning */
    bool        mark_unused_now;
    /* whether to attempt freezing tuples */
    bool        attempt_freeze;
    /* whether to attempt setting the VM */
    bool        attempt_set_vm;
    struct VacuumCutoffs *cutoffs;
    Relation    relation;
 
    /*
     * Keep the buffer, block, and page handy so that helpers needing to
     * access them don't need to make repeated calls to BufferGetBlockNumber()
     * and BufferGetPage().
     */
    BlockNumber block;
    Buffer      buffer;
    Page        page;
 
    /*-------------------------------------------------------
     * Fields describing what to do to the page
     *-------------------------------------------------------
     */
    TransactionId new_prune_xid;    /* new prune hint value */
    TransactionId latest_xid_removed;
    int         nredirected;    /* numbers of entries in arrays below */
    int         ndead;
    int         nunused;
    int         nfrozen;
    /* arrays that accumulate indexes of items to be changed */
    OffsetNumber redirected[MaxHeapTuplesPerPage * 2];
    OffsetNumber nowdead[MaxHeapTuplesPerPage];
    OffsetNumber nowunused[MaxHeapTuplesPerPage];
    HeapTupleFreeze frozen[MaxHeapTuplesPerPage];
 
    /*
     * set_all_visible and set_all_frozen indicate if the all-visible and
     * all-frozen bits in the visibility map can be set for this page after
     * pruning. They are only tracked when the caller requests VM updates
     * (attempt_set_vm); otherwise they remain false throughout.
     *
     * NOTE: set_all_visible and set_all_frozen initially don't include
     * LP_DEAD items. That's convenient for heap_page_prune_and_freeze() to
     * use them to decide whether to opportunistically freeze the page or not.
     * The set_all_visible and set_all_frozen values ultimately used to set
     * the VM are adjusted to include LP_DEAD items after we determine whether
     * or not to opportunistically freeze.
     */
    bool        set_all_visible;
    bool        set_all_frozen;
 
    /*-------------------------------------------------------
     * Working state for HOT chain processing
     *-------------------------------------------------------
     */
 
    /*
     * 'root_items' contains offsets of all LP_REDIRECT line pointers and
     * normal non-HOT tuples.  They can be stand-alone items or the first item
     * in a HOT chain.  'heaponly_items' contains heap-only tuples which can
     * only be removed as part of a HOT chain.
     */
    int         nroot_items;
    OffsetNumber root_items[MaxHeapTuplesPerPage];
    int         nheaponly_items;
    OffsetNumber heaponly_items[MaxHeapTuplesPerPage];
 
    /*
     * processed[offnum] is true if item at offnum has been processed.
     *
     * This needs to be MaxHeapTuplesPerPage + 1 long as FirstOffsetNumber is
     * 1. Otherwise every access would need to subtract 1.
     */
    bool        processed[MaxHeapTuplesPerPage + 1];
 
    /*
     * Tuple visibility is only computed once for each tuple, for correctness
     * and efficiency reasons; see comment in heap_page_prune_and_freeze() for
     * details.  This is of type int8[], instead of HTSV_Result[], so we can
     * use -1 to indicate no visibility has been computed, e.g. for LP_DEAD
     * items.
     *
     * This needs to be MaxHeapTuplesPerPage + 1 long as FirstOffsetNumber is
     * 1. Otherwise every access would need to subtract 1.
     */
    int8        htsv[MaxHeapTuplesPerPage + 1];
 
    /*-------------------------------------------------------
     * Working state for freezing
     *-------------------------------------------------------
     */
    HeapPageFreeze pagefrz;
 
    /*-------------------------------------------------------
     * Working state for visibility map processing
     *-------------------------------------------------------
     */
 
    /*
     * Caller must provide a pinned vmbuffer corresponding to the heap block
     * passed to heap_page_prune_and_freeze(). We will fix any corruption
     * found in the VM and set the VM if the page is all-visible/all-frozen.
     */
    Buffer      vmbuffer;
 
    /*
     * The state of the VM bits at the beginning of pruning and the state they
     * will be in at the end.
     */
    uint8       old_vmbits;
    uint8       new_vmbits;
 
    /* The newest xmin of live tuples on the page */
    TransactionId newest_live_xid;
 
    /*-------------------------------------------------------
     * Information about what was done
     *
     * These fields are not used by pruning itself for the most part, but are
     * used to collect information about what was pruned and what state the
     * page is in after pruning, for the benefit of the caller.  They are
     * copied to the caller's PruneFreezeResult at the end.
     * -------------------------------------------------------
     */
 
    int         ndeleted;       /* Number of tuples deleted from the page */
 
    /* Number of live and recently dead tuples, after pruning */
    int         live_tuples;
    int         recently_dead_tuples;
 
    /* Whether or not the page makes rel truncation unsafe */
    bool        hastup;
 
    /*
     * LP_DEAD items on the page after pruning.  Includes existing LP_DEAD
     * items
     */
    int         lpdead_items;   /* number of items in the array */
    OffsetNumber *deadoffsets;  /* points directly to presult->deadoffsets */
} PruneState;
 
/*
 * Type of visibility map corruption detected on a heap page and its
 * associated VM page. Passed to heap_page_fix_vm_corruption() so the caller
 * can specify what it found rather than having the function rederive the
 * corruption from page state.
 */
typedef enum VMCorruptionType
{
    /* VM bits are set but the heap page-level PD_ALL_VISIBLE flag is not */
    VM_CORRUPT_MISSING_PAGE_HINT,
    /* LP_DEAD line pointers found on a page marked all-visible */
    VM_CORRUPT_LPDEAD,
    /* Tuple not visible to all transactions on a page marked all-visible */
    VM_CORRUPT_TUPLE_VISIBILITY,
} VMCorruptionType;
 
/* Local functions */
static void prune_freeze_setup(PruneFreezeParams *params,
                               TransactionId *new_relfrozen_xid,
                               MultiXactId *new_relmin_mxid,
                               PruneFreezeResult *presult,
                               PruneState *prstate);
static void heap_page_fix_vm_corruption(PruneState *prstate,
                                        OffsetNumber offnum,
                                        VMCorruptionType corruption_type);
static void prune_freeze_fast_path(PruneState *prstate,
                                   PruneFreezeResult *presult);
static void prune_freeze_plan(PruneState *prstate,
                              OffsetNumber *off_loc);
static HTSV_Result heap_prune_satisfies_vacuum(PruneState *prstate,
                                               HeapTuple tup);
static inline HTSV_Result htsv_get_valid_status(int status);
static void heap_prune_chain(OffsetNumber maxoff,
                             OffsetNumber rootoffnum, PruneState *prstate);
static void heap_prune_record_prunable(PruneState *prstate, TransactionId xid,
                                       OffsetNumber offnum);
static void heap_prune_record_redirect(PruneState *prstate,
                                       OffsetNumber offnum, OffsetNumber rdoffnum,
                                       bool was_normal);
static void heap_prune_record_dead(PruneState *prstate, OffsetNumber offnum,
                                   bool was_normal);
static void heap_prune_record_dead_or_unused(PruneState *prstate, OffsetNumber offnum,
                                             bool was_normal);
static void heap_prune_record_unused(PruneState *prstate, OffsetNumber offnum, bool was_normal);
 
static void heap_prune_record_unchanged_lp_unused(PruneState *prstate, OffsetNumber offnum);
static void heap_prune_record_unchanged_lp_normal(PruneState *prstate, OffsetNumber offnum);
static void heap_prune_record_unchanged_lp_dead(PruneState *prstate, OffsetNumber offnum);
static void heap_prune_record_unchanged_lp_redirect(PruneState *prstate, OffsetNumber offnum);
 
static void page_verify_redirects(Page page);
 
static bool heap_page_will_freeze(bool did_tuple_hint_fpi, bool do_prune, bool do_hint_prune,
                                  PruneState *prstate);
static bool heap_page_will_set_vm(PruneState *prstate, PruneReason reason,
                                  bool do_prune, bool do_freeze);
 
 
/*
 * Optionally prune and repair fragmentation in the specified page.
 *
 * This is an opportunistic function.  It will perform housekeeping
 * only if the page heuristically looks like a candidate for pruning and we
 * can acquire buffer cleanup lock without blocking.
 *
 * Note: this is called quite often.  It's important that it fall out quickly
 * if there's not any use in pruning.
 *
 * Caller must have pin on the buffer, and must *not* have a lock on it.
 *
 * This function may pin *vmbuffer. It's passed by reference so the caller can
 * reuse the pin across calls, avoiding repeated pin/unpin cycles. If we find
 * VM corruption during pruning, we will fix it. Caller is responsible for
 * unpinning *vmbuffer.
 *
 * rel_read_only is true if we determined at plan time that the query does not
 * modify the relation. It is counterproductive to set the VM if the query
 * will immediately clear it.
 *
 * As noted in ScanRelIsReadOnly(), INSERT ... SELECT from the same table will
 * report the scan relation as read-only. This is usually harmless in
 * practice. It is useful to set scanned pages all-visible that won't be
 * inserted into. Pages it does insert to will rarely meet the criteria for
 * pruning, and those that do are likely to contain in-progress inserts which
 * make the page not fully all-visible.
 */
void
heap_page_prune_opt(Relation relation, Buffer buffer, Buffer *vmbuffer,
                    bool rel_read_only)
{
    Page        page = BufferGetPage(buffer);
    TransactionId prune_xid;
    GlobalVisState *vistest;
    Size        minfree;
 
    /*
     * We can't write WAL in recovery mode, so there's no point trying to
     * clean the page. The primary will likely issue a cleaning WAL record
     * soon anyway, so this is no particular loss.
     */
    if (RecoveryInProgress())
        return;
 
    /*
     * First check whether there's any chance there's something to prune,
     * determining the appropriate horizon is a waste if there's no prune_xid
     * (i.e. no updates/deletes left potentially dead tuples around and no
     * inserts inserted new tuples that may be visible to all).
     */
    prune_xid = PageGetPruneXid(page);
    if (!TransactionIdIsValid(prune_xid))
        return;
 
    /*
     * Check whether prune_xid indicates that there may be dead rows that can
     * be cleaned up.
     */
    vistest = GlobalVisTestFor(relation);
 
    if (!GlobalVisTestIsRemovableXid(vistest, prune_xid, true))
        return;
 
    /*
     * We prune when a previous UPDATE failed to find enough space on the page
     * for a new tuple version, or when free space falls below the relation's
     * fill-factor target (but not less than 10%).
     *
     * Checking free space here is questionable since we aren't holding any
     * lock on the buffer; in the worst case we could get a bogus answer. It's
     * unlikely to be *seriously* wrong, though, since reading either pd_lower
     * or pd_upper is probably atomic.  Avoiding taking a lock seems more
     * important than sometimes getting a wrong answer in what is after all
     * just a heuristic estimate.
     */
    minfree = RelationGetTargetPageFreeSpace(relation,
                                             HEAP_DEFAULT_FILLFACTOR);
    minfree = Max(minfree, BLCKSZ / 10);
 
    if (PageIsFull(page) || PageGetHeapFreeSpace(page) < minfree)
    {
        bool        record_free_space = false;
        Size        freespace = 0;
 
        /* OK, try to get exclusive buffer lock */
        if (!ConditionalLockBufferForCleanup(buffer))
            return;
 
        /*
         * Now that we have buffer lock, get accurate information about the
         * page's free space, and recheck the heuristic about whether to
         * prune.
         */
        if (PageIsFull(page) || PageGetHeapFreeSpace(page) < minfree)
        {
            OffsetNumber dummy_off_loc;
            PruneFreezeResult presult;
            PruneFreezeParams params;
 
            visibilitymap_pin(relation, BufferGetBlockNumber(buffer),
                              vmbuffer);
 
            params.relation = relation;
            params.buffer = buffer;
            params.vmbuffer = *vmbuffer;
            params.reason = PRUNE_ON_ACCESS;
            params.vistest = vistest;
            params.cutoffs = NULL;
 
            /*
             * We don't pass the HEAP_PAGE_PRUNE_MARK_UNUSED_NOW option
             * regardless of whether or not the relation has indexes, since we
             * cannot safely determine that during on-access pruning with the
             * current implementation.
             */
            params.options = HEAP_PAGE_PRUNE_ALLOW_FAST_PATH;
            if (rel_read_only)
                params.options |= HEAP_PAGE_PRUNE_SET_VM;
 
            heap_page_prune_and_freeze(&params, &presult, &dummy_off_loc,
                                       NULL, NULL);
 
            /*
             * Report the number of tuples reclaimed to pgstats.  This is
             * presult.ndeleted minus the number of newly-LP_DEAD-set items.
             *
             * We derive the number of dead tuples like this to avoid totally
             * forgetting about items that were set to LP_DEAD, since they
             * still need to be cleaned up by VACUUM.  We only want to count
             * heap-only tuples that just became LP_UNUSED in our report,
             * which don't.
             *
             * VACUUM doesn't have to compensate in the same way when it
             * tracks ndeleted, since it will set the same LP_DEAD items to
             * LP_UNUSED separately.
             */
            if (presult.ndeleted > presult.nnewlpdead)
                pgstat_update_heap_dead_tuples(relation,
                                               presult.ndeleted - presult.nnewlpdead);
 
            /*
             * If this prune newly set the page all-visible, VACUUM may later
             * skip the page and not update the free space map (FSM) for it.
             * Keep the FSM from going stale by recording it now. We do not
             * want to update the freespace map otherwise, to reserve
             * freespace on this page for HOT updates.
             */
            if (presult.newly_all_visible)
            {
                record_free_space = true;
                freespace = PageGetHeapFreeSpace(page);
            }
        }
 
        /* And release buffer lock */
        LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
 
        /*
         * RecordPageWithFreeSpace() only dirties the FSM when the recorded
         * free-space category actually changes. Note that vacuum will still
         * do FreeSpaceMapVacuum() for ranges of pages that are skipped, so we
         * don't have to worry about that here.
         */
        if (record_free_space)
            RecordPageWithFreeSpace(relation, BufferGetBlockNumber(buffer), freespace);
    }
}
 
/*
 * Helper for heap_page_prune_and_freeze() to initialize the PruneState using
 * the provided parameters.
 *
 * params, new_relfrozen_xid, new_relmin_mxid, and presult are input
 * parameters and are not modified by this function. Only prstate is modified.
 */
static void
prune_freeze_setup(PruneFreezeParams *params,
                   TransactionId *new_relfrozen_xid,
                   MultiXactId *new_relmin_mxid,
                   PruneFreezeResult *presult,
                   PruneState *prstate)
{
    /* Copy parameters to prstate */
    prstate->vistest = params->vistest;
    prstate->mark_unused_now =
        (params->options & HEAP_PAGE_PRUNE_MARK_UNUSED_NOW) != 0;
 
    /* cutoffs must be provided if we will attempt freezing */
    Assert(!(params->options & HEAP_PAGE_PRUNE_FREEZE) || params->cutoffs);
    prstate->attempt_freeze = (params->options & HEAP_PAGE_PRUNE_FREEZE) != 0;
    prstate->attempt_set_vm = (params->options & HEAP_PAGE_PRUNE_SET_VM) != 0;
    prstate->cutoffs = params->cutoffs;
    prstate->relation = params->relation;
    prstate->block = BufferGetBlockNumber(params->buffer);
    prstate->buffer = params->buffer;
    prstate->page = BufferGetPage(params->buffer);
 
    Assert(BufferIsValid(params->vmbuffer));
    prstate->vmbuffer = params->vmbuffer;
    prstate->new_vmbits = 0;
    prstate->old_vmbits = visibilitymap_get_status(prstate->relation,
                                                   prstate->block,
                                                   &prstate->vmbuffer);
 
    /*
     * Our strategy is to scan the page and make lists of items to change,
     * then apply the changes within a critical section.  This keeps as much
     * logic as possible out of the critical section, and also ensures that
     * WAL replay will work the same as the normal case.
     *
     * First, initialize the new pd_prune_xid value to zero (indicating no
     * prunable tuples).  If we find any tuples which may soon become
     * prunable, we will save the lowest relevant XID in new_prune_xid. Also
     * initialize the rest of our working state.
     */
    prstate->new_prune_xid = InvalidTransactionId;
    prstate->latest_xid_removed = InvalidTransactionId;
    prstate->nredirected = prstate->ndead = prstate->nunused = 0;
    prstate->nfrozen = 0;
    prstate->nroot_items = 0;
    prstate->nheaponly_items = 0;
 
    /* initialize page freezing working state */
    prstate->pagefrz.freeze_required = false;
    prstate->pagefrz.FreezePageConflictXid = InvalidTransactionId;
    if (prstate->attempt_freeze)
    {
        Assert(new_relfrozen_xid && new_relmin_mxid);
        prstate->pagefrz.FreezePageRelfrozenXid = *new_relfrozen_xid;
        prstate->pagefrz.NoFreezePageRelfrozenXid = *new_relfrozen_xid;
        prstate->pagefrz.FreezePageRelminMxid = *new_relmin_mxid;
        prstate->pagefrz.NoFreezePageRelminMxid = *new_relmin_mxid;
    }
    else
    {
        Assert(!new_relfrozen_xid && !new_relmin_mxid);
        prstate->pagefrz.FreezePageRelminMxid = InvalidMultiXactId;
        prstate->pagefrz.NoFreezePageRelminMxid = InvalidMultiXactId;
        prstate->pagefrz.FreezePageRelfrozenXid = InvalidTransactionId;
        prstate->pagefrz.NoFreezePageRelfrozenXid = InvalidTransactionId;
    }
 
    prstate->ndeleted = 0;
    prstate->live_tuples = 0;
    prstate->recently_dead_tuples = 0;
    prstate->hastup = false;
    prstate->lpdead_items = 0;
 
    /*
     * deadoffsets are filled in during pruning but are only used to populate
     * PruneFreezeResult->deadoffsets. To avoid needing two copies of the
     * array, just save a pointer to the result offsets array in the
     * PruneState.
     */
    prstate->deadoffsets = presult->deadoffsets;
 
    /*
     * We track whether the page will be all-visible/all-frozen at the end of
     * pruning and freezing. While examining tuple visibility, we'll set
     * set_all_visible to false if there are tuples on the page not visible to
     * all running and future transactions. If setting the VM is enabled for
     * this scan, we will do so if the page ends up being all-visible.
     *
     * We also keep track of the newest live XID, which is used to calculate
     * the snapshot conflict horizon for a WAL record setting the VM.
     */
    prstate->set_all_visible = prstate->attempt_set_vm;
    prstate->newest_live_xid = InvalidTransactionId;
 
    /*
     * Currently, only VACUUM performs freezing, but other callers may in the
     * future. We must initialize set_all_frozen based on whether or not the
     * caller passed HEAP_PAGE_PRUNE_FREEZE, because if they did not, we won't
     * call heap_prepare_freeze_tuple() for each tuple, and set_all_frozen
     * will never be cleared for tuples that need freezing. This would lead to
     * incorrectly setting the visibility map all-frozen for this page. We
     * can't set the page all-frozen in the VM if the caller didn't pass
     * HEAP_PAGE_PRUNE_SET_VM.
     *
     * When freezing is not required (no XIDs/MXIDs older than the freeze
     * cutoff), we may still choose to "opportunistically" freeze if doing so
     * would make the page all-frozen.
     *
     * We will not be able to freeze the whole page at the end of vacuum if
     * there are tuples present that are not visible to everyone or if there
     * are dead tuples which will not be removable. However, dead tuples that
     * will be removed by the end of vacuum should not prevent this
     * opportunistic freezing.
     *
     * Therefore, we do not clear set_all_visible and set_all_frozen when we
     * encounter LP_DEAD items. Instead, we correct them after deciding
     * whether to freeze, but before updating the VM, to avoid setting the VM
     * bits incorrectly.
     */
    prstate->set_all_frozen = prstate->attempt_freeze && prstate->attempt_set_vm;
}
 
/*
 * Helper for heap_page_prune_and_freeze(). Iterates over every tuple on the
 * page, examines its visibility information, and determines the appropriate
 * action for each tuple. All tuples are processed and classified during this
 * phase, but no modifications are made to the page until the later execution
 * stage.
 *
 * *off_loc is used for error callback and cleared before returning.
 */
static void
prune_freeze_plan(PruneState *prstate, OffsetNumber *off_loc)
{
    Page        page = prstate->page;
    BlockNumber blockno = prstate->block;
    OffsetNumber maxoff = PageGetMaxOffsetNumber(prstate->page);
    OffsetNumber offnum;
    HeapTupleData tup;
 
    tup.t_tableOid = RelationGetRelid(prstate->relation);
 
    /*
     * Determine HTSV for all tuples, and queue them up for processing as HOT
     * chain roots or as heap-only items.
     *
     * Determining HTSV only once for each tuple is required for correctness,
     * to deal with cases where running HTSV twice could result in different
     * results.  For example, RECENTLY_DEAD can turn to DEAD if another
     * checked item causes GlobalVisTestIsRemovableFullXid() to update the
     * horizon, or INSERT_IN_PROGRESS can change to DEAD if the inserting
     * transaction aborts.
     *
     * It's also good for performance. Most commonly tuples within a page are
     * stored at decreasing offsets (while the items are stored at increasing
     * offsets). When processing all tuples on a page this leads to reading
     * memory at decreasing offsets within a page, with a variable stride.
     * That's hard for CPU prefetchers to deal with. Processing the items in
     * reverse order (and thus the tuples in increasing order) increases
     * prefetching efficiency significantly / decreases the number of cache
     * misses.
     */
    for (offnum = maxoff;
         offnum >= FirstOffsetNumber;
         offnum = OffsetNumberPrev(offnum))
    {
        ItemId      itemid = PageGetItemId(page, offnum);
        HeapTupleHeader htup;
 
        /*
         * Set the offset number so that we can display it along with any
         * error that occurred while processing this tuple.
         */
        *off_loc = offnum;
 
        prstate->processed[offnum] = false;
        prstate->htsv[offnum] = -1;
 
        /* Nothing to do if slot doesn't contain a tuple */
        if (!ItemIdIsUsed(itemid))
        {
            heap_prune_record_unchanged_lp_unused(prstate, offnum);
            continue;
        }
 
        if (ItemIdIsDead(itemid))
        {
            /*
             * If the caller set mark_unused_now true, we can set dead line
             * pointers LP_UNUSED now.
             */
            if (unlikely(prstate->mark_unused_now))
                heap_prune_record_unused(prstate, offnum, false);
            else
                heap_prune_record_unchanged_lp_dead(prstate, offnum);
            continue;
        }
 
        if (ItemIdIsRedirected(itemid))
        {
            /* This is the start of a HOT chain */
            prstate->root_items[prstate->nroot_items++] = offnum;
            continue;
        }
 
        Assert(ItemIdIsNormal(itemid));
 
        /*
         * Get the tuple's visibility status and queue it up for processing.
         */
        htup = (HeapTupleHeader) PageGetItem(page, itemid);
        tup.t_data = htup;
        tup.t_len = ItemIdGetLength(itemid);
        ItemPointerSet(&tup.t_self, blockno, offnum);
 
        prstate->htsv[offnum] = heap_prune_satisfies_vacuum(prstate, &tup);
 
        if (!HeapTupleHeaderIsHeapOnly(htup))
            prstate->root_items[prstate->nroot_items++] = offnum;
        else
            prstate->heaponly_items[prstate->nheaponly_items++] = offnum;
    }
 
    /*
     * Process HOT chains.
     *
     * We added the items to the array starting from 'maxoff', so by
     * processing the array in reverse order, we process the items in
     * ascending offset number order.  The order doesn't matter for
     * correctness, but some quick micro-benchmarking suggests that this is
     * faster.  (Earlier PostgreSQL versions, which scanned all the items on
     * the page instead of using the root_items array, also did it in
     * ascending offset number order.)
     */
    for (int i = prstate->nroot_items - 1; i >= 0; i--)
    {
        offnum = prstate->root_items[i];
 
        /* Ignore items already processed as part of an earlier chain */
        if (prstate->processed[offnum])
            continue;
 
        /* see preceding loop */
        *off_loc = offnum;
 
        /* Process this item or chain of items */
        heap_prune_chain(maxoff, offnum, prstate);
    }
 
    /*
     * Process any heap-only tuples that were not already processed as part of
     * a HOT chain.
     */
    for (int i = prstate->nheaponly_items - 1; i >= 0; i--)
    {
        offnum = prstate->heaponly_items[i];
 
        if (prstate->processed[offnum])
            continue;
 
        /* see preceding loop */
        *off_loc = offnum;
 
        /*
         * If the tuple is DEAD and doesn't chain to anything else, mark it
         * unused.  (If it does chain, we can only remove it as part of
         * pruning its chain.)
         *
         * We need this primarily to handle aborted HOT updates, that is,
         * XMIN_INVALID heap-only tuples.  Those might not be linked to by any
         * chain, since the parent tuple might be re-updated before any
         * pruning occurs.  So we have to be able to reap them separately from
         * chain-pruning.  (Note that HeapTupleHeaderIsHotUpdated will never
         * return true for an XMIN_INVALID tuple, so this code will work even
         * when there were sequential updates within the aborted transaction.)
         */
        if (prstate->htsv[offnum] == HEAPTUPLE_DEAD)
        {
            ItemId      itemid = PageGetItemId(page, offnum);
            HeapTupleHeader htup = (HeapTupleHeader) PageGetItem(page, itemid);
 
            if (likely(!HeapTupleHeaderIsHotUpdated(htup)))
            {
                HeapTupleHeaderAdvanceConflictHorizon(htup,
                                                      &prstate->latest_xid_removed);
                heap_prune_record_unused(prstate, offnum, true);
            }
            else
            {
                /*
                 * This tuple should've been processed and removed as part of
                 * a HOT chain, so something's wrong.  To preserve evidence,
                 * we don't dare to remove it.  We cannot leave behind a DEAD
                 * tuple either, because that will cause VACUUM to error out.
                 * Throwing an error with a distinct error message seems like
                 * the least bad option.
                 */
                elog(ERROR, "dead heap-only tuple (%u, %d) is not linked to from any HOT chain",
                     blockno, offnum);
            }
        }
        else
            heap_prune_record_unchanged_lp_normal(prstate, offnum);
    }
 
    /* We should now have processed every tuple exactly once  */
#ifdef USE_ASSERT_CHECKING
    for (offnum = FirstOffsetNumber;
         offnum <= maxoff;
         offnum = OffsetNumberNext(offnum))
    {
        *off_loc = offnum;
 
        Assert(prstate->processed[offnum]);
    }
#endif
 
    /* Clear the offset information once we have processed the given page. */
    *off_loc = InvalidOffsetNumber;
}
 
/*
 * Decide whether to proceed with freezing according to the freeze plans
 * prepared for the current heap buffer. If freezing is chosen, this function
 * performs several pre-freeze checks.
 *
 * The values of do_prune, do_hint_prune, and did_tuple_hint_fpi must be
 * determined before calling this function.
 *
 * prstate is both an input and output parameter.
 *
 * Returns true if we should apply the freeze plans and freeze tuples on the
 * page, and false otherwise.
 */
static bool
heap_page_will_freeze(bool did_tuple_hint_fpi,
                      bool do_prune,
                      bool do_hint_prune,
                      PruneState *prstate)
{
    bool        do_freeze = false;
 
    /*
     * If the caller specified we should not attempt to freeze any tuples,
     * validate that everything is in the right state and return.
     */
    if (!prstate->attempt_freeze)
    {
        Assert(!prstate->set_all_frozen && prstate->nfrozen == 0);
        return false;
    }
 
    if (prstate->pagefrz.freeze_required)
    {
        /*
         * heap_prepare_freeze_tuple indicated that at least one XID/MXID from
         * before FreezeLimit/MultiXactCutoff is present.  Must freeze to
         * advance relfrozenxid/relminmxid.
         */
        do_freeze = true;
    }
    else
    {
        /*
         * Opportunistically freeze the page if we are generating an FPI
         * anyway and if doing so means that we can set the page all-frozen
         * afterwards (might not happen until VACUUM's final heap pass).
         *
         * XXX: Previously, we knew if pruning emitted an FPI by checking
         * pgWalUsage.wal_fpi before and after pruning.  Once the freeze and
         * prune records were combined, this heuristic couldn't be used
         * anymore.  The opportunistic freeze heuristic must be improved;
         * however, for now, try to approximate the old logic.
         */
        if (prstate->set_all_frozen && prstate->nfrozen > 0)
        {
            Assert(prstate->set_all_visible);
 
            /*
             * Freezing would make the page all-frozen.  Have already emitted
             * an FPI or will do so anyway?
             */
            if (RelationNeedsWAL(prstate->relation))
            {
                if (did_tuple_hint_fpi)
                    do_freeze = true;
                else if (do_prune)
                {
                    if (XLogCheckBufferNeedsBackup(prstate->buffer))
                        do_freeze = true;
                }
                else if (do_hint_prune)
                {
                    if (XLogHintBitIsNeeded() &&
                        XLogCheckBufferNeedsBackup(prstate->buffer))
                        do_freeze = true;
                }
            }
        }
    }
 
    if (do_freeze)
    {
        /*
         * Validate the tuples we will be freezing before entering the
         * critical section.
         */
        heap_pre_freeze_checks(prstate->buffer, prstate->frozen, prstate->nfrozen);
        Assert(TransactionIdPrecedes(prstate->pagefrz.FreezePageConflictXid,
                                     prstate->cutoffs->OldestXmin));
    }
    else if (prstate->nfrozen > 0)
    {
        /*
         * The page contained some tuples that were not already frozen, and we
         * chose not to freeze them now.  The page won't be all-frozen then.
         */
        Assert(!prstate->pagefrz.freeze_required);
 
        prstate->set_all_frozen = false;
        prstate->nfrozen = 0;   /* avoid miscounts in instrumentation */
    }
    else
    {
        /*
         * We have no freeze plans to execute.  The page might already be
         * all-frozen (perhaps only following pruning), though.  Such pages
         * can be marked all-frozen in the VM by our caller, even though none
         * of its tuples were newly frozen here.
         */
    }
 
    return do_freeze;
}
 
/*
 * Emit a warning about and fix visibility map corruption on the given page.
 *
 * The caller specifies the type of corruption it has already detected via
 * corruption_type, so that we can emit the appropriate warning. All cases
 * result in the VM bits being cleared; corruption types where PD_ALL_VISIBLE
 * is incorrectly set also clear PD_ALL_VISIBLE.
 *
 * Must be called while holding an exclusive lock on the heap buffer. Dead
 * items and not all-visible tuples must have been discovered under that same
 * lock. Although we do not hold a lock on the VM buffer, it is pinned, and
 * the heap buffer is exclusively locked, ensuring that no other backend can
 * update the VM bits corresponding to this heap page.
 *
 * This function makes changes to the VM and, potentially, the heap page, but
 * it does not need to be done in a critical section.
 */
static void
heap_page_fix_vm_corruption(PruneState *prstate, OffsetNumber offnum,
                            VMCorruptionType corruption_type)
{
    const char *relname = RelationGetRelationName(prstate->relation);
    bool        do_clear_vm = false;
    bool        do_clear_heap = false;
 
    Assert(BufferIsLockedByMeInMode(prstate->buffer, BUFFER_LOCK_EXCLUSIVE));
 
    switch (corruption_type)
    {
        case VM_CORRUPT_LPDEAD:
            ereport(WARNING,
                    (errcode(ERRCODE_DATA_CORRUPTED),
                     errmsg("dead line pointer found on page marked all-visible"),
                     errcontext("relation \"%s\", page %u, tuple %u",
                                relname, prstate->block, offnum)));
            do_clear_vm = true;
            do_clear_heap = true;
            break;
 
        case VM_CORRUPT_TUPLE_VISIBILITY:
 
            /*
             * A HEAPTUPLE_LIVE tuple on an all-visible page can appear to not
             * be visible to everyone when
             * GetOldestNonRemovableTransactionId() returns a conservative
             * value that's older than the real safe xmin. That is not
             * corruption -- the PD_ALL_VISIBLE flag is still correct.
             *
             * However, dead tuple versions, in-progress inserts, and
             * in-progress deletes should never appear on a page marked
             * all-visible. That indicates real corruption. PD_ALL_VISIBLE
             * should have been cleared by the DML operation that deleted or
             * inserted the tuple.
             */
            ereport(WARNING,
                    (errcode(ERRCODE_DATA_CORRUPTED),
                     errmsg("tuple not visible to all transactions found on page marked all-visible"),
                     errcontext("relation \"%s\", page %u, tuple %u",
                                relname, prstate->block, offnum)));
            do_clear_vm = true;
            do_clear_heap = true;
            break;
 
        case VM_CORRUPT_MISSING_PAGE_HINT:
 
            /*
             * As of PostgreSQL 9.2, the visibility map bit should never be
             * set if the page-level bit is clear. However, for vacuum, it's
             * possible that the bit got cleared after
             * heap_vac_scan_next_block() was called, so we must recheck now
             * that we have the buffer lock before concluding that the VM is
             * corrupt.
             */
            Assert(!PageIsAllVisible(prstate->page));
            Assert(prstate->old_vmbits & VISIBILITYMAP_VALID_BITS);
            ereport(WARNING,
                    (errcode(ERRCODE_DATA_CORRUPTED),
                     errmsg("page is not marked all-visible but visibility map bit is set"),
                     errcontext("relation \"%s\", page %u",
                                relname, prstate->block)));
            do_clear_vm = true;
            break;
    }
 
    Assert(do_clear_heap || do_clear_vm);
 
    /* Avoid marking the buffer dirty if PD_ALL_VISIBLE is already clear */
    if (do_clear_heap)
    {
        Assert(PageIsAllVisible(prstate->page));
        PageClearAllVisible(prstate->page);
        MarkBufferDirtyHint(prstate->buffer, true);
    }
 
    if (do_clear_vm)
    {
        LockBuffer(prstate->vmbuffer, BUFFER_LOCK_EXCLUSIVE);
        visibilitymap_clear(prstate->relation->rd_locator, prstate->block,
                            prstate->vmbuffer,
                            VISIBILITYMAP_VALID_BITS);
        LockBuffer(prstate->vmbuffer, BUFFER_LOCK_UNLOCK);
        prstate->old_vmbits = 0;
    }
}
 
/*
 * Decide whether to set the visibility map bits (all-visible and all-frozen)
 * for the current page using information from the PruneState and VM.
 *
 * This function does not actually set the VM bits or page-level visibility
 * hint, PD_ALL_VISIBLE.
 *
 * This should be called only after do_freeze has been decided (and do_prune
 * has been set), as these factor into our heuristic-based decision.
 *
 * Returns true if one or both VM bits should be set and false otherwise.
 */
static bool
heap_page_will_set_vm(PruneState *prstate, PruneReason reason,
                      bool do_prune, bool do_freeze)
{
    if (!prstate->attempt_set_vm)
        return false;
 
    if (!prstate->set_all_visible)
        return false;
 
    /*
     * If this is an on-access call and we're not actually pruning, avoid
     * setting the visibility map if it would newly dirty the heap page or, if
     * the page is already dirty, if doing so would require including a
     * full-page image (FPI) of the heap page in the WAL.
     */
    if (reason == PRUNE_ON_ACCESS && !do_prune && !do_freeze &&
        (!BufferIsDirty(prstate->buffer) || XLogCheckBufferNeedsBackup(prstate->buffer)))
    {
        prstate->set_all_visible = prstate->set_all_frozen = false;
        return false;
    }
 
    prstate->new_vmbits = VISIBILITYMAP_ALL_VISIBLE;
 
    if (prstate->set_all_frozen)
        prstate->new_vmbits |= VISIBILITYMAP_ALL_FROZEN;
 
    if (prstate->new_vmbits == prstate->old_vmbits)
    {
        prstate->new_vmbits = 0;
        return false;
    }
 
    return true;
}
 
/*
 * If the page is already all-frozen, or already all-visible and freezing
 * won't be attempted, there is no remaining work and we can use the fast path
 * to avoid the expensive overhead of heap_page_prune_and_freeze().
 *
 * This can happen when the page has a stale prune hint, or if VACUUM is
 * scanning an already all-frozen page due to SKIP_PAGES_THRESHOLD.
 *
 * The caller must already have examined the visibility map and saved the
 * status of the page's VM bits in prstate->old_vmbits. Caller must hold a
 * content lock on the heap page since it will examine line pointers.
 *
 * Before calling prune_freeze_fast_path(), the caller should first
 * check for and fix any discrepancy between the page-level visibility hint
 * and the visibility map. Otherwise, the fast path will always prevent us
 * from getting them in sync. Note that if there are tuples on the page that
 * are not visible to all but the VM is incorrectly marked
 * all-visible/all-frozen, we will not get the chance to fix that corruption
 * when using the fast path.
 */
static void
prune_freeze_fast_path(PruneState *prstate, PruneFreezeResult *presult)
{
    OffsetNumber maxoff = PageGetMaxOffsetNumber(prstate->page);
    Page        page = prstate->page;
 
    Assert((prstate->old_vmbits & VISIBILITYMAP_ALL_FROZEN) ||
           ((prstate->old_vmbits & VISIBILITYMAP_ALL_VISIBLE) &&
            !prstate->attempt_freeze));
 
    /* We'll fill in presult for the caller */
    memset(presult, 0, sizeof(PruneFreezeResult));
 
    /* Clear any stale prune hint */
    if (TransactionIdIsValid(PageGetPruneXid(page)))
    {
        PageClearPrunable(page);
        MarkBufferDirtyHint(prstate->buffer, true);
    }
 
    if (PageIsEmpty(page))
        return;
 
    /*
     * Since the page is all-visible, a count of the normal ItemIds on the
     * page should be sufficient for vacuum's live tuple count.
     */
    for (OffsetNumber off = FirstOffsetNumber;
         off <= maxoff;
         off = OffsetNumberNext(off))
    {
        ItemId      lp = PageGetItemId(page, off);
 
        if (!ItemIdIsUsed(lp))
            continue;
 
        presult->hastup = true;
 
        if (ItemIdIsNormal(lp))
            prstate->live_tuples++;
    }
 
    presult->live_tuples = prstate->live_tuples;
}
 
/*
 * Prune and repair fragmentation and potentially freeze tuples on the
 * specified page. If the page's visibility status has changed, update it in
 * the VM.
 *
 * Caller must have pin and buffer cleanup lock on the page.  Note that we
 * don't update the FSM information for page on caller's behalf.  Caller might
 * also need to account for a reduction in the length of the line pointer
 * array following array truncation by us.
 *
 * params contains the input parameters used to control freezing and pruning
 * behavior. See the definition of PruneFreezeParams for more on what each
 * parameter does.
 *
 * If the HEAP_PAGE_PRUNE_FREEZE option is set in params, we will freeze
 * tuples if it's required in order to advance relfrozenxid / relminmxid, or
 * if it's considered advantageous for overall system performance to do so
 * now.  The 'params.cutoffs', 'presult', 'new_relfrozen_xid' and
 * 'new_relmin_mxid' arguments are required when freezing.
 *
 * A vmbuffer corresponding to the heap page is also passed and if the page is
 * found to be all-visible/all-frozen, we will set it in the VM.
 *
 * presult contains output parameters needed by callers, such as the number of
 * tuples removed and the offsets of dead items on the page after pruning.
 * heap_page_prune_and_freeze() is responsible for initializing it.  Required
 * by all callers.
 *
 * off_loc is the offset location required by the caller to use in error
 * callback.
 *
 * new_relfrozen_xid and new_relmin_mxid must be provided by the caller if the
 * HEAP_PAGE_PRUNE_FREEZE option is set in params.  On entry, they contain the
 * oldest XID and multi-XID seen on the relation so far.  They will be updated
 * with the oldest values present on the page after pruning.  After processing
 * the whole relation, VACUUM can use these values as the new
 * relfrozenxid/relminmxid for the relation.
 */
void
heap_page_prune_and_freeze(PruneFreezeParams *params,
                           PruneFreezeResult *presult,
                           OffsetNumber *off_loc,
                           TransactionId *new_relfrozen_xid,
                           MultiXactId *new_relmin_mxid)
{
    PruneState  prstate;
    bool        do_freeze;
    bool        do_prune;
    bool        do_hint_prune;
    bool        do_set_vm;
    bool        did_tuple_hint_fpi;
    int64       fpi_before = pgWalUsage.wal_fpi;
    TransactionId conflict_xid;
 
    /* Initialize prstate */
    prune_freeze_setup(params,
                       new_relfrozen_xid, new_relmin_mxid,
                       presult, &prstate);
 
    /*
     * If the VM is set but PD_ALL_VISIBLE is clear, fix that corruption
     * before pruning and freezing so that the page and VM start out in a
     * consistent state.
     */
    if ((prstate.old_vmbits & VISIBILITYMAP_VALID_BITS) &&
        !PageIsAllVisible(prstate.page))
        heap_page_fix_vm_corruption(&prstate, InvalidOffsetNumber,
                                    VM_CORRUPT_MISSING_PAGE_HINT);
 
    /*
     * If the page is already all-frozen, or already all-visible when freezing
     * is not being attempted, take the fast path, skipping pruning and
     * freezing code entirely. This must be done after fixing any discrepancy
     * between the page-level visibility hint and the VM, since that may have
     * cleared old_vmbits.
     */
    if ((params->options & HEAP_PAGE_PRUNE_ALLOW_FAST_PATH) != 0 &&
        ((prstate.old_vmbits & VISIBILITYMAP_ALL_FROZEN) ||
         ((prstate.old_vmbits & VISIBILITYMAP_ALL_VISIBLE) &&
          !prstate.attempt_freeze)))
    {
        prune_freeze_fast_path(&prstate, presult);
        return;
    }
 
    /*
     * Examine all line pointers and tuple visibility information to determine
     * which line pointers should change state and which tuples may be frozen.
     * Prepare queue of state changes to later be executed in a critical
     * section.
     */
    prune_freeze_plan(&prstate, off_loc);
 
    /*
     * After processing all the live tuples on the page, if the newest xmin
     * amongst them may be considered running by any snapshot, the page cannot
     * be all-visible. This should be done before determining whether or not
     * to opportunistically freeze.
     */
    if (prstate.set_all_visible &&
        TransactionIdIsNormal(prstate.newest_live_xid) &&
        GlobalVisTestXidConsideredRunning(prstate.vistest,
                                          prstate.newest_live_xid,
                                          true))
        prstate.set_all_visible = prstate.set_all_frozen = false;
 
    /*
     * If checksums are enabled, calling heap_prune_satisfies_vacuum() while
     * checking tuple visibility information in prune_freeze_plan() may have
     * caused an FPI to be emitted.
     */
    did_tuple_hint_fpi = fpi_before != pgWalUsage.wal_fpi;
 
    do_prune = prstate.nredirected > 0 ||
        prstate.ndead > 0 ||
        prstate.nunused > 0;
 
    /*
     * Even if we don't prune anything, if we found a new value for the
     * pd_prune_xid field or the page was marked full, we will update the hint
     * bit.
     */
    do_hint_prune = PageGetPruneXid(prstate.page) != prstate.new_prune_xid ||
        PageIsFull(prstate.page);
 
    /*
     * Decide if we want to go ahead with freezing according to the freeze
     * plans we prepared, or not.
     */
    do_freeze = heap_page_will_freeze(did_tuple_hint_fpi,
                                      do_prune,
                                      do_hint_prune,
                                      &prstate);
 
    /*
     * While scanning the line pointers, we did not clear
     * set_all_visible/set_all_frozen when encountering LP_DEAD items because
     * we wanted the decision whether or not to freeze the page to be
     * unaffected by the short-term presence of LP_DEAD items.  These LP_DEAD
     * items are effectively assumed to be LP_UNUSED items in the making.  It
     * doesn't matter which vacuum heap pass (initial pass or final pass) ends
     * up setting the page all-frozen, as long as the ongoing VACUUM does it.
     *
     * Now that we finished determining whether or not to freeze the page,
     * update set_all_visible and set_all_frozen so that they reflect the true
     * state of the page for setting PD_ALL_VISIBLE and VM bits.
     */
    if (prstate.lpdead_items > 0)
        prstate.set_all_visible = prstate.set_all_frozen = false;
 
    Assert(!prstate.set_all_frozen || prstate.set_all_visible);
    Assert(!prstate.set_all_visible || prstate.attempt_set_vm);
    Assert(!prstate.set_all_visible || (prstate.lpdead_items == 0));
 
    do_set_vm = heap_page_will_set_vm(&prstate, params->reason, do_prune, do_freeze);
 
    /*
     * new_vmbits should be 0 regardless of whether or not the page is
     * all-visible if we do not intend to set the VM.
     */
    Assert(do_set_vm || prstate.new_vmbits == 0);
 
    /*
     * The snapshot conflict horizon for the whole record is the most
     * conservative (newest) horizon required by any change in the record.
     */
    conflict_xid = InvalidTransactionId;
    if (do_set_vm)
        conflict_xid = prstate.newest_live_xid;
    if (do_freeze && TransactionIdFollows(prstate.pagefrz.FreezePageConflictXid, conflict_xid))
        conflict_xid = prstate.pagefrz.FreezePageConflictXid;
    if (do_prune && TransactionIdFollows(prstate.latest_xid_removed, conflict_xid))
        conflict_xid = prstate.latest_xid_removed;
 
    /* Lock vmbuffer before entering a critical section */
    if (do_set_vm)
        LockBuffer(prstate.vmbuffer, BUFFER_LOCK_EXCLUSIVE);
 
    /* Any error while applying the changes is critical */
    START_CRIT_SECTION();
 
    if (do_hint_prune)
    {
        /*
         * Update the page's pd_prune_xid field to either zero, or the lowest
         * XID of any soon-prunable tuple.
         */
        ((PageHeader) prstate.page)->pd_prune_xid = prstate.new_prune_xid;
 
        /*
         * Also clear the "page is full" flag, since there's no point in
         * repeating the prune/defrag process until something else happens to
         * the page.
         */
        PageClearFull(prstate.page);
 
        /*
         * If that's all we had to do to the page, this is a non-WAL-logged
         * hint. If we are going to freeze or prune the page or set
         * PD_ALL_VISIBLE, we will mark the buffer dirty below.
         *
         * Setting PD_ALL_VISIBLE is fully WAL-logged because it is forbidden
         * for the VM to be set and PD_ALL_VISIBLE to be clear.
         */
        if (!do_freeze && !do_prune && !do_set_vm)
            MarkBufferDirtyHint(prstate.buffer, true);
    }
 
    if (do_prune || do_freeze || do_set_vm)
    {
        /* Apply the planned item changes and repair page fragmentation. */
        if (do_prune)
        {
            heap_page_prune_execute(prstate.buffer, false,
                                    prstate.redirected, prstate.nredirected,
                                    prstate.nowdead, prstate.ndead,
                                    prstate.nowunused, prstate.nunused);
        }
 
        if (do_freeze)
            heap_freeze_prepared_tuples(prstate.buffer, prstate.frozen, prstate.nfrozen);
 
        /* Set the visibility map and page visibility hint */
        if (do_set_vm)
        {
            /*
             * While it is valid for PD_ALL_VISIBLE to be set when the
             * corresponding VM bit is clear, we strongly prefer to keep them
             * in sync.
             *
             * The heap buffer must be marked dirty before adding it to the
             * WAL chain when setting the VM. We don't worry about
             * unnecessarily dirtying the heap buffer if PD_ALL_VISIBLE is
             * already set, though. It is extremely rare to have a clean heap
             * buffer with PD_ALL_VISIBLE already set and the VM bits clear,
             * so there is no point in optimizing it.
             */
            PageSetAllVisible(prstate.page);
            PageClearPrunable(prstate.page);
            visibilitymap_set(prstate.block, prstate.vmbuffer, prstate.new_vmbits,
                              prstate.relation->rd_locator);
        }
 
        MarkBufferDirty(prstate.buffer);
 
        /*
         * Emit a WAL XLOG_HEAP2_PRUNE* record showing what we did
         */
        if (RelationNeedsWAL(prstate.relation))
        {
            log_heap_prune_and_freeze(prstate.relation, prstate.buffer,
                                      do_set_vm ? prstate.vmbuffer : InvalidBuffer,
                                      do_set_vm ? prstate.new_vmbits : 0,
                                      conflict_xid,
                                      true, /* cleanup lock */
                                      params->reason,
                                      prstate.frozen, prstate.nfrozen,
                                      prstate.redirected, prstate.nredirected,
                                      prstate.nowdead, prstate.ndead,
                                      prstate.nowunused, prstate.nunused);
        }
    }
 
    END_CRIT_SECTION();
 
    if (do_set_vm)
        LockBuffer(prstate.vmbuffer, BUFFER_LOCK_UNLOCK);
 
    /*
     * During its second pass over the heap, VACUUM calls
     * heap_page_would_be_all_visible() to determine whether a page is
     * all-visible and all-frozen. The logic here is similar. After completing
     * pruning and freezing, use an assertion to verify that our results
     * remain consistent with heap_page_would_be_all_visible(). It's also a
     * valuable cross-check of the page state after pruning and freezing.
     */
#ifdef USE_ASSERT_CHECKING
    if (prstate.set_all_visible)
    {
        TransactionId debug_cutoff;
        bool        debug_all_frozen;
 
        Assert(prstate.lpdead_items == 0);
 
        Assert(heap_page_is_all_visible(prstate.relation, prstate.buffer,
                                        prstate.vistest,
                                        &debug_all_frozen,
                                        &debug_cutoff, off_loc));
 
        Assert(!TransactionIdIsValid(debug_cutoff) ||
               debug_cutoff == prstate.newest_live_xid);
 
        /*
         * It's possible the page is composed entirely of frozen tuples but is
         * not set all-frozen in the VM and did not pass
         * HEAP_PAGE_PRUNE_FREEZE. In this case, it's possible
         * heap_page_is_all_visible() finds the page completely frozen, even
         * though prstate.set_all_frozen is false.
         */
        Assert(!prstate.set_all_frozen || debug_all_frozen);
    }
#endif
 
    /* Copy information back for caller */
    presult->ndeleted = prstate.ndeleted;
    presult->nnewlpdead = prstate.ndead;
    presult->nfrozen = prstate.nfrozen;
    presult->live_tuples = prstate.live_tuples;
    presult->recently_dead_tuples = prstate.recently_dead_tuples;
    presult->hastup = prstate.hastup;
 
    presult->lpdead_items = prstate.lpdead_items;
    /* the presult->deadoffsets array was already filled in */
 
    presult->newly_all_visible = false;
    presult->newly_all_frozen = false;
    presult->newly_all_visible_frozen = false;
    if (do_set_vm)
    {
        if ((prstate.old_vmbits & VISIBILITYMAP_ALL_VISIBLE) == 0)
        {
            presult->newly_all_visible = true;
            if (prstate.set_all_frozen)
                presult->newly_all_visible_frozen = true;
        }
        else if ((prstate.old_vmbits & VISIBILITYMAP_ALL_FROZEN) == 0 &&
                 prstate.set_all_frozen)
            presult->newly_all_frozen = true;
    }
 
    if (prstate.attempt_freeze)
    {
        if (presult->nfrozen > 0)
        {
            *new_relfrozen_xid = prstate.pagefrz.FreezePageRelfrozenXid;
            *new_relmin_mxid = prstate.pagefrz.FreezePageRelminMxid;
        }
        else
        {
            *new_relfrozen_xid = prstate.pagefrz.NoFreezePageRelfrozenXid;
            *new_relmin_mxid = prstate.pagefrz.NoFreezePageRelminMxid;
        }
    }
}
 
 
/*
 * Perform visibility checks for heap pruning.
 */
static HTSV_Result
heap_prune_satisfies_vacuum(PruneState *prstate, HeapTuple tup)
{
    HTSV_Result res;
    TransactionId dead_after;
 
    res = HeapTupleSatisfiesVacuumHorizon(tup, prstate->buffer, &dead_after);
 
    if (res != HEAPTUPLE_RECENTLY_DEAD)
        return res;
 
    /*
     * For VACUUM, we must be sure to prune tuples with xmax older than
     * OldestXmin -- a visibility cutoff determined at the beginning of
     * vacuuming the relation. OldestXmin is used for freezing determination
     * and we cannot freeze dead tuples' xmaxes.
     */
    if (prstate->cutoffs &&
        TransactionIdIsValid(prstate->cutoffs->OldestXmin) &&
        NormalTransactionIdPrecedes(dead_after, prstate->cutoffs->OldestXmin))
        return HEAPTUPLE_DEAD;
 
    /*
     * Determine whether or not the tuple is considered dead when compared
     * with the provided GlobalVisState. On-access pruning does not provide
     * VacuumCutoffs. And for vacuum, even if the tuple's xmax is not older
     * than OldestXmin, GlobalVisTestIsRemovableXid() could find the row dead
     * if the GlobalVisState has been updated since the beginning of vacuuming
     * the relation.
     */
    if (GlobalVisTestIsRemovableXid(prstate->vistest, dead_after, true))
        return HEAPTUPLE_DEAD;
 
    return res;
}
 
 
/*
 * Pruning calculates tuple visibility once and saves the results in an array
 * of int8.  See PruneState.htsv for details.  This helper function is meant
 * to guard against examining visibility status array members which have not
 * yet been computed.
 */
static inline HTSV_Result
htsv_get_valid_status(int status)
{
    Assert(status >= HEAPTUPLE_DEAD &&
           status <= HEAPTUPLE_DELETE_IN_PROGRESS);
    return (HTSV_Result) status;
}
 
/*
 * Prune specified line pointer or a HOT chain originating at line pointer.
 *
 * Tuple visibility information is provided in prstate->htsv.
 *
 * If the item is an index-referenced tuple (i.e. not a heap-only tuple),
 * the HOT chain is pruned by removing all DEAD tuples at the start of the HOT
 * chain.  We also prune any RECENTLY_DEAD tuples preceding a DEAD tuple.
 * This is OK because a RECENTLY_DEAD tuple preceding a DEAD tuple is really
 * DEAD, our visibility test is just too coarse to detect it.
 *
 * Pruning must never leave behind a DEAD tuple that still has tuple storage.
 * VACUUM isn't prepared to deal with that case.
 *
 * The root line pointer is redirected to the tuple immediately after the
 * latest DEAD tuple.  If all tuples in the chain are DEAD, the root line
 * pointer is marked LP_DEAD.  (This includes the case of a DEAD simple
 * tuple, which we treat as a chain of length 1.)
 *
 * We don't actually change the page here. We just add entries to the arrays in
 * prstate showing the changes to be made.  Items to be redirected are added
 * to the redirected[] array (two entries per redirection); items to be set to
 * LP_DEAD state are added to nowdead[]; and items to be set to LP_UNUSED
 * state are added to nowunused[].  We perform bookkeeping of live tuples,
 * visibility etc. based on what the page will look like after the changes
 * applied.  All that bookkeeping is performed in the heap_prune_record_*()
 * subroutines.  The division of labor is that heap_prune_chain() decides the
 * fate of each tuple, ie. whether it's going to be removed, redirected or
 * left unchanged, and the heap_prune_record_*() subroutines update PruneState
 * based on that outcome.
 */
static void
heap_prune_chain(OffsetNumber maxoff, OffsetNumber rootoffnum,
                 PruneState *prstate)
{
    TransactionId priorXmax = InvalidTransactionId;
    ItemId      rootlp;
    OffsetNumber offnum;
    OffsetNumber chainitems[MaxHeapTuplesPerPage];
    Page        page = prstate->page;
 
    /*
     * After traversing the HOT chain, ndeadchain is the index in chainitems
     * of the first live successor after the last dead item.
     */
    int         ndeadchain = 0,
                nchain = 0;
 
    rootlp = PageGetItemId(page, rootoffnum);
 
    /* Start from the root tuple */
    offnum = rootoffnum;
 
    /* while not end of the chain */
    for (;;)
    {
        HeapTupleHeader htup;
        ItemId      lp;
 
        /* Sanity check (pure paranoia) */
        if (offnum < FirstOffsetNumber)
            break;
 
        /*
         * An offset past the end of page's line pointer array is possible
         * when the array was truncated (original item must have been unused)
         */
        if (offnum > maxoff)
            break;
 
        /* If item is already processed, stop --- it must not be same chain */
        if (prstate->processed[offnum])
            break;
 
        lp = PageGetItemId(page, offnum);
 
        /*
         * Unused item obviously isn't part of the chain. Likewise, a dead
         * line pointer can't be part of the chain.  Both of those cases were
         * already marked as processed.
         */
        Assert(ItemIdIsUsed(lp));
        Assert(!ItemIdIsDead(lp));
 
        /*
         * If we are looking at the redirected root line pointer, jump to the
         * first normal tuple in the chain.  If we find a redirect somewhere
         * else, stop --- it must not be same chain.
         */
        if (ItemIdIsRedirected(lp))
        {
            if (nchain > 0)
                break;          /* not at start of chain */
            chainitems[nchain++] = offnum;
            offnum = ItemIdGetRedirect(rootlp);
            continue;
        }
 
        Assert(ItemIdIsNormal(lp));
 
        htup = (HeapTupleHeader) PageGetItem(page, lp);
 
        /*
         * Check the tuple XMIN against prior XMAX, if any
         */
        if (TransactionIdIsValid(priorXmax) &&
            !TransactionIdEquals(HeapTupleHeaderGetXmin(htup), priorXmax))
            break;
 
        /*
         * OK, this tuple is indeed a member of the chain.
         */
        chainitems[nchain++] = offnum;
 
        switch (htsv_get_valid_status(prstate->htsv[offnum]))
        {
            case HEAPTUPLE_DEAD:
 
                /* Remember the last DEAD tuple seen */
                ndeadchain = nchain;
                HeapTupleHeaderAdvanceConflictHorizon(htup,
                                                      &prstate->latest_xid_removed);
                /* Advance to next chain member */
                break;
 
            case HEAPTUPLE_RECENTLY_DEAD:
 
                /*
                 * We don't need to advance the conflict horizon for
                 * RECENTLY_DEAD tuples, even if we are removing them.  This
                 * is because we only remove RECENTLY_DEAD tuples if they
                 * precede a DEAD tuple, and the DEAD tuple must have been
                 * inserted by a newer transaction than the RECENTLY_DEAD
                 * tuple by virtue of being later in the chain.  We will have
                 * advanced the conflict horizon for the DEAD tuple.
                 */
 
                /*
                 * Advance past RECENTLY_DEAD tuples just in case there's a
                 * DEAD one after them.  We have to make sure that we don't
                 * miss any DEAD tuples, since DEAD tuples that still have
                 * tuple storage after pruning will confuse VACUUM.
                 */
                break;
 
            case HEAPTUPLE_DELETE_IN_PROGRESS:
            case HEAPTUPLE_LIVE:
            case HEAPTUPLE_INSERT_IN_PROGRESS:
                goto process_chain;
 
            default:
                elog(ERROR, "unexpected HeapTupleSatisfiesVacuum result");
                goto process_chain;
        }
 
        /*
         * If the tuple is not HOT-updated, then we are at the end of this
         * HOT-update chain.
         */
        if (!HeapTupleHeaderIsHotUpdated(htup))
            goto process_chain;
 
        /* HOT implies it can't have moved to different partition */
        Assert(!HeapTupleHeaderIndicatesMovedPartitions(htup));
 
        /*
         * Advance to next chain member.
         */
        Assert(ItemPointerGetBlockNumber(&htup->t_ctid) == prstate->block);
        offnum = ItemPointerGetOffsetNumber(&htup->t_ctid);
        priorXmax = HeapTupleHeaderGetUpdateXid(htup);
    }
 
    if (ItemIdIsRedirected(rootlp) && nchain < 2)
    {
        /*
         * We found a redirect item that doesn't point to a valid follow-on
         * item.  This can happen if the loop in heap_page_prune_and_freeze()
         * caused us to visit the dead successor of a redirect item before
         * visiting the redirect item.  We can clean up by setting the
         * redirect item to LP_DEAD state or LP_UNUSED if the caller
         * indicated.
         */
        heap_prune_record_dead_or_unused(prstate, rootoffnum, false);
        return;
    }
 
process_chain:
 
    if (ndeadchain == 0)
    {
        /*
         * No DEAD tuple was found, so the chain is entirely composed of
         * normal, unchanged tuples.  Leave it alone.
         */
        int         i = 0;
 
        if (ItemIdIsRedirected(rootlp))
        {
            heap_prune_record_unchanged_lp_redirect(prstate, rootoffnum);
            i++;
        }
        for (; i < nchain; i++)
            heap_prune_record_unchanged_lp_normal(prstate, chainitems[i]);
    }
    else if (ndeadchain == nchain)
    {
        /*
         * The entire chain is dead.  Mark the root line pointer LP_DEAD, and
         * fully remove the other tuples in the chain.
         */
        heap_prune_record_dead_or_unused(prstate, rootoffnum, ItemIdIsNormal(rootlp));
        for (int i = 1; i < nchain; i++)
            heap_prune_record_unused(prstate, chainitems[i], true);
    }
    else
    {
        /*
         * We found a DEAD tuple in the chain.  Redirect the root line pointer
         * to the first non-DEAD tuple, and mark as unused each intermediate
         * item that we are able to remove from the chain.
         */
        heap_prune_record_redirect(prstate, rootoffnum, chainitems[ndeadchain],
                                   ItemIdIsNormal(rootlp));
        for (int i = 1; i < ndeadchain; i++)
            heap_prune_record_unused(prstate, chainitems[i], true);
 
        /* the rest of tuples in the chain are normal, unchanged tuples */
        for (int i = ndeadchain; i < nchain; i++)
            heap_prune_record_unchanged_lp_normal(prstate, chainitems[i]);
    }
}
 
/* Record lowest soon-prunable XID */
static void
heap_prune_record_prunable(PruneState *prstate, TransactionId xid,
                           OffsetNumber offnum)
{
    /*
     * This should exactly match the PageSetPrunable macro.  We can't store
     * directly into the page header yet, so we update working state.
     */
    Assert(TransactionIdIsNormal(xid));
    if (!TransactionIdIsValid(prstate->new_prune_xid) ||
        TransactionIdPrecedes(xid, prstate->new_prune_xid))
        prstate->new_prune_xid = xid;
 
    /*
     * It's incorrect for a page to be marked all-visible if it contains
     * prunable items.
     */
    if (PageIsAllVisible(prstate->page))
        heap_page_fix_vm_corruption(prstate, offnum,
                                    VM_CORRUPT_TUPLE_VISIBILITY);
}
 
/* Record line pointer to be redirected */
static void
heap_prune_record_redirect(PruneState *prstate,
                           OffsetNumber offnum, OffsetNumber rdoffnum,
                           bool was_normal)
{
    Assert(!prstate->processed[offnum]);
    prstate->processed[offnum] = true;
 
    /*
     * Do not mark the redirect target here.  It needs to be counted
     * separately as an unchanged tuple.
     */
 
    Assert(prstate->nredirected < MaxHeapTuplesPerPage);
    prstate->redirected[prstate->nredirected * 2] = offnum;
    prstate->redirected[prstate->nredirected * 2 + 1] = rdoffnum;
 
    prstate->nredirected++;
 
    /*
     * If the root entry had been a normal tuple, we are deleting it, so count
     * it in the result.  But changing a redirect (even to DEAD state) doesn't
     * count.
     */
    if (was_normal)
        prstate->ndeleted++;
 
    prstate->hastup = true;
}
 
/* Record line pointer to be marked dead */
static void
heap_prune_record_dead(PruneState *prstate, OffsetNumber offnum,
                       bool was_normal)
{
    Assert(!prstate->processed[offnum]);
    prstate->processed[offnum] = true;
 
    Assert(prstate->ndead < MaxHeapTuplesPerPage);
    prstate->nowdead[prstate->ndead] = offnum;
    prstate->ndead++;
 
    /*
     * Deliberately delay unsetting set_all_visible and set_all_frozen until
     * later during pruning. Removable dead tuples shouldn't preclude freezing
     * the page.
     */
 
    /* Record the dead offset for vacuum */
    prstate->deadoffsets[prstate->lpdead_items++] = offnum;
 
    /*
     * If the root entry had been a normal tuple, we are deleting it, so count
     * it in the result.  But changing a redirect (even to DEAD state) doesn't
     * count.
     */
    if (was_normal)
        prstate->ndeleted++;
}
 
/*
 * Depending on whether or not the caller set mark_unused_now to true, record that a
 * line pointer should be marked LP_DEAD or LP_UNUSED. There are other cases in
 * which we will mark line pointers LP_UNUSED, but we will not mark line
 * pointers LP_DEAD if mark_unused_now is true.
 */
static void
heap_prune_record_dead_or_unused(PruneState *prstate, OffsetNumber offnum,
                                 bool was_normal)
{
    /*
     * If the caller set mark_unused_now to true, we can remove dead tuples
     * during pruning instead of marking their line pointers dead. Set this
     * tuple's line pointer LP_UNUSED. We hint that this option is less
     * likely.
     */
    if (unlikely(prstate->mark_unused_now))
        heap_prune_record_unused(prstate, offnum, was_normal);
    else
        heap_prune_record_dead(prstate, offnum, was_normal);
 
    /*
     * It's incorrect for the page to be set all-visible if it contains dead
     * items. Fix that on the heap page and check the VM for corruption as
     * well. Do that here rather than in heap_prune_record_dead() so we also
     * cover tuples that are directly marked LP_UNUSED via mark_unused_now.
     */
    if (PageIsAllVisible(prstate->page))
        heap_page_fix_vm_corruption(prstate, offnum, VM_CORRUPT_LPDEAD);
}
 
/* Record line pointer to be marked unused */
static void
heap_prune_record_unused(PruneState *prstate, OffsetNumber offnum, bool was_normal)
{
    Assert(!prstate->processed[offnum]);
    prstate->processed[offnum] = true;
 
    Assert(prstate->nunused < MaxHeapTuplesPerPage);
    prstate->nowunused[prstate->nunused] = offnum;
    prstate->nunused++;
 
    /*
     * If the root entry had been a normal tuple, we are deleting it, so count
     * it in the result.  But changing a redirect (even to DEAD state) doesn't
     * count.
     */
    if (was_normal)
        prstate->ndeleted++;
}
 
/*
 * Record an unused line pointer that is left unchanged.
 */
static void
heap_prune_record_unchanged_lp_unused(PruneState *prstate, OffsetNumber offnum)
{
    Assert(!prstate->processed[offnum]);
    prstate->processed[offnum] = true;
}
 
/*
 * Record line pointer that is left unchanged.  We consider freezing it, and
 * update bookkeeping of tuple counts and page visibility.
 */
static void
heap_prune_record_unchanged_lp_normal(PruneState *prstate, OffsetNumber offnum)
{
    HeapTupleHeader htup;
    TransactionId xmin;
    Page        page = prstate->page;
 
    Assert(!prstate->processed[offnum]);
    prstate->processed[offnum] = true;
 
    prstate->hastup = true;     /* the page is not empty */
 
    /*
     * The criteria for counting a tuple as live in this block need to match
     * what analyze.c's acquire_sample_rows() does, otherwise VACUUM and
     * ANALYZE may produce wildly different reltuples values, e.g. when there
     * are many recently-dead tuples.
     *
     * The logic here is a bit simpler than acquire_sample_rows(), as VACUUM
     * can't run inside a transaction block, which makes some cases impossible
     * (e.g. in-progress insert from the same transaction).
     *
     * HEAPTUPLE_DEAD are handled by the other heap_prune_record_*()
     * subroutines.  They don't count dead items like acquire_sample_rows()
     * does, because we assume that all dead items will become LP_UNUSED
     * before VACUUM finishes.  This difference is only superficial.  VACUUM
     * effectively agrees with ANALYZE about DEAD items, in the end.  VACUUM
     * won't remember LP_DEAD items, but only because they're not supposed to
     * be left behind when it is done. (Cases where we bypass index vacuuming
     * will violate this optimistic assumption, but the overall impact of that
     * should be negligible.)
     */
    htup = (HeapTupleHeader) PageGetItem(page, PageGetItemId(page, offnum));
 
    switch (prstate->htsv[offnum])
    {
        case HEAPTUPLE_LIVE:
 
            /*
             * Count it as live.  Not only is this natural, but it's also what
             * acquire_sample_rows() does.
             */
            prstate->live_tuples++;
 
            /*
             * Is the tuple definitely visible to all transactions?
             *
             * NB: Like with per-tuple hint bits, we can't set the
             * PD_ALL_VISIBLE flag if the inserter committed asynchronously.
             * See SetHintBits for more info.  Check that the tuple is hinted
             * xmin-committed because of that.
             */
            if (!HeapTupleHeaderXminCommitted(htup))
            {
                prstate->set_all_visible = false;
                prstate->set_all_frozen = false;
                break;
            }
 
            /*
             * The inserter definitely committed. But we don't know if it is
             * old enough that everyone sees it as committed. Later, after
             * processing all the tuples on the page, we'll check if there is
             * any snapshot that still considers the newest xid on the page to
             * be running. If so, we don't consider the page all-visible.
             */
            xmin = HeapTupleHeaderGetXmin(htup);
 
            /* Track newest xmin on page. */
            if (TransactionIdFollows(xmin, prstate->newest_live_xid) &&
                TransactionIdIsNormal(xmin))
                prstate->newest_live_xid = xmin;
 
            break;
 
        case HEAPTUPLE_RECENTLY_DEAD:
            prstate->recently_dead_tuples++;
            prstate->set_all_visible = false;
            prstate->set_all_frozen = false;
 
            /*
             * This tuple will soon become DEAD.  Update the hint field so
             * that the page is reconsidered for pruning in future.
             */
            heap_prune_record_prunable(prstate,
                                       HeapTupleHeaderGetUpdateXid(htup),
                                       offnum);
            break;
 
        case HEAPTUPLE_INSERT_IN_PROGRESS:
 
            /*
             * We do not count these rows as live, because we expect the
             * inserting transaction to update the counters at commit, and we
             * assume that will happen only after we report our results.  This
             * assumption is a bit shaky, but it is what acquire_sample_rows()
             * does, so be consistent.
             */
            prstate->set_all_visible = false;
            prstate->set_all_frozen = false;
 
            /*
             * Though there is nothing "prunable" on the page, we maintain
             * pd_prune_xid for inserts so that we have the opportunity to
             * mark them all-visible during the next round of pruning.
             */
            heap_prune_record_prunable(prstate,
                                       HeapTupleHeaderGetXmin(htup),
                                       offnum);
            break;
 
        case HEAPTUPLE_DELETE_IN_PROGRESS:
 
            /*
             * This an expected case during concurrent vacuum.  Count such
             * rows as live.  As above, we assume the deleting transaction
             * will commit and update the counters after we report.
             */
            prstate->live_tuples++;
            prstate->set_all_visible = false;
            prstate->set_all_frozen = false;
 
            /*
             * This tuple may soon become DEAD.  Update the hint field so that
             * the page is reconsidered for pruning in future.
             */
            heap_prune_record_prunable(prstate,
                                       HeapTupleHeaderGetUpdateXid(htup),
                                       offnum);
            break;
 
        default:
 
            /*
             * DEAD tuples should've been passed to heap_prune_record_dead()
             * or heap_prune_record_unused() instead.
             */
            elog(ERROR, "unexpected HeapTupleSatisfiesVacuum result %d",
                 prstate->htsv[offnum]);
            break;
    }
 
    /* Consider freezing any normal tuples which will not be removed */
    if (prstate->attempt_freeze)
    {
        bool        totally_frozen;
 
        if ((heap_prepare_freeze_tuple(htup,
                                       prstate->cutoffs,
                                       &prstate->pagefrz,
                                       &prstate->frozen[prstate->nfrozen],
                                       &totally_frozen)))
        {
            /* Save prepared freeze plan for later */
            prstate->frozen[prstate->nfrozen++].offset = offnum;
        }
 
        /*
         * If any tuple isn't either totally frozen already or eligible to
         * become totally frozen (according to its freeze plan), then the page
         * definitely cannot be set all-frozen in the visibility map later on.
         */
        if (!totally_frozen)
            prstate->set_all_frozen = false;
    }
}
 
 
/*
 * Record line pointer that was already LP_DEAD and is left unchanged.
 */
static void
heap_prune_record_unchanged_lp_dead(PruneState *prstate, OffsetNumber offnum)
{
    Assert(!prstate->processed[offnum]);
    prstate->processed[offnum] = true;
 
    /*
     * Deliberately don't set hastup for LP_DEAD items.  We make the soft
     * assumption that any LP_DEAD items encountered here will become
     * LP_UNUSED later on, before count_nondeletable_pages is reached.  If we
     * don't make this assumption then rel truncation will only happen every
     * other VACUUM, at most.  Besides, VACUUM must treat
     * hastup/nonempty_pages as provisional no matter how LP_DEAD items are
     * handled (handled here, or handled later on).
     *
     * Similarly, don't unset set_all_visible and set_all_frozen until later,
     * at the end of heap_page_prune_and_freeze().  This will allow us to
     * attempt to freeze the page after pruning.  As long as we unset it
     * before updating the visibility map, this will be correct.
     */
 
    /* Record the dead offset for vacuum */
    prstate->deadoffsets[prstate->lpdead_items++] = offnum;
 
    /*
     * It's incorrect for a page to be marked all-visible if it contains dead
     * items.
     */
    if (PageIsAllVisible(prstate->page))
        heap_page_fix_vm_corruption(prstate, offnum, VM_CORRUPT_LPDEAD);
}
 
/*
 * Record LP_REDIRECT that is left unchanged.
 */
static void
heap_prune_record_unchanged_lp_redirect(PruneState *prstate, OffsetNumber offnum)
{
    /*
     * A redirect line pointer doesn't count as a live tuple.
     *
     * If we leave a redirect line pointer in place, there will be another
     * tuple on the page that it points to.  We will do the bookkeeping for
     * that separately.  So we have nothing to do here, except remember that
     * we processed this item.
     */
    Assert(!prstate->processed[offnum]);
    prstate->processed[offnum] = true;
}
 
/*
 * Perform the actual page changes needed by heap_page_prune_and_freeze().
 *
 * If 'lp_truncate_only' is set, we are merely marking LP_DEAD line pointers
 * as unused, not redirecting or removing anything else.  The
 * PageRepairFragmentation() call is skipped in that case.
 *
 * If 'lp_truncate_only' is not set, the caller must hold a cleanup lock on
 * the buffer.  If it is set, an ordinary exclusive lock suffices.
 */
void
heap_page_prune_execute(Buffer buffer, bool lp_truncate_only,
                        OffsetNumber *redirected, int nredirected,
                        OffsetNumber *nowdead, int ndead,
                        OffsetNumber *nowunused, int nunused)
{
    Page        page = BufferGetPage(buffer);
    OffsetNumber *offnum;
    HeapTupleHeader htup PG_USED_FOR_ASSERTS_ONLY;
 
    /* Shouldn't be called unless there's something to do */
    Assert(nredirected > 0 || ndead > 0 || nunused > 0);
 
    /* If 'lp_truncate_only', we can only remove already-dead line pointers */
    Assert(!lp_truncate_only || (nredirected == 0 && ndead == 0));
 
    /* Update all redirected line pointers */
    offnum = redirected;
    for (int i = 0; i < nredirected; i++)
    {
        OffsetNumber fromoff = *offnum++;
        OffsetNumber tooff = *offnum++;
        ItemId      fromlp = PageGetItemId(page, fromoff);
        ItemId      tolp PG_USED_FOR_ASSERTS_ONLY;
 
#ifdef USE_ASSERT_CHECKING
 
        /*
         * Any existing item that we set as an LP_REDIRECT (any 'from' item)
         * must be the first item from a HOT chain.  If the item has tuple
         * storage then it can't be a heap-only tuple.  Otherwise we are just
         * maintaining an existing LP_REDIRECT from an existing HOT chain that
         * has been pruned at least once before now.
         */
        if (!ItemIdIsRedirected(fromlp))
        {
            Assert(ItemIdHasStorage(fromlp) && ItemIdIsNormal(fromlp));
 
            htup = (HeapTupleHeader) PageGetItem(page, fromlp);
            Assert(!HeapTupleHeaderIsHeapOnly(htup));
        }
        else
        {
            /* We shouldn't need to redundantly set the redirect */
            Assert(ItemIdGetRedirect(fromlp) != tooff);
        }
 
        /*
         * The item that we're about to set as an LP_REDIRECT (the 'from'
         * item) will point to an existing item (the 'to' item) that is
         * already a heap-only tuple.  There can be at most one LP_REDIRECT
         * item per HOT chain.
         *
         * We need to keep around an LP_REDIRECT item (after original
         * non-heap-only root tuple gets pruned away) so that it's always
         * possible for VACUUM to easily figure out what TID to delete from
         * indexes when an entire HOT chain becomes dead.  A heap-only tuple
         * can never become LP_DEAD; an LP_REDIRECT item or a regular heap
         * tuple can.
         *
         * This check may miss problems, e.g. the target of a redirect could
         * be marked as unused subsequently. The page_verify_redirects() check
         * below will catch such problems.
         */
        tolp = PageGetItemId(page, tooff);
        Assert(ItemIdHasStorage(tolp) && ItemIdIsNormal(tolp));
        htup = (HeapTupleHeader) PageGetItem(page, tolp);
        Assert(HeapTupleHeaderIsHeapOnly(htup));
#endif
 
        ItemIdSetRedirect(fromlp, tooff);
    }
 
    /* Update all now-dead line pointers */
    offnum = nowdead;
    for (int i = 0; i < ndead; i++)
    {
        OffsetNumber off = *offnum++;
        ItemId      lp = PageGetItemId(page, off);
 
#ifdef USE_ASSERT_CHECKING
 
        /*
         * An LP_DEAD line pointer must be left behind when the original item
         * (which is dead to everybody) could still be referenced by a TID in
         * an index.  This should never be necessary with any individual
         * heap-only tuple item, though. (It's not clear how much of a problem
         * that would be, but there is no reason to allow it.)
         */
        if (ItemIdHasStorage(lp))
        {
            Assert(ItemIdIsNormal(lp));
            htup = (HeapTupleHeader) PageGetItem(page, lp);
            Assert(!HeapTupleHeaderIsHeapOnly(htup));
        }
        else
        {
            /* Whole HOT chain becomes dead */
            Assert(ItemIdIsRedirected(lp));
        }
#endif
 
        ItemIdSetDead(lp);
    }
 
    /* Update all now-unused line pointers */
    offnum = nowunused;
    for (int i = 0; i < nunused; i++)
    {
        OffsetNumber off = *offnum++;
        ItemId      lp = PageGetItemId(page, off);
 
#ifdef USE_ASSERT_CHECKING
 
        if (lp_truncate_only)
        {
            /* Setting LP_DEAD to LP_UNUSED in vacuum's second pass */
            Assert(ItemIdIsDead(lp) && !ItemIdHasStorage(lp));
        }
        else
        {
            /*
             * When heap_page_prune_and_freeze() was called, mark_unused_now
             * may have been passed as true, which allows would-be LP_DEAD
             * items to be made LP_UNUSED instead.  This is only possible if
             * the relation has no indexes.  If there are any dead items, then
             * mark_unused_now was not true and every item being marked
             * LP_UNUSED must refer to a heap-only tuple.
             */
            if (ndead > 0)
            {
                Assert(ItemIdHasStorage(lp) && ItemIdIsNormal(lp));
                htup = (HeapTupleHeader) PageGetItem(page, lp);
                Assert(HeapTupleHeaderIsHeapOnly(htup));
            }
            else
                Assert(ItemIdIsUsed(lp));
        }
 
#endif
 
        ItemIdSetUnused(lp);
    }
 
    if (lp_truncate_only)
        PageTruncateLinePointerArray(page);
    else
    {
        /*
         * Finally, repair any fragmentation, and update the page's hint bit
         * about whether it has free pointers.
         */
        PageRepairFragmentation(page);
 
        /*
         * Now that the page has been modified, assert that redirect items
         * still point to valid targets.
         */
        page_verify_redirects(page);
    }
}
 
 
/*
 * If built with assertions, verify that all LP_REDIRECT items point to a
 * valid item.
 *
 * One way that bugs related to HOT pruning show is redirect items pointing to
 * removed tuples. It's not trivial to reliably check that marking an item
 * unused will not orphan a redirect item during heap_prune_chain() /
 * heap_page_prune_execute(), so we additionally check the whole page after
 * pruning. Without this check such bugs would typically only cause asserts
 * later, potentially well after the corruption has been introduced.
 *
 * Also check comments in heap_page_prune_execute()'s redirection loop.
 */
static void
page_verify_redirects(Page page)
{
#ifdef USE_ASSERT_CHECKING
    OffsetNumber offnum;
    OffsetNumber maxoff;
 
    maxoff = PageGetMaxOffsetNumber(page);
    for (offnum = FirstOffsetNumber;
         offnum <= maxoff;
         offnum = OffsetNumberNext(offnum))
    {
        ItemId      itemid = PageGetItemId(page, offnum);
        OffsetNumber targoff;
        ItemId      targitem;
        HeapTupleHeader htup;
 
        if (!ItemIdIsRedirected(itemid))
            continue;
 
        targoff = ItemIdGetRedirect(itemid);
        targitem = PageGetItemId(page, targoff);
 
        Assert(ItemIdIsUsed(targitem));
        Assert(ItemIdIsNormal(targitem));
        Assert(ItemIdHasStorage(targitem));
        htup = (HeapTupleHeader) PageGetItem(page, targitem);
        Assert(HeapTupleHeaderIsHeapOnly(htup));
    }
#endif
}
 
 
/*
 * For all items in this page, find their respective root line pointers.
 * If item k is part of a HOT-chain with root at item j, then we set
 * root_offsets[k - 1] = j.
 *
 * The passed-in root_offsets array must have MaxHeapTuplesPerPage entries.
 * Unused entries are filled with InvalidOffsetNumber (zero).
 *
 * The function must be called with at least share lock on the buffer, to
 * prevent concurrent prune operations.
 *
 * Note: The information collected here is valid only as long as the caller
 * holds a pin on the buffer. Once pin is released, a tuple might be pruned
 * and reused by a completely unrelated tuple.
 */
void
heap_get_root_tuples(Page page, OffsetNumber *root_offsets)
{
    OffsetNumber offnum,
                maxoff;
 
    MemSet(root_offsets, InvalidOffsetNumber,
           MaxHeapTuplesPerPage * sizeof(OffsetNumber));
 
    maxoff = PageGetMaxOffsetNumber(page);
    for (offnum = FirstOffsetNumber; offnum <= maxoff; offnum = OffsetNumberNext(offnum))
    {
        ItemId      lp = PageGetItemId(page, offnum);
        HeapTupleHeader htup;
        OffsetNumber nextoffnum;
        TransactionId priorXmax;
 
        /* skip unused and dead items */
        if (!ItemIdIsUsed(lp) || ItemIdIsDead(lp))
            continue;
 
        if (ItemIdIsNormal(lp))
        {
            htup = (HeapTupleHeader) PageGetItem(page, lp);
 
            /*
             * Check if this tuple is part of a HOT-chain rooted at some other
             * tuple. If so, skip it for now; we'll process it when we find
             * its root.
             */
            if (HeapTupleHeaderIsHeapOnly(htup))
                continue;
 
            /*
             * This is either a plain tuple or the root of a HOT-chain.
             * Remember it in the mapping.
             */
            root_offsets[offnum - 1] = offnum;
 
            /* If it's not the start of a HOT-chain, we're done with it */
            if (!HeapTupleHeaderIsHotUpdated(htup))
                continue;
 
            /* Set up to scan the HOT-chain */
            nextoffnum = ItemPointerGetOffsetNumber(&htup->t_ctid);
            priorXmax = HeapTupleHeaderGetUpdateXid(htup);
        }
        else
        {
            /* Must be a redirect item. We do not set its root_offsets entry */
            Assert(ItemIdIsRedirected(lp));
            /* Set up to scan the HOT-chain */
            nextoffnum = ItemIdGetRedirect(lp);
            priorXmax = InvalidTransactionId;
        }
 
        /*
         * Now follow the HOT-chain and collect other tuples in the chain.
         *
         * Note: Even though this is a nested loop, the complexity of the
         * function is O(N) because a tuple in the page should be visited not
         * more than twice, once in the outer loop and once in HOT-chain
         * chases.
         */
        for (;;)
        {
            /* Sanity check (pure paranoia) */
            if (nextoffnum < FirstOffsetNumber)
                break;
 
            /*
             * An offset past the end of page's line pointer array is possible
             * when the array was truncated
             */
            if (nextoffnum > maxoff)
                break;
 
            lp = PageGetItemId(page, nextoffnum);
 
            /* Check for broken chains */
            if (!ItemIdIsNormal(lp))
                break;
 
            htup = (HeapTupleHeader) PageGetItem(page, lp);
 
            if (TransactionIdIsValid(priorXmax) &&
                !TransactionIdEquals(priorXmax, HeapTupleHeaderGetXmin(htup)))
                break;
 
            /* Remember the root line pointer for this item */
            root_offsets[nextoffnum - 1] = offnum;
 
            /* Advance to next chain member, if any */
            if (!HeapTupleHeaderIsHotUpdated(htup))
                break;
 
            /* HOT implies it can't have moved to different partition */
            Assert(!HeapTupleHeaderIndicatesMovedPartitions(htup));
 
            nextoffnum = ItemPointerGetOffsetNumber(&htup->t_ctid);
            priorXmax = HeapTupleHeaderGetUpdateXid(htup);
        }
    }
}
 
 
/*
 * Compare fields that describe actions required to freeze tuple with caller's
 * open plan.  If everything matches then the frz tuple plan is equivalent to
 * caller's plan.
 */
static inline bool
heap_log_freeze_eq(xlhp_freeze_plan *plan, HeapTupleFreeze *frz)
{
    if (plan->xmax == frz->xmax &&
        plan->t_infomask2 == frz->t_infomask2 &&
        plan->t_infomask == frz->t_infomask &&
        plan->frzflags == frz->frzflags)
        return true;
 
    /* Caller must call heap_log_freeze_new_plan again for frz */
    return false;
}
 
/*
 * Comparator used to deduplicate the freeze plans used in WAL records.
 */
static int
heap_log_freeze_cmp(const void *arg1, const void *arg2)
{
    const HeapTupleFreeze *frz1 = arg1;
    const HeapTupleFreeze *frz2 = arg2;
 
    if (frz1->xmax < frz2->xmax)
        return -1;
    else if (frz1->xmax > frz2->xmax)
        return 1;
 
    if (frz1->t_infomask2 < frz2->t_infomask2)
        return -1;
    else if (frz1->t_infomask2 > frz2->t_infomask2)
        return 1;
 
    if (frz1->t_infomask < frz2->t_infomask)
        return -1;
    else if (frz1->t_infomask > frz2->t_infomask)
        return 1;
 
    if (frz1->frzflags < frz2->frzflags)
        return -1;
    else if (frz1->frzflags > frz2->frzflags)
        return 1;
 
    /*
     * heap_log_freeze_eq would consider these tuple-wise plans to be equal.
     * (So the tuples will share a single canonical freeze plan.)
     *
     * We tiebreak on page offset number to keep each freeze plan's page
     * offset number array individually sorted. (Unnecessary, but be tidy.)
     */
    if (frz1->offset < frz2->offset)
        return -1;
    else if (frz1->offset > frz2->offset)
        return 1;
 
    Assert(false);
    return 0;
}
 
/*
 * Start new plan initialized using tuple-level actions.  At least one tuple
 * will have steps required to freeze described by caller's plan during REDO.
 */
static inline void
heap_log_freeze_new_plan(xlhp_freeze_plan *plan, HeapTupleFreeze *frz)
{
    plan->xmax = frz->xmax;
    plan->t_infomask2 = frz->t_infomask2;
    plan->t_infomask = frz->t_infomask;
    plan->frzflags = frz->frzflags;
    plan->ntuples = 1;          /* for now */
}
 
/*
 * Deduplicate tuple-based freeze plans so that each distinct set of
 * processing steps is only stored once in the WAL record.
 * Called during original execution of freezing (for logged relations).
 *
 * Return value is number of plans set in *plans_out for caller.  Also writes
 * an array of offset numbers into *offsets_out output argument for caller
 * (actually there is one array per freeze plan, but that's not of immediate
 * concern to our caller).
 */
static int
heap_log_freeze_plan(HeapTupleFreeze *tuples, int ntuples,
                     xlhp_freeze_plan *plans_out,
                     OffsetNumber *offsets_out)
{
    int         nplans = 0;
 
    /* Sort tuple-based freeze plans in the order required to deduplicate */
    qsort(tuples, ntuples, sizeof(HeapTupleFreeze), heap_log_freeze_cmp);
 
    for (int i = 0; i < ntuples; i++)
    {
        HeapTupleFreeze *frz = tuples + i;
 
        if (i == 0)
        {
            /* New canonical freeze plan starting with first tup */
            heap_log_freeze_new_plan(plans_out, frz);
            nplans++;
        }
        else if (heap_log_freeze_eq(plans_out, frz))
        {
            /* tup matches open canonical plan -- include tup in it */
            Assert(offsets_out[i - 1] < frz->offset);
            plans_out->ntuples++;
        }
        else
        {
            /* Tup doesn't match current plan -- done with it now */
            plans_out++;
 
            /* New canonical freeze plan starting with this tup */
            heap_log_freeze_new_plan(plans_out, frz);
            nplans++;
        }
 
        /*
         * Save page offset number in dedicated buffer in passing.
         *
         * REDO routine relies on the record's offset numbers array grouping
         * offset numbers by freeze plan.  The sort order within each grouping
         * is ascending offset number order, just to keep things tidy.
         */
        offsets_out[i] = frz->offset;
    }
 
    Assert(nplans > 0 && nplans <= ntuples);
 
    return nplans;
}
 
/*
 * Write an XLOG_HEAP2_PRUNE* WAL record
 *
 * This is used for several different page maintenance operations:
 *
 * - Page pruning, in VACUUM's 1st pass or on access: Some items are
 *   redirected, some marked dead, and some removed altogether.
 *
 * - Freezing: Items are marked as 'frozen'.
 *
 * - Vacuum, 2nd pass: Items that are already LP_DEAD are marked as unused.
 *
 * They have enough commonalities that we use a single WAL record for them
 * all.
 *
 * If replaying the record requires a cleanup lock, pass cleanup_lock = true.
 * Replaying 'redirected' or 'dead' items always requires a cleanup lock, but
 * replaying 'unused' items depends on whether they were all previously marked
 * as dead.
 *
 * If the VM is being updated, vmflags will contain the bits to set. In this
 * case, vmbuffer should already have been updated and marked dirty and should
 * still be pinned and locked.
 *
 * Note: This function scribbles on the 'frozen' array.
 *
 * Note: This is called in a critical section, so careful what you do here.
 */
void
log_heap_prune_and_freeze(Relation relation, Buffer buffer,
                          Buffer vmbuffer, uint8 vmflags,
                          TransactionId conflict_xid,
                          bool cleanup_lock,
                          PruneReason reason,
                          HeapTupleFreeze *frozen, int nfrozen,
                          OffsetNumber *redirected, int nredirected,
                          OffsetNumber *dead, int ndead,
                          OffsetNumber *unused, int nunused)
{
    xl_heap_prune xlrec;
    XLogRecPtr  recptr;
    uint8       info;
    uint8       regbuf_flags_heap;
 
    Page        heap_page = BufferGetPage(buffer);
 
    /* The following local variables hold data registered in the WAL record: */
    xlhp_freeze_plan plans[MaxHeapTuplesPerPage];
    xlhp_freeze_plans freeze_plans;
    xlhp_prune_items redirect_items;
    xlhp_prune_items dead_items;
    xlhp_prune_items unused_items;
    OffsetNumber frz_offsets[MaxHeapTuplesPerPage];
    bool        do_prune = nredirected > 0 || ndead > 0 || nunused > 0;
    bool        do_set_vm = vmflags & VISIBILITYMAP_VALID_BITS;
    bool        heap_fpi_allowed = true;
 
    Assert((vmflags & VISIBILITYMAP_VALID_BITS) == vmflags);
 
    xlrec.flags = 0;
    regbuf_flags_heap = REGBUF_STANDARD;
 
    /*
     * We can avoid an FPI of the heap page if the only modification we are
     * making to it is to set PD_ALL_VISIBLE and checksums/wal_log_hints are
     * disabled.
     *
     * However, if the page has never been WAL-logged (LSN is invalid), we
     * must force an FPI regardless.  This can happen when another backend
     * extends the heap, initializes the page, and then fails before WAL-
     * logging it.  Since heap extension is not WAL-logged, recovery might try
     * to replay our record and find that the page isn't initialized, which
     * would cause a PANIC.
     */
    if (!XLogRecPtrIsValid(PageGetLSN(heap_page)))
        regbuf_flags_heap |= REGBUF_FORCE_IMAGE;
    else if (!do_prune && nfrozen == 0 && (!do_set_vm || !XLogHintBitIsNeeded()))
    {
        regbuf_flags_heap |= REGBUF_NO_IMAGE;
        heap_fpi_allowed = false;
    }
 
    /*
     * Prepare data for the buffer.  The arrays are not actually in the
     * buffer, but we pretend that they are.  When XLogInsert stores a full
     * page image, the arrays can be omitted.
     */
    XLogBeginInsert();
    XLogRegisterBuffer(0, buffer, regbuf_flags_heap);
 
    if (do_set_vm)
        XLogRegisterBuffer(1, vmbuffer, 0);
 
    if (nfrozen > 0)
    {
        int         nplans;
 
        xlrec.flags |= XLHP_HAS_FREEZE_PLANS;
 
        /*
         * Prepare deduplicated representation for use in the WAL record. This
         * destructively sorts frozen tuples array in-place.
         */
        nplans = heap_log_freeze_plan(frozen, nfrozen, plans, frz_offsets);
 
        freeze_plans.nplans = nplans;
        XLogRegisterBufData(0, &freeze_plans,
                            offsetof(xlhp_freeze_plans, plans));
        XLogRegisterBufData(0, plans,
                            sizeof(xlhp_freeze_plan) * nplans);
    }
    if (nredirected > 0)
    {
        xlrec.flags |= XLHP_HAS_REDIRECTIONS;
 
        redirect_items.ntargets = nredirected;
        XLogRegisterBufData(0, &redirect_items,
                            offsetof(xlhp_prune_items, data));
        XLogRegisterBufData(0, redirected,
                            sizeof(OffsetNumber[2]) * nredirected);
    }
    if (ndead > 0)
    {
        xlrec.flags |= XLHP_HAS_DEAD_ITEMS;
 
        dead_items.ntargets = ndead;
        XLogRegisterBufData(0, &dead_items,
                            offsetof(xlhp_prune_items, data));
        XLogRegisterBufData(0, dead,
                            sizeof(OffsetNumber) * ndead);
    }
    if (nunused > 0)
    {
        xlrec.flags |= XLHP_HAS_NOW_UNUSED_ITEMS;
 
        unused_items.ntargets = nunused;
        XLogRegisterBufData(0, &unused_items,
                            offsetof(xlhp_prune_items, data));
        XLogRegisterBufData(0, unused,
                            sizeof(OffsetNumber) * nunused);
    }
    if (nfrozen > 0)
        XLogRegisterBufData(0, frz_offsets,
                            sizeof(OffsetNumber) * nfrozen);
 
    /*
     * Prepare the main xl_heap_prune record.  We already set the XLHP_HAS_*
     * flag above.
     */
    if (vmflags & VISIBILITYMAP_ALL_VISIBLE)
    {
        xlrec.flags |= XLHP_VM_ALL_VISIBLE;
        if (vmflags & VISIBILITYMAP_ALL_FROZEN)
            xlrec.flags |= XLHP_VM_ALL_FROZEN;
    }
    if (RelationIsAccessibleInLogicalDecoding(relation))
        xlrec.flags |= XLHP_IS_CATALOG_REL;
    if (TransactionIdIsValid(conflict_xid))
        xlrec.flags |= XLHP_HAS_CONFLICT_HORIZON;
    if (cleanup_lock)
        xlrec.flags |= XLHP_CLEANUP_LOCK;
    else
    {
        Assert(nredirected == 0 && ndead == 0);
        /* also, any items in 'unused' must've been LP_DEAD previously */
    }
    XLogRegisterData(&xlrec, SizeOfHeapPrune);
    if (TransactionIdIsValid(conflict_xid))
        XLogRegisterData(&conflict_xid, sizeof(TransactionId));
 
    switch (reason)
    {
        case PRUNE_ON_ACCESS:
            info = XLOG_HEAP2_PRUNE_ON_ACCESS;
            break;
        case PRUNE_VACUUM_SCAN:
            info = XLOG_HEAP2_PRUNE_VACUUM_SCAN;
            break;
        case PRUNE_VACUUM_CLEANUP:
            info = XLOG_HEAP2_PRUNE_VACUUM_CLEANUP;
            break;
        default:
            elog(ERROR, "unrecognized prune reason: %d", (int) reason);
            break;
    }
    recptr = XLogInsert(RM_HEAP2_ID, info);
 
    if (do_set_vm)
    {
        Assert(BufferIsDirty(vmbuffer));
        PageSetLSN(BufferGetPage(vmbuffer), recptr);
    }
 
    /*
     * If we explicitly skip an FPI, we must not stamp the heap page with this
     * record's LSN. Recovery skips records <= the stamped LSN, so this could
     * lead to skipping an earlier FPI needed to repair a torn page.
     */
    if (heap_fpi_allowed)
    {
        Assert(BufferIsDirty(buffer));
        PageSetLSN(heap_page, recptr);
    }
}