autovacuum.c

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/*-------------------------------------------------------------------------
 *
 * autovacuum.c
 *
 * PostgreSQL Integrated Autovacuum Daemon
 *
 * The autovacuum system is structured in two different kinds of processes: the
 * autovacuum launcher and the autovacuum worker.  The launcher is an
 * always-running process, started by the postmaster when the autovacuum GUC
 * parameter is set.  The launcher schedules autovacuum workers to be started
 * when appropriate.  The workers are the processes which execute the actual
 * vacuuming; they connect to a database as determined in the launcher, and
 * once connected they examine the catalogs to select the tables to vacuum.
 *
 * The autovacuum launcher cannot start the worker processes by itself,
 * because doing so would cause robustness issues (namely, failure to shut
 * them down on exceptional conditions, and also, since the launcher is
 * connected to shared memory and is thus subject to corruption there, it is
 * not as robust as the postmaster).  So it leaves that task to the postmaster.
 *
 * There is an autovacuum shared memory area, where the launcher stores
 * information about the database it wants vacuumed.  When it wants a new
 * worker to start, it sets a flag in shared memory and sends a signal to the
 * postmaster.  Then postmaster knows nothing more than it must start a worker;
 * so it forks a new child, which turns into a worker.  This new process
 * connects to shared memory, and there it can inspect the information that the
 * launcher has set up.
 *
 * If the fork() call fails in the postmaster, it sets a flag in the shared
 * memory area, and sends a signal to the launcher.  The launcher, upon
 * noticing the flag, can try starting the worker again by resending the
 * signal.  Note that the failure can only be transient (fork failure due to
 * high load, memory pressure, too many processes, etc); more permanent
 * problems, like failure to connect to a database, are detected later in the
 * worker and dealt with just by having the worker exit normally.  The launcher
 * will launch a new worker again later, per schedule.
 *
 * When the worker is done vacuuming it sends SIGUSR2 to the launcher.  The
 * launcher then wakes up and is able to launch another worker, if the schedule
 * is so tight that a new worker is needed immediately.  At this time the
 * launcher can also balance the settings for the various remaining workers'
 * cost-based vacuum delay feature.
 *
 * Note that there can be more than one worker in a database concurrently.
 * They will store the table they are currently vacuuming in shared memory, so
 * that other workers avoid being blocked waiting for the vacuum lock for that
 * table.  They will also reload the pgstats data just before vacuuming each
 * table, to avoid vacuuming a table that was just finished being vacuumed by
 * another worker and thus is no longer noted in shared memory.  However,
 * there is a window (caused by pgstat delay) on which a worker may choose a
 * table that was already vacuumed; this is a bug in the current design.
 *
 * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/postmaster/autovacuum.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include <signal.h>
#include <sys/time.h>
#include <unistd.h>

#include "access/heapam.h"
#include "access/htup_details.h"
#include "access/multixact.h"
#include "access/reloptions.h"
#include "access/tableam.h"
#include "access/transam.h"
#include "access/xact.h"
#include "catalog/dependency.h"
#include "catalog/namespace.h"
#include "catalog/pg_database.h"
#include "catalog/pg_inherits.h"
#include "commands/dbcommands.h"
#include "commands/vacuum.h"
#include "lib/ilist.h"
#include "libpq/pqsignal.h"
#include "miscadmin.h"
#include "nodes/makefuncs.h"
#include "pgstat.h"
#include "postmaster/autovacuum.h"
#include "postmaster/fork_process.h"
#include "postmaster/interrupt.h"
#include "postmaster/postmaster.h"
#include "storage/bufmgr.h"
#include "storage/ipc.h"
#include "storage/latch.h"
#include "storage/lmgr.h"
#include "storage/pmsignal.h"
#include "storage/proc.h"
#include "storage/procsignal.h"
#include "storage/sinvaladt.h"
#include "storage/smgr.h"
#include "tcop/tcopprot.h"
#include "utils/fmgroids.h"
#include "utils/fmgrprotos.h"
#include "utils/lsyscache.h"
#include "utils/memutils.h"
#include "utils/ps_status.h"
#include "utils/rel.h"
#include "utils/snapmgr.h"
#include "utils/syscache.h"
#include "utils/timeout.h"
#include "utils/timestamp.h"


/*
 * GUC parameters
 */
bool        autovacuum_start_daemon = false;
int         autovacuum_max_workers;
int         autovacuum_work_mem = -1;
int         autovacuum_naptime;
int         autovacuum_vac_thresh;
double      autovacuum_vac_scale;
int         autovacuum_vac_ins_thresh;
double      autovacuum_vac_ins_scale;
int         autovacuum_anl_thresh;
double      autovacuum_anl_scale;
int         autovacuum_freeze_max_age;
int         autovacuum_multixact_freeze_max_age;

double      autovacuum_vac_cost_delay;
int         autovacuum_vac_cost_limit;

int         Log_autovacuum_min_duration = -1;

/* how long to keep pgstat data in the launcher, in milliseconds */
#define STATS_READ_DELAY 1000

/* the minimum allowed time between two awakenings of the launcher */
#define MIN_AUTOVAC_SLEEPTIME 100.0 /* milliseconds */
#define MAX_AUTOVAC_SLEEPTIME 300   /* seconds */

/* Flags to tell if we are in an autovacuum process */
static bool am_autovacuum_launcher = false;
static bool am_autovacuum_worker = false;

/* Flags set by signal handlers */
static volatile sig_atomic_t got_SIGUSR2 = false;

/* Comparison points for determining whether freeze_max_age is exceeded */
static TransactionId recentXid;
static MultiXactId recentMulti;

/* Default freeze ages to use for autovacuum (varies by database) */
static int  default_freeze_min_age;
static int  default_freeze_table_age;
static int  default_multixact_freeze_min_age;
static int  default_multixact_freeze_table_age;

/* Memory context for long-lived data */
static MemoryContext AutovacMemCxt;

/* struct to keep track of databases in launcher */
typedef struct avl_dbase
{
    Oid         adl_datid;      /* hash key -- must be first */
    TimestampTz adl_next_worker;
    int         adl_score;
    dlist_node  adl_node;
} avl_dbase;

/* struct to keep track of databases in worker */
typedef struct avw_dbase
{
    Oid         adw_datid;
    char       *adw_name;
    TransactionId adw_frozenxid;
    MultiXactId adw_minmulti;
    PgStat_StatDBEntry *adw_entry;
} avw_dbase;

/* struct to keep track of tables to vacuum and/or analyze, in 1st pass */
typedef struct av_relation
{
    Oid         ar_toastrelid;  /* hash key - must be first */
    Oid         ar_relid;
    bool        ar_hasrelopts;
    AutoVacOpts ar_reloptions;  /* copy of AutoVacOpts from the main table's
                                 * reloptions, or NULL if none */
} av_relation;

/* struct to keep track of tables to vacuum and/or analyze, after rechecking */
typedef struct autovac_table
{
    Oid         at_relid;
    VacuumParams at_params;
    double      at_vacuum_cost_delay;
    int         at_vacuum_cost_limit;
    bool        at_dobalance;
    bool        at_sharedrel;
    char       *at_relname;
    char       *at_nspname;
    char       *at_datname;
} autovac_table;

/*-------------
 * This struct holds information about a single worker's whereabouts.  We keep
 * an array of these in shared memory, sized according to
 * autovacuum_max_workers.
 *
 * wi_links     entry into free list or running list
 * wi_dboid     OID of the database this worker is supposed to work on
 * wi_tableoid  OID of the table currently being vacuumed, if any
 * wi_sharedrel flag indicating whether table is marked relisshared
 * wi_proc      pointer to PGPROC of the running worker, NULL if not started
 * wi_launchtime Time at which this worker was launched
 * wi_cost_*    Vacuum cost-based delay parameters current in this worker
 *
 * All fields are protected by AutovacuumLock, except for wi_tableoid and
 * wi_sharedrel which are protected by AutovacuumScheduleLock (note these
 * two fields are read-only for everyone except that worker itself).
 *-------------
 */
typedef struct WorkerInfoData
{
    dlist_node  wi_links;
    Oid         wi_dboid;
    Oid         wi_tableoid;
    PGPROC     *wi_proc;
    TimestampTz wi_launchtime;
    bool        wi_dobalance;
    bool        wi_sharedrel;
    double      wi_cost_delay;
    int         wi_cost_limit;
    int         wi_cost_limit_base;
} WorkerInfoData;

typedef struct WorkerInfoData *WorkerInfo;

/*
 * Possible signals received by the launcher from remote processes.  These are
 * stored atomically in shared memory so that other processes can set them
 * without locking.
 */
typedef enum
{
    AutoVacForkFailed,          /* failed trying to start a worker */
    AutoVacRebalance,           /* rebalance the cost limits */
    AutoVacNumSignals           /* must be last */
}           AutoVacuumSignal;

/*
 * Autovacuum workitem array, stored in AutoVacuumShmem->av_workItems.  This
 * list is mostly protected by AutovacuumLock, except that if an item is
 * marked 'active' other processes must not modify the work-identifying
 * members.
 */
typedef struct AutoVacuumWorkItem
{
    AutoVacuumWorkItemType avw_type;
    bool        avw_used;       /* below data is valid */
    bool        avw_active;     /* being processed */
    Oid         avw_database;
    Oid         avw_relation;
    BlockNumber avw_blockNumber;
} AutoVacuumWorkItem;

#define NUM_WORKITEMS   256

/*-------------
 * The main autovacuum shmem struct.  On shared memory we store this main
 * struct and the array of WorkerInfo structs.  This struct keeps:
 *
 * av_signal        set by other processes to indicate various conditions
 * av_launcherpid   the PID of the autovacuum launcher
 * av_freeWorkers   the WorkerInfo freelist
 * av_runningWorkers the WorkerInfo non-free queue
 * av_startingWorker pointer to WorkerInfo currently being started (cleared by
 *                  the worker itself as soon as it's up and running)
 * av_workItems     work item array
 *
 * This struct is protected by AutovacuumLock, except for av_signal and parts
 * of the worker list (see above).
 *-------------
 */
typedef struct
{
    sig_atomic_t av_signal[AutoVacNumSignals];
    pid_t       av_launcherpid;
    dlist_head  av_freeWorkers;
    dlist_head  av_runningWorkers;
    WorkerInfo  av_startingWorker;
    AutoVacuumWorkItem av_workItems[NUM_WORKITEMS];
} AutoVacuumShmemStruct;

static AutoVacuumShmemStruct *AutoVacuumShmem;

/*
 * the database list (of avl_dbase elements) in the launcher, and the context
 * that contains it
 */
static dlist_head DatabaseList = DLIST_STATIC_INIT(DatabaseList);
static MemoryContext DatabaseListCxt = NULL;

/* Pointer to my own WorkerInfo, valid on each worker */
static WorkerInfo MyWorkerInfo = NULL;

/* PID of launcher, valid only in worker while shutting down */
int         AutovacuumLauncherPid = 0;

#ifdef EXEC_BACKEND
static pid_t avlauncher_forkexec(void);
static pid_t avworker_forkexec(void);
#endif
NON_EXEC_STATIC void AutoVacWorkerMain(int argc, char *argv[]) pg_attribute_noreturn();
NON_EXEC_STATIC void AutoVacLauncherMain(int argc, char *argv[]) pg_attribute_noreturn();

static Oid  do_start_worker(void);
static void HandleAutoVacLauncherInterrupts(void);
static void AutoVacLauncherShutdown(void) pg_attribute_noreturn();
static void launcher_determine_sleep(bool canlaunch, bool recursing,
                                     struct timeval *nap);
static void launch_worker(TimestampTz now);
static List *get_database_list(void);
static void rebuild_database_list(Oid newdb);
static int  db_comparator(const void *a, const void *b);
static void autovac_balance_cost(void);

static void do_autovacuum(void);
static void FreeWorkerInfo(int code, Datum arg);

static autovac_table *table_recheck_autovac(Oid relid, HTAB *table_toast_map,
                                            TupleDesc pg_class_desc,
                                            int effective_multixact_freeze_max_age);
static void recheck_relation_needs_vacanalyze(Oid relid, AutoVacOpts *avopts,
                                              Form_pg_class classForm,
                                              int effective_multixact_freeze_max_age,
                                              bool *dovacuum, bool *doanalyze, bool *wraparound);
static void relation_needs_vacanalyze(Oid relid, AutoVacOpts *relopts,
                                      Form_pg_class classForm,
                                      PgStat_StatTabEntry *tabentry,
                                      int effective_multixact_freeze_max_age,
                                      bool *dovacuum, bool *doanalyze, bool *wraparound);

static void autovacuum_do_vac_analyze(autovac_table *tab,
                                      BufferAccessStrategy bstrategy);
static AutoVacOpts *extract_autovac_opts(HeapTuple tup,
                                         TupleDesc pg_class_desc);
static PgStat_StatTabEntry *get_pgstat_tabentry_relid(Oid relid, bool isshared,
                                                      PgStat_StatDBEntry *shared,
                                                      PgStat_StatDBEntry *dbentry);
static void perform_work_item(AutoVacuumWorkItem *workitem);
static void autovac_report_activity(autovac_table *tab);
static void autovac_report_workitem(AutoVacuumWorkItem *workitem,
                                    const char *nspname, const char *relname);
static void avl_sigusr2_handler(SIGNAL_ARGS);
static void autovac_refresh_stats(void);



/********************************************************************
 *                    AUTOVACUUM LAUNCHER CODE
 ********************************************************************/

#ifdef EXEC_BACKEND
/*
 * forkexec routine for the autovacuum launcher process.
 *
 * Format up the arglist, then fork and exec.
 */
static pid_t
avlauncher_forkexec(void)
{
    char       *av[10];
    int         ac = 0;

    av[ac++] = "postgres";
    av[ac++] = "--forkavlauncher";
    av[ac++] = NULL;            /* filled in by postmaster_forkexec */
    av[ac] = NULL;

    Assert(ac < lengthof(av));

    return postmaster_forkexec(ac, av);
}

/*
 * We need this set from the outside, before InitProcess is called
 */
void
AutovacuumLauncherIAm(void)
{
    am_autovacuum_launcher = true;
}
#endif

/*
 * Main entry point for autovacuum launcher process, to be called from the
 * postmaster.
 */
int
StartAutoVacLauncher(void)
{
    pid_t       AutoVacPID;

#ifdef EXEC_BACKEND
    switch ((AutoVacPID = avlauncher_forkexec()))
#else
    switch ((AutoVacPID = fork_process()))
#endif
    {
        case -1:
            ereport(LOG,
                    (errmsg("could not fork autovacuum launcher process: %m")));
            return 0;

#ifndef EXEC_BACKEND
        case 0:
            /* in postmaster child ... */
            InitPostmasterChild();

            /* Close the postmaster's sockets */
            ClosePostmasterPorts(false);

            AutoVacLauncherMain(0, NULL);
            break;
#endif
        default:
            return (int) AutoVacPID;
    }

    /* shouldn't get here */
    return 0;
}

/*
 * Main loop for the autovacuum launcher process.
 */
NON_EXEC_STATIC void
AutoVacLauncherMain(int argc, char *argv[])
{
    sigjmp_buf  local_sigjmp_buf;

    am_autovacuum_launcher = true;

    MyBackendType = B_AUTOVAC_LAUNCHER;
    init_ps_display(NULL);

    ereport(DEBUG1,
            (errmsg_internal("autovacuum launcher started")));

    if (PostAuthDelay)
        pg_usleep(PostAuthDelay * 1000000L);

    SetProcessingMode(InitProcessing);

    /*
     * Set up signal handlers.  We operate on databases much like a regular
     * backend, so we use the same signal handling.  See equivalent code in
     * tcop/postgres.c.
     */
    pqsignal(SIGHUP, SignalHandlerForConfigReload);
    pqsignal(SIGINT, StatementCancelHandler);
    pqsignal(SIGTERM, SignalHandlerForShutdownRequest);
    /* SIGQUIT handler was already set up by InitPostmasterChild */

    InitializeTimeouts();       /* establishes SIGALRM handler */

    pqsignal(SIGPIPE, SIG_IGN);
    pqsignal(SIGUSR1, procsignal_sigusr1_handler);
    pqsignal(SIGUSR2, avl_sigusr2_handler);
    pqsignal(SIGFPE, FloatExceptionHandler);
    pqsignal(SIGCHLD, SIG_DFL);

    /* Early initialization */
    BaseInit();

    /*
     * Create a per-backend PGPROC struct in shared memory, except in the
     * EXEC_BACKEND case where this was done in SubPostmasterMain. We must do
     * this before we can use LWLocks (and in the EXEC_BACKEND case we already
     * had to do some stuff with LWLocks).
     */
#ifndef EXEC_BACKEND
    InitProcess();
#endif

    InitPostgres(NULL, InvalidOid, NULL, InvalidOid, NULL, false);

    SetProcessingMode(NormalProcessing);

    /*
     * Create a memory context that we will do all our work in.  We do this so
     * that we can reset the context during error recovery and thereby avoid
     * possible memory leaks.
     */
    AutovacMemCxt = AllocSetContextCreate(TopMemoryContext,
                                          "Autovacuum Launcher",
                                          ALLOCSET_DEFAULT_SIZES);
    MemoryContextSwitchTo(AutovacMemCxt);

    /*
     * If an exception is encountered, processing resumes here.
     *
     * This code is a stripped down version of PostgresMain error recovery.
     *
     * Note that we use sigsetjmp(..., 1), so that the prevailing signal mask
     * (to wit, BlockSig) will be restored when longjmp'ing to here.  Thus,
     * signals other than SIGQUIT will be blocked until we complete error
     * recovery.  It might seem that this policy makes the HOLD_INTERRUPTS()
     * call redundant, but it is not since InterruptPending might be set
     * already.
     */
    if (sigsetjmp(local_sigjmp_buf, 1) != 0)
    {
        /* since not using PG_TRY, must reset error stack by hand */
        error_context_stack = NULL;

        /* Prevents interrupts while cleaning up */
        HOLD_INTERRUPTS();

        /* Forget any pending QueryCancel or timeout request */
        disable_all_timeouts(false);
        QueryCancelPending = false; /* second to avoid race condition */

        /* Report the error to the server log */
        EmitErrorReport();

        /* Abort the current transaction in order to recover */
        AbortCurrentTransaction();

        /*
         * Release any other resources, for the case where we were not in a
         * transaction.
         */
        LWLockReleaseAll();
        pgstat_report_wait_end();
        AbortBufferIO();
        UnlockBuffers();
        /* this is probably dead code, but let's be safe: */
        if (AuxProcessResourceOwner)
            ReleaseAuxProcessResources(false);
        AtEOXact_Buffers(false);
        AtEOXact_SMgr();
        AtEOXact_Files(false);
        AtEOXact_HashTables(false);

        /*
         * Now return to normal top-level context and clear ErrorContext for
         * next time.
         */
        MemoryContextSwitchTo(AutovacMemCxt);
        FlushErrorState();

        /* Flush any leaked data in the top-level context */
        MemoryContextResetAndDeleteChildren(AutovacMemCxt);

        /* don't leave dangling pointers to freed memory */
        DatabaseListCxt = NULL;
        dlist_init(&DatabaseList);

        /*
         * Make sure pgstat also considers our stat data as gone.  Note: we
         * mustn't use autovac_refresh_stats here.
         */
        pgstat_clear_snapshot();

        /* Now we can allow interrupts again */
        RESUME_INTERRUPTS();

        /* if in shutdown mode, no need for anything further; just go away */
        if (ShutdownRequestPending)
            AutoVacLauncherShutdown();

        /*
         * Sleep at least 1 second after any error.  We don't want to be
         * filling the error logs as fast as we can.
         */
        pg_usleep(1000000L);
    }

    /* We can now handle ereport(ERROR) */
    PG_exception_stack = &local_sigjmp_buf;

    /* must unblock signals before calling rebuild_database_list */
    PG_SETMASK(&UnBlockSig);

    /*
     * Set always-secure search path.  Launcher doesn't connect to a database,
     * so this has no effect.
     */
    SetConfigOption("search_path", "", PGC_SUSET, PGC_S_OVERRIDE);

    /*
     * Force zero_damaged_pages OFF in the autovac process, even if it is set
     * in postgresql.conf.  We don't really want such a dangerous option being
     * applied non-interactively.
     */
    SetConfigOption("zero_damaged_pages", "false", PGC_SUSET, PGC_S_OVERRIDE);

    /*
     * Force settable timeouts off to avoid letting these settings prevent
     * regular maintenance from being executed.
     */
    SetConfigOption("statement_timeout", "0", PGC_SUSET, PGC_S_OVERRIDE);
    SetConfigOption("lock_timeout", "0", PGC_SUSET, PGC_S_OVERRIDE);
    SetConfigOption("idle_in_transaction_session_timeout", "0",
                    PGC_SUSET, PGC_S_OVERRIDE);

    /*
     * Force default_transaction_isolation to READ COMMITTED.  We don't want
     * to pay the overhead of serializable mode, nor add any risk of causing
     * deadlocks or delaying other transactions.
     */
    SetConfigOption("default_transaction_isolation", "read committed",
                    PGC_SUSET, PGC_S_OVERRIDE);

    /*
     * In emergency mode, just start a worker (unless shutdown was requested)
     * and go away.
     */
    if (!AutoVacuumingActive())
    {
        if (!ShutdownRequestPending)
            do_start_worker();
        proc_exit(0);           /* done */
    }

    AutoVacuumShmem->av_launcherpid = MyProcPid;

    /*
     * Create the initial database list.  The invariant we want this list to
     * keep is that it's ordered by decreasing next_time.  As soon as an entry
     * is updated to a higher time, it will be moved to the front (which is
     * correct because the only operation is to add autovacuum_naptime to the
     * entry, and time always increases).
     */
    rebuild_database_list(InvalidOid);

    /* loop until shutdown request */
    while (!ShutdownRequestPending)
    {
        struct timeval nap;
        TimestampTz current_time = 0;
        bool        can_launch;

        /*
         * This loop is a bit different from the normal use of WaitLatch,
         * because we'd like to sleep before the first launch of a child
         * process.  So it's WaitLatch, then ResetLatch, then check for
         * wakening conditions.
         */

        launcher_determine_sleep(!dlist_is_empty(&AutoVacuumShmem->av_freeWorkers),
                                 false, &nap);

        /*
         * Wait until naptime expires or we get some type of signal (all the
         * signal handlers will wake us by calling SetLatch).
         */
        (void) WaitLatch(MyLatch,
                         WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH,
                         (nap.tv_sec * 1000L) + (nap.tv_usec / 1000L),
                         WAIT_EVENT_AUTOVACUUM_MAIN);

        ResetLatch(MyLatch);

        HandleAutoVacLauncherInterrupts();

        /*
         * a worker finished, or postmaster signaled failure to start a worker
         */
        if (got_SIGUSR2)
        {
            got_SIGUSR2 = false;

            /* rebalance cost limits, if needed */
            if (AutoVacuumShmem->av_signal[AutoVacRebalance])
            {
                LWLockAcquire(AutovacuumLock, LW_EXCLUSIVE);
                AutoVacuumShmem->av_signal[AutoVacRebalance] = false;
                autovac_balance_cost();
                LWLockRelease(AutovacuumLock);
            }

            if (AutoVacuumShmem->av_signal[AutoVacForkFailed])
            {
                /*
                 * If the postmaster failed to start a new worker, we sleep
                 * for a little while and resend the signal.  The new worker's
                 * state is still in memory, so this is sufficient.  After
                 * that, we restart the main loop.
                 *
                 * XXX should we put a limit to the number of times we retry?
                 * I don't think it makes much sense, because a future start
                 * of a worker will continue to fail in the same way.
                 */
                AutoVacuumShmem->av_signal[AutoVacForkFailed] = false;
                pg_usleep(1000000L);    /* 1s */
                SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_WORKER);
                continue;
            }
        }

        /*
         * There are some conditions that we need to check before trying to
         * start a worker.  First, we need to make sure that there is a worker
         * slot available.  Second, we need to make sure that no other worker
         * failed while starting up.
         */

        current_time = GetCurrentTimestamp();
        LWLockAcquire(AutovacuumLock, LW_SHARED);

        can_launch = !dlist_is_empty(&AutoVacuumShmem->av_freeWorkers);

        if (AutoVacuumShmem->av_startingWorker != NULL)
        {
            int         waittime;
            WorkerInfo  worker = AutoVacuumShmem->av_startingWorker;

            /*
             * We can't launch another worker when another one is still
             * starting up (or failed while doing so), so just sleep for a bit
             * more; that worker will wake us up again as soon as it's ready.
             * We will only wait autovacuum_naptime seconds (up to a maximum
             * of 60 seconds) for this to happen however.  Note that failure
             * to connect to a particular database is not a problem here,
             * because the worker removes itself from the startingWorker
             * pointer before trying to connect.  Problems detected by the
             * postmaster (like fork() failure) are also reported and handled
             * differently.  The only problems that may cause this code to
             * fire are errors in the earlier sections of AutoVacWorkerMain,
             * before the worker removes the WorkerInfo from the
             * startingWorker pointer.
             */
            waittime = Min(autovacuum_naptime, 60) * 1000;
            if (TimestampDifferenceExceeds(worker->wi_launchtime, current_time,
                                           waittime))
            {
                LWLockRelease(AutovacuumLock);
                LWLockAcquire(AutovacuumLock, LW_EXCLUSIVE);

                /*
                 * No other process can put a worker in starting mode, so if
                 * startingWorker is still INVALID after exchanging our lock,
                 * we assume it's the same one we saw above (so we don't
                 * recheck the launch time).
                 */
                if (AutoVacuumShmem->av_startingWorker != NULL)
                {
                    worker = AutoVacuumShmem->av_startingWorker;
                    worker->wi_dboid = InvalidOid;
                    worker->wi_tableoid = InvalidOid;
                    worker->wi_sharedrel = false;
                    worker->wi_proc = NULL;
                    worker->wi_launchtime = 0;
                    dlist_push_head(&AutoVacuumShmem->av_freeWorkers,
                                    &worker->wi_links);
                    AutoVacuumShmem->av_startingWorker = NULL;
                    elog(WARNING, "worker took too long to start; canceled");
                }
            }
            else
                can_launch = false;
        }
        LWLockRelease(AutovacuumLock);  /* either shared or exclusive */

        /* if we can't do anything, just go back to sleep */
        if (!can_launch)
            continue;

        /* We're OK to start a new worker */

        if (dlist_is_empty(&DatabaseList))
        {
            /*
             * Special case when the list is empty: start a worker right away.
             * This covers the initial case, when no database is in pgstats
             * (thus the list is empty).  Note that the constraints in
             * launcher_determine_sleep keep us from starting workers too
             * quickly (at most once every autovacuum_naptime when the list is
             * empty).
             */
            launch_worker(current_time);
        }
        else
        {
            /*
             * because rebuild_database_list constructs a list with most
             * distant adl_next_worker first, we obtain our database from the
             * tail of the list.
             */
            avl_dbase  *avdb;

            avdb = dlist_tail_element(avl_dbase, adl_node, &DatabaseList);

            /*
             * launch a worker if next_worker is right now or it is in the
             * past
             */
            if (TimestampDifferenceExceeds(avdb->adl_next_worker,
                                           current_time, 0))
                launch_worker(current_time);
        }
    }

    AutoVacLauncherShutdown();
}

/*
 * Process any new interrupts.
 */
static void
HandleAutoVacLauncherInterrupts(void)
{
    /* the normal shutdown case */
    if (ShutdownRequestPending)
        AutoVacLauncherShutdown();

    if (ConfigReloadPending)
    {
        ConfigReloadPending = false;
        ProcessConfigFile(PGC_SIGHUP);

        /* shutdown requested in config file? */
        if (!AutoVacuumingActive())
            AutoVacLauncherShutdown();

        /* rebalance in case the default cost parameters changed */
        LWLockAcquire(AutovacuumLock, LW_EXCLUSIVE);
        autovac_balance_cost();
        LWLockRelease(AutovacuumLock);

        /* rebuild the list in case the naptime changed */
        rebuild_database_list(InvalidOid);
    }

    /* Process barrier events */
    if (ProcSignalBarrierPending)
        ProcessProcSignalBarrier();

    /* Process sinval catchup interrupts that happened while sleeping */
    ProcessCatchupInterrupt();
}

/*
 * Perform a normal exit from the autovac launcher.
 */
static void
AutoVacLauncherShutdown(void)
{
    ereport(DEBUG1,
            (errmsg_internal("autovacuum launcher shutting down")));
    AutoVacuumShmem->av_launcherpid = 0;

    proc_exit(0);               /* done */
}

/*
 * Determine the time to sleep, based on the database list.
 *
 * The "canlaunch" parameter indicates whether we can start a worker right now,
 * for example due to the workers being all busy.  If this is false, we will
 * cause a long sleep, which will be interrupted when a worker exits.
 */
static void
launcher_determine_sleep(bool canlaunch, bool recursing, struct timeval *nap)
{
    /*
     * We sleep until the next scheduled vacuum.  We trust that when the
     * database list was built, care was taken so that no entries have times
     * in the past; if the first entry has too close a next_worker value, or a
     * time in the past, we will sleep a small nominal time.
     */
    if (!canlaunch)
    {
        nap->tv_sec = autovacuum_naptime;
        nap->tv_usec = 0;
    }
    else if (!dlist_is_empty(&DatabaseList))
    {
        TimestampTz current_time = GetCurrentTimestamp();
        TimestampTz next_wakeup;
        avl_dbase  *avdb;
        long        secs;
        int         usecs;

        avdb = dlist_tail_element(avl_dbase, adl_node, &DatabaseList);

        next_wakeup = avdb->adl_next_worker;
        TimestampDifference(current_time, next_wakeup, &secs, &usecs);

        nap->tv_sec = secs;
        nap->tv_usec = usecs;
    }
    else
    {
        /* list is empty, sleep for whole autovacuum_naptime seconds  */
        nap->tv_sec = autovacuum_naptime;
        nap->tv_usec = 0;
    }

    /*
     * If the result is exactly zero, it means a database had an entry with
     * time in the past.  Rebuild the list so that the databases are evenly
     * distributed again, and recalculate the time to sleep.  This can happen
     * if there are more tables needing vacuum than workers, and they all take
     * longer to vacuum than autovacuum_naptime.
     *
     * We only recurse once.  rebuild_database_list should always return times
     * in the future, but it seems best not to trust too much on that.
     */
    if (nap->tv_sec == 0 && nap->tv_usec == 0 && !recursing)
    {
        rebuild_database_list(InvalidOid);
        launcher_determine_sleep(canlaunch, true, nap);
        return;
    }

    /* The smallest time we'll allow the launcher to sleep. */
    if (nap->tv_sec <= 0 && nap->tv_usec <= MIN_AUTOVAC_SLEEPTIME * 1000)
    {
        nap->tv_sec = 0;
        nap->tv_usec = MIN_AUTOVAC_SLEEPTIME * 1000;
    }

    /*
     * If the sleep time is too large, clamp it to an arbitrary maximum (plus
     * any fractional seconds, for simplicity).  This avoids an essentially
     * infinite sleep in strange cases like the system clock going backwards a
     * few years.
     */
    if (nap->tv_sec > MAX_AUTOVAC_SLEEPTIME)
        nap->tv_sec = MAX_AUTOVAC_SLEEPTIME;
}

/*
 * Build an updated DatabaseList.  It must only contain databases that appear
 * in pgstats, and must be sorted by next_worker from highest to lowest,
 * distributed regularly across the next autovacuum_naptime interval.
 *
 * Receives the Oid of the database that made this list be generated (we call
 * this the "new" database, because when the database was already present on
 * the list, we expect that this function is not called at all).  The
 * preexisting list, if any, will be used to preserve the order of the
 * databases in the autovacuum_naptime period.  The new database is put at the
 * end of the interval.  The actual values are not saved, which should not be
 * much of a problem.
 */
static void
rebuild_database_list(Oid newdb)
{
    List       *dblist;
    ListCell   *cell;
    MemoryContext newcxt;
    MemoryContext oldcxt;
    MemoryContext tmpcxt;
    HASHCTL     hctl;
    int         score;
    int         nelems;
    HTAB       *dbhash;
    dlist_iter  iter;

    /* use fresh stats */
    autovac_refresh_stats();

    newcxt = AllocSetContextCreate(AutovacMemCxt,
                                   "AV dblist",
                                   ALLOCSET_DEFAULT_SIZES);
    tmpcxt = AllocSetContextCreate(newcxt,
                                   "tmp AV dblist",
                                   ALLOCSET_DEFAULT_SIZES);
    oldcxt = MemoryContextSwitchTo(tmpcxt);

    /*
     * Implementing this is not as simple as it sounds, because we need to put
     * the new database at the end of the list; next the databases that were
     * already on the list, and finally (at the tail of the list) all the
     * other databases that are not on the existing list.
     *
     * To do this, we build an empty hash table of scored databases.  We will
     * start with the lowest score (zero) for the new database, then
     * increasing scores for the databases in the existing list, in order, and
     * lastly increasing scores for all databases gotten via
     * get_database_list() that are not already on the hash.
     *
     * Then we will put all the hash elements into an array, sort the array by
     * score, and finally put the array elements into the new doubly linked
     * list.
     */
    hctl.keysize = sizeof(Oid);
    hctl.entrysize = sizeof(avl_dbase);
    hctl.hcxt = tmpcxt;
    dbhash = hash_create("db hash", 20, &hctl,  /* magic number here FIXME */
                         HASH_ELEM | HASH_BLOBS | HASH_CONTEXT);

    /* start by inserting the new database */
    score = 0;
    if (OidIsValid(newdb))
    {
        avl_dbase  *db;
        PgStat_StatDBEntry *entry;

        /* only consider this database if it has a pgstat entry */
        entry = pgstat_fetch_stat_dbentry(newdb);
        if (entry != NULL)
        {
            /* we assume it isn't found because the hash was just created */
            db = hash_search(dbhash, &newdb, HASH_ENTER, NULL);

            /* hash_search already filled in the key */
            db->adl_score = score++;
            /* next_worker is filled in later */
        }
    }

    /* Now insert the databases from the existing list */
    dlist_foreach(iter, &DatabaseList)
    {
        avl_dbase  *avdb = dlist_container(avl_dbase, adl_node, iter.cur);
        avl_dbase  *db;
        bool        found;
        PgStat_StatDBEntry *entry;

        /*
         * skip databases with no stat entries -- in particular, this gets rid
         * of dropped databases
         */
        entry = pgstat_fetch_stat_dbentry(avdb->adl_datid);
        if (entry == NULL)
            continue;

        db = hash_search(dbhash, &(avdb->adl_datid), HASH_ENTER, &found);

        if (!found)
        {
            /* hash_search already filled in the key */
            db->adl_score = score++;
            /* next_worker is filled in later */
        }
    }

    /* finally, insert all qualifying databases not previously inserted */
    dblist = get_database_list();
    foreach(cell, dblist)
    {
        avw_dbase  *avdb = lfirst(cell);
        avl_dbase  *db;
        bool        found;
        PgStat_StatDBEntry *entry;

        /* only consider databases with a pgstat entry */
        entry = pgstat_fetch_stat_dbentry(avdb->adw_datid);
        if (entry == NULL)
            continue;

        db = hash_search(dbhash, &(avdb->adw_datid), HASH_ENTER, &found);
        /* only update the score if the database was not already on the hash */
        if (!found)
        {
            /* hash_search already filled in the key */
            db->adl_score = score++;
            /* next_worker is filled in later */
        }
    }
    nelems = score;

    /* from here on, the allocated memory belongs to the new list */
    MemoryContextSwitchTo(newcxt);
    dlist_init(&DatabaseList);

    if (nelems > 0)
    {
        TimestampTz current_time;
        int         millis_increment;
        avl_dbase  *dbary;
        avl_dbase  *db;
        HASH_SEQ_STATUS seq;
        int         i;

        /* put all the hash elements into an array */
        dbary = palloc(nelems * sizeof(avl_dbase));

        i = 0;
        hash_seq_init(&seq, dbhash);
        while ((db = hash_seq_search(&seq)) != NULL)
            memcpy(&(dbary[i++]), db, sizeof(avl_dbase));

        /* sort the array */
        qsort(dbary, nelems, sizeof(avl_dbase), db_comparator);

        /*
         * Determine the time interval between databases in the schedule. If
         * we see that the configured naptime would take us to sleep times
         * lower than our min sleep time (which launcher_determine_sleep is
         * coded not to allow), silently use a larger naptime (but don't touch
         * the GUC variable).
         */
        millis_increment = 1000.0 * autovacuum_naptime / nelems;
        if (millis_increment <= MIN_AUTOVAC_SLEEPTIME)
            millis_increment = MIN_AUTOVAC_SLEEPTIME * 1.1;

        current_time = GetCurrentTimestamp();

        /*
         * move the elements from the array into the dlist, setting the
         * next_worker while walking the array
         */
        for (i = 0; i < nelems; i++)
        {
            avl_dbase  *db = &(dbary[i]);

            current_time = TimestampTzPlusMilliseconds(current_time,
                                                       millis_increment);
            db->adl_next_worker = current_time;

            /* later elements should go closer to the head of the list */
            dlist_push_head(&DatabaseList, &db->adl_node);
        }
    }

    /* all done, clean up memory */
    if (DatabaseListCxt != NULL)
        MemoryContextDelete(DatabaseListCxt);
    MemoryContextDelete(tmpcxt);
    DatabaseListCxt = newcxt;
    MemoryContextSwitchTo(oldcxt);
}

/* qsort comparator for avl_dbase, using adl_score */
static int
db_comparator(const void *a, const void *b)
{
    if (((const avl_dbase *) a)->adl_score == ((const avl_dbase *) b)->adl_score)
        return 0;
    else
        return (((const avl_dbase *) a)->adl_score < ((const avl_dbase *) b)->adl_score) ? 1 : -1;
}

/*
 * do_start_worker
 *
 * Bare-bones procedure for starting an autovacuum worker from the launcher.
 * It determines what database to work on, sets up shared memory stuff and
 * signals postmaster to start the worker.  It fails gracefully if invoked when
 * autovacuum_workers are already active.
 *
 * Return value is the OID of the database that the worker is going to process,
 * or InvalidOid if no worker was actually started.
 */
static Oid
do_start_worker(void)
{
    List       *dblist;
    ListCell   *cell;
    TransactionId xidForceLimit;
    MultiXactId multiForceLimit;
    bool        for_xid_wrap;
    bool        for_multi_wrap;
    avw_dbase  *avdb;
    TimestampTz current_time;
    bool        skipit = false;
    Oid         retval = InvalidOid;
    MemoryContext tmpcxt,
                oldcxt;

    /* return quickly when there are no free workers */
    LWLockAcquire(AutovacuumLock, LW_SHARED);
    if (dlist_is_empty(&AutoVacuumShmem->av_freeWorkers))
    {
        LWLockRelease(AutovacuumLock);
        return InvalidOid;
    }
    LWLockRelease(AutovacuumLock);

    /*
     * Create and switch to a temporary context to avoid leaking the memory
     * allocated for the database list.
     */
    tmpcxt = AllocSetContextCreate(CurrentMemoryContext,
                                   "Start worker tmp cxt",
                                   ALLOCSET_DEFAULT_SIZES);
    oldcxt = MemoryContextSwitchTo(tmpcxt);

    /* use fresh stats */
    autovac_refresh_stats();

    /* Get a list of databases */
    dblist = get_database_list();

    /*
     * Determine the oldest datfrozenxid/relfrozenxid that we will allow to
     * pass without forcing a vacuum.  (This limit can be tightened for
     * particular tables, but not loosened.)
     */
    recentXid = ReadNextTransactionId();
    xidForceLimit = recentXid - autovacuum_freeze_max_age;
    /* ensure it's a "normal" XID, else TransactionIdPrecedes misbehaves */
    /* this can cause the limit to go backwards by 3, but that's OK */
    if (xidForceLimit < FirstNormalTransactionId)
        xidForceLimit -= FirstNormalTransactionId;

    /* Also determine the oldest datminmxid we will consider. */
    recentMulti = ReadNextMultiXactId();
    multiForceLimit = recentMulti - MultiXactMemberFreezeThreshold();
    if (multiForceLimit < FirstMultiXactId)
        multiForceLimit -= FirstMultiXactId;

    /*
     * Choose a database to connect to.  We pick the database that was least
     * recently auto-vacuumed, or one that needs vacuuming to prevent Xid
     * wraparound-related data loss.  If any db at risk of Xid wraparound is
     * found, we pick the one with oldest datfrozenxid, independently of
     * autovacuum times; similarly we pick the one with the oldest datminmxid
     * if any is in MultiXactId wraparound.  Note that those in Xid wraparound
     * danger are given more priority than those in multi wraparound danger.
     *
     * Note that a database with no stats entry is not considered, except for
     * Xid wraparound purposes.  The theory is that if no one has ever
     * connected to it since the stats were last initialized, it doesn't need
     * vacuuming.
     *
     * XXX This could be improved if we had more info about whether it needs
     * vacuuming before connecting to it.  Perhaps look through the pgstats
     * data for the database's tables?  One idea is to keep track of the
     * number of new and dead tuples per database in pgstats.  However it
     * isn't clear how to construct a metric that measures that and not cause
     * starvation for less busy databases.
     */
    avdb = NULL;
    for_xid_wrap = false;
    for_multi_wrap = false;
    current_time = GetCurrentTimestamp();
    foreach(cell, dblist)
    {
        avw_dbase  *tmp = lfirst(cell);
        dlist_iter  iter;

        /* Check to see if this one is at risk of wraparound */
        if (TransactionIdPrecedes(tmp->adw_frozenxid, xidForceLimit))
        {
            if (avdb == NULL ||
                TransactionIdPrecedes(tmp->adw_frozenxid,
                                      avdb->adw_frozenxid))
                avdb = tmp;
            for_xid_wrap = true;
            continue;
        }
        else if (for_xid_wrap)
            continue;           /* ignore not-at-risk DBs */
        else if (MultiXactIdPrecedes(tmp->adw_minmulti, multiForceLimit))
        {
            if (avdb == NULL ||
                MultiXactIdPrecedes(tmp->adw_minmulti, avdb->adw_minmulti))
                avdb = tmp;
            for_multi_wrap = true;
            continue;
        }
        else if (for_multi_wrap)
            continue;           /* ignore not-at-risk DBs */

        /* Find pgstat entry if any */
        tmp->adw_entry = pgstat_fetch_stat_dbentry(tmp->adw_datid);

        /*
         * Skip a database with no pgstat entry; it means it hasn't seen any
         * activity.
         */
        if (!tmp->adw_entry)
            continue;

        /*
         * Also, skip a database that appears on the database list as having
         * been processed recently (less than autovacuum_naptime seconds ago).
         * We do this so that we don't select a database which we just
         * selected, but that pgstat hasn't gotten around to updating the last
         * autovacuum time yet.
         */
        skipit = false;

        dlist_reverse_foreach(iter, &DatabaseList)
        {
            avl_dbase  *dbp = dlist_container(avl_dbase, adl_node, iter.cur);

            if (dbp->adl_datid == tmp->adw_datid)
            {
                /*
                 * Skip this database if its next_worker value falls between
                 * the current time and the current time plus naptime.
                 */
                if (!TimestampDifferenceExceeds(dbp->adl_next_worker,
                                                current_time, 0) &&
                    !TimestampDifferenceExceeds(current_time,
                                                dbp->adl_next_worker,
                                                autovacuum_naptime * 1000))
                    skipit = true;

                break;
            }
        }
        if (skipit)
            continue;

        /*
         * Remember the db with oldest autovac time.  (If we are here, both
         * tmp->entry and db->entry must be non-null.)
         */
        if (avdb == NULL ||
            tmp->adw_entry->last_autovac_time < avdb->adw_entry->last_autovac_time)
            avdb = tmp;
    }

    /* Found a database -- process it */
    if (avdb != NULL)
    {
        WorkerInfo  worker;
        dlist_node *wptr;

        LWLockAcquire(AutovacuumLock, LW_EXCLUSIVE);

        /*
         * Get a worker entry from the freelist.  We checked above, so there
         * really should be a free slot.
         */
        wptr = dlist_pop_head_node(&AutoVacuumShmem->av_freeWorkers);

        worker = dlist_container(WorkerInfoData, wi_links, wptr);
        worker->wi_dboid = avdb->adw_datid;
        worker->wi_proc = NULL;
        worker->wi_launchtime = GetCurrentTimestamp();

        AutoVacuumShmem->av_startingWorker = worker;

        LWLockRelease(AutovacuumLock);

        SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_WORKER);

        retval = avdb->adw_datid;
    }
    else if (skipit)
    {
        /*
         * If we skipped all databases on the list, rebuild it, because it
         * probably contains a dropped database.
         */
        rebuild_database_list(InvalidOid);
    }

    MemoryContextSwitchTo(oldcxt);
    MemoryContextDelete(tmpcxt);

    return retval;
}

/*
 * launch_worker
 *
 * Wrapper for starting a worker from the launcher.  Besides actually starting
 * it, update the database list to reflect the next time that another one will
 * need to be started on the selected database.  The actual database choice is
 * left to do_start_worker.
 *
 * This routine is also expected to insert an entry into the database list if
 * the selected database was previously absent from the list.
 */
static void
launch_worker(TimestampTz now)
{
    Oid         dbid;
    dlist_iter  iter;

    dbid = do_start_worker();
    if (OidIsValid(dbid))
    {
        bool        found = false;

        /*
         * Walk the database list and update the corresponding entry.  If the
         * database is not on the list, we'll recreate the list.
         */
        dlist_foreach(iter, &DatabaseList)
        {
            avl_dbase  *avdb = dlist_container(avl_dbase, adl_node, iter.cur);

            if (avdb->adl_datid == dbid)
            {
                found = true;

                /*
                 * add autovacuum_naptime seconds to the current time, and use
                 * that as the new "next_worker" field for this database.
                 */
                avdb->adl_next_worker =
                    TimestampTzPlusMilliseconds(now, autovacuum_naptime * 1000);

                dlist_move_head(&DatabaseList, iter.cur);
                break;
            }
        }

        /*
         * If the database was not present in the database list, we rebuild
         * the list.  It's possible that the database does not get into the
         * list anyway, for example if it's a database that doesn't have a
         * pgstat entry, but this is not a problem because we don't want to
         * schedule workers regularly into those in any case.
         */
        if (!found)
            rebuild_database_list(dbid);
    }
}

/*
 * Called from postmaster to signal a failure to fork a process to become
 * worker.  The postmaster should kill(SIGUSR2) the launcher shortly
 * after calling this function.
 */
void
AutoVacWorkerFailed(void)
{
    AutoVacuumShmem->av_signal[AutoVacForkFailed] = true;
}

/* SIGUSR2: a worker is up and running, or just finished, or failed to fork */
static void
avl_sigusr2_handler(SIGNAL_ARGS)
{
    int         save_errno = errno;

    got_SIGUSR2 = true;
    SetLatch(MyLatch);

    errno = save_errno;
}


/********************************************************************
 *                    AUTOVACUUM WORKER CODE
 ********************************************************************/

#ifdef EXEC_BACKEND
/*
 * forkexec routines for the autovacuum worker.
 *
 * Format up the arglist, then fork and exec.
 */
static pid_t
avworker_forkexec(void)
{
    char       *av[10];
    int         ac = 0;

    av[ac++] = "postgres";
    av[ac++] = "--forkavworker";
    av[ac++] = NULL;            /* filled in by postmaster_forkexec */
    av[ac] = NULL;

    Assert(ac < lengthof(av));

    return postmaster_forkexec(ac, av);
}

/*
 * We need this set from the outside, before InitProcess is called
 */
void
AutovacuumWorkerIAm(void)
{
    am_autovacuum_worker = true;
}
#endif

/*
 * Main entry point for autovacuum worker process.
 *
 * This code is heavily based on pgarch.c, q.v.
 */
int
StartAutoVacWorker(void)
{
    pid_t       worker_pid;

#ifdef EXEC_BACKEND
    switch ((worker_pid = avworker_forkexec()))
#else
    switch ((worker_pid = fork_process()))
#endif
    {
        case -1:
            ereport(LOG,
                    (errmsg("could not fork autovacuum worker process: %m")));
            return 0;

#ifndef EXEC_BACKEND
        case 0:
            /* in postmaster child ... */
            InitPostmasterChild();

            /* Close the postmaster's sockets */
            ClosePostmasterPorts(false);

            AutoVacWorkerMain(0, NULL);
            break;
#endif
        default:
            return (int) worker_pid;
    }

    /* shouldn't get here */
    return 0;
}

/*
 * AutoVacWorkerMain
 */
NON_EXEC_STATIC void
AutoVacWorkerMain(int argc, char *argv[])
{
    sigjmp_buf  local_sigjmp_buf;
    Oid         dbid;

    am_autovacuum_worker = true;

    MyBackendType = B_AUTOVAC_WORKER;
    init_ps_display(NULL);

    SetProcessingMode(InitProcessing);

    /*
     * Set up signal handlers.  We operate on databases much like a regular
     * backend, so we use the same signal handling.  See equivalent code in
     * tcop/postgres.c.
     */
    pqsignal(SIGHUP, SignalHandlerForConfigReload);

    /*
     * SIGINT is used to signal canceling the current table's vacuum; SIGTERM
     * means abort and exit cleanly, and SIGQUIT means abandon ship.
     */
    pqsignal(SIGINT, StatementCancelHandler);
    pqsignal(SIGTERM, die);
    /* SIGQUIT handler was already set up by InitPostmasterChild */

    InitializeTimeouts();       /* establishes SIGALRM handler */

    pqsignal(SIGPIPE, SIG_IGN);
    pqsignal(SIGUSR1, procsignal_sigusr1_handler);
    pqsignal(SIGUSR2, SIG_IGN);
    pqsignal(SIGFPE, FloatExceptionHandler);
    pqsignal(SIGCHLD, SIG_DFL);

    /* Early initialization */
    BaseInit();

    /*
     * Create a per-backend PGPROC struct in shared memory, except in the
     * EXEC_BACKEND case where this was done in SubPostmasterMain. We must do
     * this before we can use LWLocks (and in the EXEC_BACKEND case we already
     * had to do some stuff with LWLocks).
     */
#ifndef EXEC_BACKEND
    InitProcess();
#endif

    /*
     * If an exception is encountered, processing resumes here.
     *
     * Unlike most auxiliary processes, we don't attempt to continue
     * processing after an error; we just clean up and exit.  The autovac
     * launcher is responsible for spawning another worker later.
     *
     * Note that we use sigsetjmp(..., 1), so that the prevailing signal mask
     * (to wit, BlockSig) will be restored when longjmp'ing to here.  Thus,
     * signals other than SIGQUIT will be blocked until we exit.  It might
     * seem that this policy makes the HOLD_INTERRUPTS() call redundant, but
     * it is not since InterruptPending might be set already.
     */
    if (sigsetjmp(local_sigjmp_buf, 1) != 0)
    {
        /* since not using PG_TRY, must reset error stack by hand */
        error_context_stack = NULL;

        /* Prevents interrupts while cleaning up */
        HOLD_INTERRUPTS();

        /* Report the error to the server log */
        EmitErrorReport();

        /*
         * We can now go away.  Note that because we called InitProcess, a
         * callback was registered to do ProcKill, which will clean up
         * necessary state.
         */
        proc_exit(0);
    }

    /* We can now handle ereport(ERROR) */
    PG_exception_stack = &local_sigjmp_buf;

    PG_SETMASK(&UnBlockSig);

    /*
     * Set always-secure search path, so malicious users can't redirect user
     * code (e.g. pg_index.indexprs).  (That code runs in a
     * SECURITY_RESTRICTED_OPERATION sandbox, so malicious users could not
     * take control of the entire autovacuum worker in any case.)
     */
    SetConfigOption("search_path", "", PGC_SUSET, PGC_S_OVERRIDE);

    /*
     * Force zero_damaged_pages OFF in the autovac process, even if it is set
     * in postgresql.conf.  We don't really want such a dangerous option being
     * applied non-interactively.
     */
    SetConfigOption("zero_damaged_pages", "false", PGC_SUSET, PGC_S_OVERRIDE);

    /*
     * Force settable timeouts off to avoid letting these settings prevent
     * regular maintenance from being executed.
     */
    SetConfigOption("statement_timeout", "0", PGC_SUSET, PGC_S_OVERRIDE);
    SetConfigOption("lock_timeout", "0", PGC_SUSET, PGC_S_OVERRIDE);
    SetConfigOption("idle_in_transaction_session_timeout", "0",
                    PGC_SUSET, PGC_S_OVERRIDE);

    /*
     * Force default_transaction_isolation to READ COMMITTED.  We don't want
     * to pay the overhead of serializable mode, nor add any risk of causing
     * deadlocks or delaying other transactions.
     */
    SetConfigOption("default_transaction_isolation", "read committed",
                    PGC_SUSET, PGC_S_OVERRIDE);

    /*
     * Force synchronous replication off to allow regular maintenance even if
     * we are waiting for standbys to connect. This is important to ensure we
     * aren't blocked from performing anti-wraparound tasks.
     */
    if (synchronous_commit > SYNCHRONOUS_COMMIT_LOCAL_FLUSH)
        SetConfigOption("synchronous_commit", "local",
                        PGC_SUSET, PGC_S_OVERRIDE);

    /*
     * Get the info about the database we're going to work on.
     */
    LWLockAcquire(AutovacuumLock, LW_EXCLUSIVE);

    /*
     * beware of startingWorker being INVALID; this should normally not
     * happen, but if a worker fails after forking and before this, the
     * launcher might have decided to remove it from the queue and start
     * again.
     */
    if (AutoVacuumShmem->av_startingWorker != NULL)
    {
        MyWorkerInfo = AutoVacuumShmem->av_startingWorker;
        dbid = MyWorkerInfo->wi_dboid;
        MyWorkerInfo->wi_proc = MyProc;

        /* insert into the running list */
        dlist_push_head(&AutoVacuumShmem->av_runningWorkers,
                        &MyWorkerInfo->wi_links);

        /*
         * remove from the "starting" pointer, so that the launcher can start
         * a new worker if required
         */
        AutoVacuumShmem->av_startingWorker = NULL;
        LWLockRelease(AutovacuumLock);

        on_shmem_exit(FreeWorkerInfo, 0);

        /* wake up the launcher */
        if (AutoVacuumShmem->av_launcherpid != 0)
            kill(AutoVacuumShmem->av_launcherpid, SIGUSR2);
    }
    else
    {
        /* no worker entry for me, go away */
        elog(WARNING, "autovacuum worker started without a worker entry");
        dbid = InvalidOid;
        LWLockRelease(AutovacuumLock);
    }

    if (OidIsValid(dbid))
    {
        char        dbname[NAMEDATALEN];

        /*
         * Report autovac startup to the stats collector.  We deliberately do
         * this before InitPostgres, so that the last_autovac_time will get
         * updated even if the connection attempt fails.  This is to prevent
         * autovac from getting "stuck" repeatedly selecting an unopenable
         * database, rather than making any progress on stuff it can connect
         * to.
         */
        pgstat_report_autovac(dbid);

        /*
         * Connect to the selected database
         *
         * Note: if we have selected a just-deleted database (due to using
         * stale stats info), we'll fail and exit here.
         */
        InitPostgres(NULL, dbid, NULL, InvalidOid, dbname, false);
        SetProcessingMode(NormalProcessing);
        set_ps_display(dbname);
        ereport(DEBUG1,
                (errmsg_internal("autovacuum: processing database \"%s\"", dbname)));

        if (PostAuthDelay)
            pg_usleep(PostAuthDelay * 1000000L);

        /* And do an appropriate amount of work */
        recentXid = ReadNextTransactionId();
        recentMulti = ReadNextMultiXactId();
        do_autovacuum();
    }

    /*
     * The launcher will be notified of my death in ProcKill, *if* we managed
     * to get a worker slot at all
     */

    /* All done, go away */
    proc_exit(0);
}

/*
 * Return a WorkerInfo to the free list
 */
static void
FreeWorkerInfo(int code, Datum arg)
{
    if (MyWorkerInfo != NULL)
    {
        LWLockAcquire(AutovacuumLock, LW_EXCLUSIVE);

        /*
         * Wake the launcher up so that he can launch a new worker immediately
         * if required.  We only save the launcher's PID in local memory here;
         * the actual signal will be sent when the PGPROC is recycled.  Note
         * that we always do this, so that the launcher can rebalance the cost
         * limit setting of the remaining workers.
         *
         * We somewhat ignore the risk that the launcher changes its PID
         * between us reading it and the actual kill; we expect ProcKill to be
         * called shortly after us, and we assume that PIDs are not reused too
         * quickly after a process exits.
         */
        AutovacuumLauncherPid = AutoVacuumShmem->av_launcherpid;

        dlist_delete(&MyWorkerInfo->wi_links);
        MyWorkerInfo->wi_dboid = InvalidOid;
        MyWorkerInfo->wi_tableoid = InvalidOid;
        MyWorkerInfo->wi_sharedrel = false;
        MyWorkerInfo->wi_proc = NULL;
        MyWorkerInfo->wi_launchtime = 0;
        MyWorkerInfo->wi_dobalance = false;
        MyWorkerInfo->wi_cost_delay = 0;
        MyWorkerInfo->wi_cost_limit = 0;
        MyWorkerInfo->wi_cost_limit_base = 0;
        dlist_push_head(&AutoVacuumShmem->av_freeWorkers,
                        &MyWorkerInfo->wi_links);
        /* not mine anymore */
        MyWorkerInfo = NULL;

        /*
         * now that we're inactive, cause a rebalancing of the surviving
         * workers
         */
        AutoVacuumShmem->av_signal[AutoVacRebalance] = true;
        LWLockRelease(AutovacuumLock);
    }
}

/*
 * Update the cost-based delay parameters, so that multiple workers consume
 * each a fraction of the total available I/O.
 */
void
AutoVacuumUpdateDelay(void)
{
    if (MyWorkerInfo)
    {
        VacuumCostDelay = MyWorkerInfo->wi_cost_delay;
        VacuumCostLimit = MyWorkerInfo->wi_cost_limit;
    }
}

/*
 * autovac_balance_cost
 *      Recalculate the cost limit setting for each active worker.
 *
 * Caller must hold the AutovacuumLock in exclusive mode.
 */
static void
autovac_balance_cost(void)
{
    /*
     * The idea here is that we ration out I/O equally.  The amount of I/O
     * that a worker can consume is determined by cost_limit/cost_delay, so we
     * try to equalize those ratios rather than the raw limit settings.
     *
     * note: in cost_limit, zero also means use value from elsewhere, because
     * zero is not a valid value.
     */
    int         vac_cost_limit = (autovacuum_vac_cost_limit > 0 ?
                                  autovacuum_vac_cost_limit : VacuumCostLimit);
    double      vac_cost_delay = (autovacuum_vac_cost_delay >= 0 ?
                                  autovacuum_vac_cost_delay : VacuumCostDelay);
    double      cost_total;
    double      cost_avail;
    dlist_iter  iter;

    /* not set? nothing to do */
    if (vac_cost_limit <= 0 || vac_cost_delay <= 0)
        return;

    /* calculate the total base cost limit of participating active workers */
    cost_total = 0.0;
    dlist_foreach(iter, &AutoVacuumShmem->av_runningWorkers)
    {
        WorkerInfo  worker = dlist_container(WorkerInfoData, wi_links, iter.cur);

        if (worker->wi_proc != NULL &&
            worker->wi_dobalance &&
            worker->wi_cost_limit_base > 0 && worker->wi_cost_delay > 0)
            cost_total +=
                (double) worker->wi_cost_limit_base / worker->wi_cost_delay;
    }

    /* there are no cost limits -- nothing to do */
    if (cost_total <= 0)
        return;

    /*
     * Adjust cost limit of each active worker to balance the total of cost
     * limit to autovacuum_vacuum_cost_limit.
     */
    cost_avail = (double) vac_cost_limit / vac_cost_delay;
    dlist_foreach(iter, &AutoVacuumShmem->av_runningWorkers)
    {
        WorkerInfo  worker = dlist_container(WorkerInfoData, wi_links, iter.cur);

        if (worker->wi_proc != NULL &&
            worker->wi_dobalance &&
            worker->wi_cost_limit_base > 0 && worker->wi_cost_delay > 0)
        {
            int         limit = (int)
            (cost_avail * worker->wi_cost_limit_base / cost_total);

            /*
             * We put a lower bound of 1 on the cost_limit, to avoid division-
             * by-zero in the vacuum code.  Also, in case of roundoff trouble
             * in these calculations, let's be sure we don't ever set
             * cost_limit to more than the base value.
             */
            worker->wi_cost_limit = Max(Min(limit,
                                            worker->wi_cost_limit_base),
                                        1);
        }

        if (worker->wi_proc != NULL)
            elog(DEBUG2, "autovac_balance_cost(pid=%d db=%u, rel=%u, dobalance=%s cost_limit=%d, cost_limit_base=%d, cost_delay=%g)",
                 worker->wi_proc->pid, worker->wi_dboid, worker->wi_tableoid,
                 worker->wi_dobalance ? "yes" : "no",
                 worker->wi_cost_limit, worker->wi_cost_limit_base,
                 worker->wi_cost_delay);
    }
}

/*
 * get_database_list
 *      Return a list of all databases found in pg_database.
 *
 * The list and associated data is allocated in the caller's memory context,
 * which is in charge of ensuring that it's properly cleaned up afterwards.
 *
 * Note: this is the only function in which the autovacuum launcher uses a
 * transaction.  Although we aren't attached to any particular database and
 * therefore can't access most catalogs, we do have enough infrastructure
 * to do a seqscan on pg_database.
 */
static List *
get_database_list(void)
{
    List       *dblist = NIL;
    Relation    rel;
    TableScanDesc scan;
    HeapTuple   tup;
    MemoryContext resultcxt;

    /* This is the context that we will allocate our output data in */
    resultcxt = CurrentMemoryContext;

    /*
     * Start a transaction so we can access pg_database, and get a snapshot.
     * We don't have a use for the snapshot itself, but we're interested in
     * the secondary effect that it sets RecentGlobalXmin.  (This is critical
     * for anything that reads heap pages, because HOT may decide to prune
     * them even if the process doesn't attempt to modify any tuples.)
     *
     * FIXME: This comment is inaccurate / the code buggy. A snapshot that is
     * not pushed/active does not reliably prevent HOT pruning (->xmin could
     * e.g. be cleared when cache invalidations are processed).
     */
    StartTransactionCommand();
    (void) GetTransactionSnapshot();

    rel = table_open(DatabaseRelationId, AccessShareLock);
    scan = table_beginscan_catalog(rel, 0, NULL);

    while (HeapTupleIsValid(tup = heap_getnext(scan, ForwardScanDirection)))
    {
        Form_pg_database pgdatabase = (Form_pg_database) GETSTRUCT(tup);
        avw_dbase  *avdb;
        MemoryContext oldcxt;

        /*
         * Allocate our results in the caller's context, not the
         * transaction's. We do this inside the loop, and restore the original
         * context at the end, so that leaky things like heap_getnext() are
         * not called in a potentially long-lived context.
         */
        oldcxt = MemoryContextSwitchTo(resultcxt);

        avdb = (avw_dbase *) palloc(sizeof(avw_dbase));

        avdb->adw_datid = pgdatabase->oid;
        avdb->adw_name = pstrdup(NameStr(pgdatabase->datname));
        avdb->adw_frozenxid = pgdatabase->datfrozenxid;
        avdb->adw_minmulti = pgdatabase->datminmxid;
        /* this gets set later: */
        avdb->adw_entry = NULL;

        dblist = lappend(dblist, avdb);
        MemoryContextSwitchTo(oldcxt);
    }

    table_endscan(scan);
    table_close(rel, AccessShareLock);

    CommitTransactionCommand();

    return dblist;
}

/*
 * Process a database table-by-table
 *
 * Note that CHECK_FOR_INTERRUPTS is supposed to be used in certain spots in
 * order not to ignore shutdown commands for too long.
 */
static void
do_autovacuum(void)
{
    Relation    classRel;
    HeapTuple   tuple;
    TableScanDesc relScan;
    Form_pg_database dbForm;
    List       *table_oids = NIL;
    List       *orphan_oids = NIL;
    HASHCTL     ctl;
    HTAB       *table_toast_map;
    ListCell   *volatile cell;
    PgStat_StatDBEntry *shared;
    PgStat_StatDBEntry *dbentry;
    BufferAccessStrategy bstrategy;
    ScanKeyData key;
    TupleDesc   pg_class_desc;
    int         effective_multixact_freeze_max_age;
    bool        did_vacuum = false;
    bool        found_concurrent_worker = false;
    bool        updated = false;
    int         i;

    /*
     * StartTransactionCommand and CommitTransactionCommand will automatically
     * switch to other contexts.  We need this one to keep the list of
     * relations to vacuum/analyze across transactions.
     */
    AutovacMemCxt = AllocSetContextCreate(TopMemoryContext,
                                          "AV worker",
                                          ALLOCSET_DEFAULT_SIZES);
    MemoryContextSwitchTo(AutovacMemCxt);

    /*
     * may be NULL if we couldn't find an entry (only happens if we are
     * forcing a vacuum for anti-wrap purposes).
     */
    dbentry = pgstat_fetch_stat_dbentry(MyDatabaseId);

    /* Start a transaction so our commands have one to play into. */
    StartTransactionCommand();

    /*
     * Clean up any dead statistics collector entries for this DB. We always
     * want to do this exactly once per DB-processing cycle, even if we find
     * nothing worth vacuuming in the database.
     */
    pgstat_vacuum_stat();

    /*
     * Compute the multixact age for which freezing is urgent.  This is
     * normally autovacuum_multixact_freeze_max_age, but may be less if we are
     * short of multixact member space.
     */
    effective_multixact_freeze_max_age = MultiXactMemberFreezeThreshold();

    /*
     * Find the pg_database entry and select the default freeze ages. We use
     * zero in template and nonconnectable databases, else the system-wide
     * default.
     */
    tuple = SearchSysCache1(DATABASEOID, ObjectIdGetDatum(MyDatabaseId));
    if (!HeapTupleIsValid(tuple))
        elog(ERROR, "cache lookup failed for database %u", MyDatabaseId);
    dbForm = (Form_pg_database) GETSTRUCT(tuple);

    if (dbForm->datistemplate || !dbForm->datallowconn)
    {
        default_freeze_min_age = 0;
        default_freeze_table_age = 0;
        default_multixact_freeze_min_age = 0;
        default_multixact_freeze_table_age = 0;
    }
    else
    {
        default_freeze_min_age = vacuum_freeze_min_age;
        default_freeze_table_age = vacuum_freeze_table_age;
        default_multixact_freeze_min_age = vacuum_multixact_freeze_min_age;
        default_multixact_freeze_table_age = vacuum_multixact_freeze_table_age;
    }

    ReleaseSysCache(tuple);

    /* StartTransactionCommand changed elsewhere */
    MemoryContextSwitchTo(AutovacMemCxt);

    /* The database hash where pgstat keeps shared relations */
    shared = pgstat_fetch_stat_dbentry(InvalidOid);

    classRel = table_open(RelationRelationId, AccessShareLock);

    /* create a copy so we can use it after closing pg_class */
    pg_class_desc = CreateTupleDescCopy(RelationGetDescr(classRel));

    /* create hash table for toast <-> main relid mapping */
    ctl.keysize = sizeof(Oid);
    ctl.entrysize = sizeof(av_relation);

    table_toast_map = hash_create("TOAST to main relid map",
                                  100,
                                  &ctl,
                                  HASH_ELEM | HASH_BLOBS);

    /*
     * Scan pg_class to determine which tables to vacuum.
     *
     * We do this in three passes: First we let pgstat collector know about
     * the partitioned table ancestors of all partitions that have recently
     * acquired rows for analyze.  This informs the second pass about the
     * total number of tuple count in partitioning hierarchies.
     *
     * On the second pass, we collect the list of plain relations,
     * materialized views and partitioned tables.  On the third one we collect
     * TOAST tables.
     *
     * The reason for doing the third pass is that during it we want to use
     * the main relation's pg_class.reloptions entry if the TOAST table does
     * not have any, and we cannot obtain it unless we know beforehand what's
     * the main table OID.
     *
     * We need to check TOAST tables separately because in cases with short,
     * wide tables there might be proportionally much more activity in the
     * TOAST table than in its parent.
     */
    relScan = table_beginscan_catalog(classRel, 0, NULL);

    /*
     * First pass: before collecting the list of tables to vacuum, let stat
     * collector know about partitioned-table ancestors of each partition.
     */
    while ((tuple = heap_getnext(relScan, ForwardScanDirection)) != NULL)
    {
        Form_pg_class classForm = (Form_pg_class) GETSTRUCT(tuple);
        Oid         relid = classForm->oid;
        PgStat_StatTabEntry *tabentry;

        /* Only consider permanent leaf partitions */
        if (!classForm->relispartition ||
            classForm->relkind == RELKIND_PARTITIONED_TABLE ||
            classForm->relpersistence == RELPERSISTENCE_TEMP)
            continue;

        /*
         * No need to do this for partitions that haven't acquired any rows.
         */
        tabentry = pgstat_fetch_stat_tabentry(relid);
        if (tabentry &&
            tabentry->changes_since_analyze -
            tabentry->changes_since_analyze_reported > 0)
        {
            pgstat_report_anl_ancestors(relid);
            updated = true;
        }
    }

    /* Acquire fresh stats for the next passes, if needed */
    if (updated)
    {
        autovac_refresh_stats();
        dbentry = pgstat_fetch_stat_dbentry(MyDatabaseId);
        shared = pgstat_fetch_stat_dbentry(InvalidOid);
    }

    /*
     * On the second pass, we collect main tables to vacuum, and also the main
     * table relid to TOAST relid mapping.
     */
    while ((tuple = heap_getnext(relScan, ForwardScanDirection)) != NULL)
    {
        Form_pg_class classForm = (Form_pg_class) GETSTRUCT(tuple);
        PgStat_StatTabEntry *tabentry;
        AutoVacOpts *relopts;
        Oid         relid;
        bool        dovacuum;
        bool        doanalyze;
        bool        wraparound;

        if (classForm->relkind != RELKIND_RELATION &&
            classForm->relkind != RELKIND_MATVIEW &&
            classForm->relkind != RELKIND_PARTITIONED_TABLE)
            continue;

        relid = classForm->oid;

        /*
         * Check if it is a temp table (presumably, of some other backend's).
         * We cannot safely process other backends' temp tables.
         */
        if (classForm->relpersistence == RELPERSISTENCE_TEMP)
        {
            /*
             * We just ignore it if the owning backend is still active and
             * using the temporary schema.  Also, for safety, ignore it if the
             * namespace doesn't exist or isn't a temp namespace after all.
             */
            if (checkTempNamespaceStatus(classForm->relnamespace) == TEMP_NAMESPACE_IDLE)
            {
                /*
                 * The table seems to be orphaned -- although it might be that
                 * the owning backend has already deleted it and exited; our
                 * pg_class scan snapshot is not necessarily up-to-date
                 * anymore, so we could be looking at a committed-dead entry.
                 * Remember it so we can try to delete it later.
                 */
                orphan_oids = lappend_oid(orphan_oids, relid);
            }
            continue;
        }

        /* Fetch reloptions and the pgstat entry for this table */
        relopts = extract_autovac_opts(tuple, pg_class_desc);
        tabentry = get_pgstat_tabentry_relid(relid, classForm->relisshared,
                                             shared, dbentry);

        /* Check if it needs vacuum or analyze */
        relation_needs_vacanalyze(relid, relopts, classForm, tabentry,
                                  effective_multixact_freeze_max_age,
                                  &dovacuum, &doanalyze, &wraparound);

        /* Relations that need work are added to table_oids */
        if (dovacuum || doanalyze)
            table_oids = lappend_oid(table_oids, relid);

        /*
         * Remember TOAST associations for the second pass.  Note: we must do
         * this whether or not the table is going to be vacuumed, because we
         * don't automatically vacuum toast tables along the parent table.
         */
        if (OidIsValid(classForm->reltoastrelid))
        {
            av_relation *hentry;
            bool        found;

            hentry = hash_search(table_toast_map,
                                 &classForm->reltoastrelid,
                                 HASH_ENTER, &found);

            if (!found)
            {
                /* hash_search already filled in the key */
                hentry->ar_relid = relid;
                hentry->ar_hasrelopts = false;
                if (relopts != NULL)
                {
                    hentry->ar_hasrelopts = true;
                    memcpy(&hentry->ar_reloptions, relopts,
                           sizeof(AutoVacOpts));
                }
            }
        }
    }

    table_endscan(relScan);

    /* third pass: check TOAST tables */
    ScanKeyInit(&key,
                Anum_pg_class_relkind,
                BTEqualStrategyNumber, F_CHAREQ,
                CharGetDatum(RELKIND_TOASTVALUE));

    relScan = table_beginscan_catalog(classRel, 1, &key);
    while ((tuple = heap_getnext(relScan, ForwardScanDirection)) != NULL)
    {
        Form_pg_class classForm = (Form_pg_class) GETSTRUCT(tuple);
        PgStat_StatTabEntry *tabentry;
        Oid         relid;
        AutoVacOpts *relopts = NULL;
        bool        dovacuum;
        bool        doanalyze;
        bool        wraparound;

        /*
         * We cannot safely process other backends' temp tables, so skip 'em.
         */
        if (classForm->relpersistence == RELPERSISTENCE_TEMP)
            continue;

        relid = classForm->oid;

        /*
         * fetch reloptions -- if this toast table does not have them, try the
         * main rel
         */
        relopts = extract_autovac_opts(tuple, pg_class_desc);
        if (relopts == NULL)
        {
            av_relation *hentry;
            bool        found;

            hentry = hash_search(table_toast_map, &relid, HASH_FIND, &found);
            if (found && hentry->ar_hasrelopts)
                relopts = &hentry->ar_reloptions;
        }

        /* Fetch the pgstat entry for this table */
        tabentry = get_pgstat_tabentry_relid(relid, classForm->relisshared,
                                             shared, dbentry);

        relation_needs_vacanalyze(relid, relopts, classForm, tabentry,
                                  effective_multixact_freeze_max_age,
                                  &dovacuum, &doanalyze, &wraparound);

        /* ignore analyze for toast tables */
        if (dovacuum)
            table_oids = lappend_oid(table_oids, relid);
    }

    table_endscan(relScan);
    table_close(classRel, AccessShareLock);

    /*
     * Recheck orphan temporary tables, and if they still seem orphaned, drop
     * them.  We'll eat a transaction per dropped table, which might seem
     * excessive, but we should only need to do anything as a result of a
     * previous backend crash, so this should not happen often enough to
     * justify "optimizing".  Using separate transactions ensures that we
     * don't bloat the lock table if there are many temp tables to be dropped,
     * and it ensures that we don't lose work if a deletion attempt fails.
     */
    foreach(cell, orphan_oids)
    {
        Oid         relid = lfirst_oid(cell);
        Form_pg_class classForm;
        ObjectAddress object;

        /*
         * Check for user-requested abort.
         */
        CHECK_FOR_INTERRUPTS();

        /*
         * Try to lock the table.  If we can't get the lock immediately,
         * somebody else is using (or dropping) the table, so it's not our
         * concern anymore.  Having the lock prevents race conditions below.
         */
        if (!ConditionalLockRelationOid(relid, AccessExclusiveLock))
            continue;

        /*
         * Re-fetch the pg_class tuple and re-check whether it still seems to
         * be an orphaned temp table.  If it's not there or no longer the same
         * relation, ignore it.
         */
        tuple = SearchSysCacheCopy1(RELOID, ObjectIdGetDatum(relid));
        if (!HeapTupleIsValid(tuple))
        {
            /* be sure to drop useless lock so we don't bloat lock table */
            UnlockRelationOid(relid, AccessExclusiveLock);
            continue;
        }
        classForm = (Form_pg_class) GETSTRUCT(tuple);

        /*
         * Make all the same tests made in the loop above.  In event of OID
         * counter wraparound, the pg_class entry we have now might be
         * completely unrelated to the one we saw before.
         */
        if (!((classForm->relkind == RELKIND_RELATION ||
               classForm->relkind == RELKIND_MATVIEW) &&
              classForm->relpersistence == RELPERSISTENCE_TEMP))
        {
            UnlockRelationOid(relid, AccessExclusiveLock);
            continue;
        }

        if (checkTempNamespaceStatus(classForm->relnamespace) != TEMP_NAMESPACE_IDLE)
        {
            UnlockRelationOid(relid, AccessExclusiveLock);
            continue;
        }

        /* OK, let's delete it */
        ereport(LOG,
                (errmsg("autovacuum: dropping orphan temp table \"%s.%s.%s\"",
                        get_database_name(MyDatabaseId),
                        get_namespace_name(classForm->relnamespace),
                        NameStr(classForm->relname))));

        object.classId = RelationRelationId;
        object.objectId = relid;
        object.objectSubId = 0;
        performDeletion(&object, DROP_CASCADE,
                        PERFORM_DELETION_INTERNAL |
                        PERFORM_DELETION_QUIETLY |
                        PERFORM_DELETION_SKIP_EXTENSIONS);

        /*
         * To commit the deletion, end current transaction and start a new
         * one.  Note this also releases the lock we took.
         */
        CommitTransactionCommand();
        StartTransactionCommand();

        /* StartTransactionCommand changed current memory context */
        MemoryContextSwitchTo(AutovacMemCxt);
    }

    /*
     * Create a buffer access strategy object for VACUUM to use.  We want to
     * use the same one across all the vacuum operations we perform, since the
     * point is for VACUUM not to blow out the shared cache.
     */
    bstrategy = GetAccessStrategy(BAS_VACUUM);

    /*
     * create a memory context to act as fake PortalContext, so that the
     * contexts created in the vacuum code are cleaned up for each table.
     */
    PortalContext = AllocSetContextCreate(AutovacMemCxt,
                                          "Autovacuum Portal",
                                          ALLOCSET_DEFAULT_SIZES);

    /*
     * Perform operations on collected tables.
     */
    foreach(cell, table_oids)
    {
        Oid         relid = lfirst_oid(cell);
        HeapTuple   classTup;
        autovac_table *tab;
        bool        isshared;
        bool        skipit;
        double      stdVacuumCostDelay;
        int         stdVacuumCostLimit;
        dlist_iter  iter;

        CHECK_FOR_INTERRUPTS();

        /*
         * Check for config changes before processing each collected table.
         */
        if (ConfigReloadPending)
        {
            ConfigReloadPending = false;
            ProcessConfigFile(PGC_SIGHUP);

            /*
             * You might be tempted to bail out if we see autovacuum is now
             * disabled.  Must resist that temptation -- this might be a
             * for-wraparound emergency worker, in which case that would be
             * entirely inappropriate.
             */
        }

        /*
         * Find out whether the table is shared or not.  (It's slightly
         * annoying to fetch the syscache entry just for this, but in typical
         * cases it adds little cost because table_recheck_autovac would
         * refetch the entry anyway.  We could buy that back by copying the
         * tuple here and passing it to table_recheck_autovac, but that
         * increases the odds of that function working with stale data.)
         */
        classTup = SearchSysCache1(RELOID, ObjectIdGetDatum(relid));
        if (!HeapTupleIsValid(classTup))
            continue;           /* somebody deleted the rel, forget it */
        isshared = ((Form_pg_class) GETSTRUCT(classTup))->relisshared;
        ReleaseSysCache(classTup);

        /*
         * Hold schedule lock from here until we've claimed the table.  We
         * also need the AutovacuumLock to walk the worker array, but that one
         * can just be a shared lock.
         */
        LWLockAcquire(AutovacuumScheduleLock, LW_EXCLUSIVE);
        LWLockAcquire(AutovacuumLock, LW_SHARED);

        /*
         * Check whether the table is being vacuumed concurrently by another
         * worker.
         */
        skipit = false;
        dlist_foreach(iter, &AutoVacuumShmem->av_runningWorkers)
        {
            WorkerInfo  worker = dlist_container(WorkerInfoData, wi_links, iter.cur);

            /* ignore myself */
            if (worker == MyWorkerInfo)
                continue;

            /* ignore workers in other databases (unless table is shared) */
            if (!worker->wi_sharedrel && worker->wi_dboid != MyDatabaseId)
                continue;

            if (worker->wi_tableoid == relid)
            {
                skipit = true;
                found_concurrent_worker = true;
                break;
            }
        }
        LWLockRelease(AutovacuumLock);
        if (skipit)
        {
            LWLockRelease(AutovacuumScheduleLock);
            continue;
        }

        /*
         * Store the table's OID in shared memory before releasing the
         * schedule lock, so that other workers don't try to vacuum it
         * concurrently.  (We claim it here so as not to hold
         * AutovacuumScheduleLock while rechecking the stats.)
         */
        MyWorkerInfo->wi_tableoid = relid;
        MyWorkerInfo->wi_sharedrel = isshared;
        LWLockRelease(AutovacuumScheduleLock);

        /*
         * Check whether pgstat data still says we need to vacuum this table.
         * It could have changed if something else processed the table while
         * we weren't looking.
         *
         * Note: we have a special case in pgstat code to ensure that the
         * stats we read are as up-to-date as possible, to avoid the problem
         * that somebody just finished vacuuming this table.  The window to
         * the race condition is not closed but it is very small.
         */
        MemoryContextSwitchTo(AutovacMemCxt);
        tab = table_recheck_autovac(relid, table_toast_map, pg_class_desc,
                                    effective_multixact_freeze_max_age);
        if (tab == NULL)
        {
            /* someone else vacuumed the table, or it went away */
            LWLockAcquire(AutovacuumScheduleLock, LW_EXCLUSIVE);
            MyWorkerInfo->wi_tableoid = InvalidOid;
            MyWorkerInfo->wi_sharedrel = false;
            LWLockRelease(AutovacuumScheduleLock);
            continue;
        }

        /*
         * Remember the prevailing values of the vacuum cost GUCs.  We have to
         * restore these at the bottom of the loop, else we'll compute wrong
         * values in the next iteration of autovac_balance_cost().
         */
        stdVacuumCostDelay = VacuumCostDelay;
        stdVacuumCostLimit = VacuumCostLimit;

        /* Must hold AutovacuumLock while mucking with cost balance info */
        LWLockAcquire(AutovacuumLock, LW_EXCLUSIVE);

        /* advertise my cost delay parameters for the balancing algorithm */
        MyWorkerInfo->wi_dobalance = tab->at_dobalance;
        MyWorkerInfo->wi_cost_delay = tab->at_vacuum_cost_delay;
        MyWorkerInfo->wi_cost_limit = tab->at_vacuum_cost_limit;
        MyWorkerInfo->wi_cost_limit_base = tab->at_vacuum_cost_limit;

        /* do a balance */
        autovac_balance_cost();

        /* set the active cost parameters from the result of that */
        AutoVacuumUpdateDelay();

        /* done */
        LWLockRelease(AutovacuumLock);

        /* clean up memory before each iteration */
        MemoryContextResetAndDeleteChildren(PortalContext);

        /*
         * Save the relation name for a possible error message, to avoid a
         * catalog lookup in case of an error.  If any of these return NULL,
         * then the relation has been dropped since last we checked; skip it.
         * Note: they must live in a long-lived memory context because we call
         * vacuum and analyze in different transactions.
         */

        tab->at_relname = get_rel_name(tab->at_relid);
        tab->at_nspname = get_namespace_name(get_rel_namespace(tab->at_relid));
        tab->at_datname = get_database_name(MyDatabaseId);
        if (!tab->at_relname || !tab->at_nspname || !tab->at_datname)
            goto deleted;

        /*
         * We will abort vacuuming the current table if something errors out,
         * and continue with the next one in schedule; in particular, this
         * happens if we are interrupted with SIGINT.
         */
        PG_TRY();
        {
            /* Use PortalContext for any per-table allocations */
            MemoryContextSwitchTo(PortalContext);

            /* have at it */
            autovacuum_do_vac_analyze(tab, bstrategy);

            /*
             * Clear a possible query-cancel signal, to avoid a late reaction
             * to an automatically-sent signal because of vacuuming the
             * current table (we're done with it, so it would make no sense to
             * cancel at this point.)
             */
            QueryCancelPending = false;
        }
        PG_CATCH();
        {
            /*
             * Abort the transaction, start a new one, and proceed with the
             * next table in our list.
             */
            HOLD_INTERRUPTS();
            if (tab->at_params.options & VACOPT_VACUUM)
                errcontext("automatic vacuum of table \"%s.%s.%s\"",
                           tab->at_datname, tab->at_nspname, tab->at_relname);
            else
                errcontext("automatic analyze of table \"%s.%s.%s\"",
                           tab->at_datname, tab->at_nspname, tab->at_relname);
            EmitErrorReport();

            /* this resets ProcGlobal->statusFlags[i] too */
            AbortOutOfAnyTransaction();
            FlushErrorState();
            MemoryContextResetAndDeleteChildren(PortalContext);

            /* restart our transaction for the following operations */
            StartTransactionCommand();
            RESUME_INTERRUPTS();
        }
        PG_END_TRY();

        /* Make sure we're back in AutovacMemCxt */
        MemoryContextSwitchTo(AutovacMemCxt);

        did_vacuum = true;

        /* ProcGlobal->statusFlags[i] are reset at the next end of xact */

        /* be tidy */
deleted:
        if (tab->at_datname != NULL)
            pfree(tab->at_datname);
        if (tab->at_nspname != NULL)
            pfree(tab->at_nspname);
        if (tab->at_relname != NULL)
            pfree(tab->at_relname);
        pfree(tab);

        /*
         * Remove my info from shared memory.  We could, but intentionally
         * don't, clear wi_cost_limit and friends --- this is on the
         * assumption that we probably have more to do with similar cost
         * settings, so we don't want to give up our share of I/O for a very
         * short interval and thereby thrash the global balance.
         */
        LWLockAcquire(AutovacuumScheduleLock, LW_EXCLUSIVE);
        MyWorkerInfo->wi_tableoid = InvalidOid;
        MyWorkerInfo->wi_sharedrel = false;
        LWLockRelease(AutovacuumScheduleLock);

        /* restore vacuum cost GUCs for the next iteration */
        VacuumCostDelay = stdVacuumCostDelay;
        VacuumCostLimit = stdVacuumCostLimit;
    }

    /*
     * Perform additional work items, as requested by backends.
     */
    LWLockAcquire(AutovacuumLock, LW_EXCLUSIVE);
    for (i = 0; i < NUM_WORKITEMS; i++)
    {
        AutoVacuumWorkItem *workitem = &AutoVacuumShmem->av_workItems[i];

        if (!workitem->avw_used)
            continue;
        if (workitem->avw_active)
            continue;
        if (workitem->avw_database != MyDatabaseId)
            continue;

        /* claim this one, and release lock while performing it */
        workitem->avw_active = true;
        LWLockRelease(AutovacuumLock);

        perform_work_item(workitem);

        /*
         * Check for config changes before acquiring lock for further jobs.
         */
        CHECK_FOR_INTERRUPTS();
        if (ConfigReloadPending)
        {
            ConfigReloadPending = false;
            ProcessConfigFile(PGC_SIGHUP);
        }

        LWLockAcquire(AutovacuumLock, LW_EXCLUSIVE);

        /* and mark it done */
        workitem->avw_active = false;
        workitem->avw_used = false;
    }
    LWLockRelease(AutovacuumLock);

    /*
     * We leak table_toast_map here (among other things), but since we're
     * going away soon, it's not a problem.
     */

    /*
     * Update pg_database.datfrozenxid, and truncate pg_xact if possible. We
     * only need to do this once, not after each table.
     *
     * Even if we didn't vacuum anything, it may still be important to do
     * this, because one indirect effect of vac_update_datfrozenxid() is to
     * update ShmemVariableCache->xidVacLimit.  That might need to be done
     * even if we haven't vacuumed anything, because relations with older
     * relfrozenxid values or other databases with older datfrozenxid values
     * might have been dropped, allowing xidVacLimit to advance.
     *
     * However, it's also important not to do this blindly in all cases,
     * because when autovacuum=off this will restart the autovacuum launcher.
     * If we're not careful, an infinite loop can result, where workers find
     * no work to do and restart the launcher, which starts another worker in
     * the same database that finds no work to do.  To prevent that, we skip
     * this if (1) we found no work to do and (2) we skipped at least one
     * table due to concurrent autovacuum activity.  In that case, the other
     * worker has already done it, or will do so when it finishes.
     */
    if (did_vacuum || !found_concurrent_worker)
        vac_update_datfrozenxid();

    /* Finally close out the last transaction. */
    CommitTransactionCommand();
}

/*
 * Execute a previously registered work item.
 */
static void
perform_work_item(AutoVacuumWorkItem *workitem)
{
    char       *cur_datname = NULL;
    char       *cur_nspname = NULL;
    char       *cur_relname = NULL;

    /*
     * Note we do not store table info in MyWorkerInfo, since this is not
     * vacuuming proper.
     */

    /*
     * Save the relation name for a possible error message, to avoid a catalog
     * lookup in case of an error.  If any of these return NULL, then the
     * relation has been dropped since last we checked; skip it.
     */
    Assert(CurrentMemoryContext == AutovacMemCxt);

    cur_relname = get_rel_name(workitem->avw_relation);
    cur_nspname = get_namespace_name(get_rel_namespace(workitem->avw_relation));
    cur_datname = get_database_name(MyDatabaseId);
    if (!cur_relname || !cur_nspname || !cur_datname)
        goto deleted2;

    autovac_report_workitem(workitem, cur_nspname, cur_relname);

    /* clean up memory before each work item */
    MemoryContextResetAndDeleteChildren(PortalContext);

    /*
     * We will abort the current work item if something errors out, and
     * continue with the next one; in particular, this happens if we are
     * interrupted with SIGINT.  Note that this means that the work item list
     * can be lossy.
     */
    PG_TRY();
    {
        /* Use PortalContext for any per-work-item allocations */
        MemoryContextSwitchTo(PortalContext);

        /* have at it */
        switch (workitem->avw_type)
        {
            case AVW_BRINSummarizeRange:
                DirectFunctionCall2(brin_summarize_range,
                                    ObjectIdGetDatum(workitem->avw_relation),
                                    Int64GetDatum((int64) workitem->avw_blockNumber));
                break;
            default:
                elog(WARNING, "unrecognized work item found: type %d",
                     workitem->avw_type);
                break;
        }

        /*
         * Clear a possible query-cancel signal, to avoid a late reaction to
         * an automatically-sent signal because of vacuuming the current table
         * (we're done with it, so it would make no sense to cancel at this
         * point.)
         */
        QueryCancelPending = false;
    }
    PG_CATCH();
    {
        /*
         * Abort the transaction, start a new one, and proceed with the next
         * table in our list.
         */
        HOLD_INTERRUPTS();
        errcontext("processing work entry for relation \"%s.%s.%s\"",
                   cur_datname, cur_nspname, cur_relname);
        EmitErrorReport();

        /* this resets ProcGlobal->statusFlags[i] too */
        AbortOutOfAnyTransaction();
        FlushErrorState();
        MemoryContextResetAndDeleteChildren(PortalContext);

        /* restart our transaction for the following operations */
        StartTransactionCommand();
        RESUME_INTERRUPTS();
    }
    PG_END_TRY();

    /* Make sure we're back in AutovacMemCxt */
    MemoryContextSwitchTo(AutovacMemCxt);

    /* We intentionally do not set did_vacuum here */

    /* be tidy */
deleted2:
    if (cur_datname)
        pfree(cur_datname);
    if (cur_nspname)
        pfree(cur_nspname);
    if (cur_relname)
        pfree(cur_relname);
}

/*
 * extract_autovac_opts
 *
 * Given a relation's pg_class tuple, return the AutoVacOpts portion of
 * reloptions, if set; otherwise, return NULL.
 *
 * Note: callers do not have a relation lock on the table at this point,
 * so the table could have been dropped, and its catalog rows gone, after
 * we acquired the pg_class row.  If pg_class had a TOAST table, this would
 * be a risk; fortunately, it doesn't.
 */
static AutoVacOpts *
extract_autovac_opts(HeapTuple tup, TupleDesc pg_class_desc)
{
    bytea      *relopts;
    AutoVacOpts *av;

    Assert(((Form_pg_class) GETSTRUCT(tup))->relkind == RELKIND_RELATION ||
           ((Form_pg_class) GETSTRUCT(tup))->relkind == RELKIND_MATVIEW ||
           ((Form_pg_class) GETSTRUCT(tup))->relkind == RELKIND_PARTITIONED_TABLE ||
           ((Form_pg_class) GETSTRUCT(tup))->relkind == RELKIND_TOASTVALUE);

    relopts = extractRelOptions(tup, pg_class_desc, NULL);
    if (relopts == NULL)
        return NULL;

    av = palloc(sizeof(AutoVacOpts));
    memcpy(av, &(((StdRdOptions *) relopts)->autovacuum), sizeof(AutoVacOpts));
    pfree(relopts);

    return av;
}

/*
 * get_pgstat_tabentry_relid
 *
 * Fetch the pgstat entry of a table, either local to a database or shared.
 */
static PgStat_StatTabEntry *
get_pgstat_tabentry_relid(Oid relid, bool isshared, PgStat_StatDBEntry *shared,
                          PgStat_StatDBEntry *dbentry)
{
    PgStat_StatTabEntry *tabentry = NULL;

    if (isshared)
    {
        if (PointerIsValid(shared))
            tabentry = hash_search(shared->tables, &relid,
                                   HASH_FIND, NULL);
    }
    else if (PointerIsValid(dbentry))
        tabentry = hash_search(dbentry->tables, &relid,
                               HASH_FIND, NULL);

    return tabentry;
}

/*
 * table_recheck_autovac
 *
 * Recheck whether a table still needs vacuum or analyze.  Return value is a
 * valid autovac_table pointer if it does, NULL otherwise.
 *
 * Note that the returned autovac_table does not have the name fields set.
 */
static autovac_table *
table_recheck_autovac(Oid relid, HTAB *table_toast_map,
                      TupleDesc pg_class_desc,
                      int effective_multixact_freeze_max_age)
{
    Form_pg_class classForm;
    HeapTuple   classTup;
    bool        dovacuum;
    bool        doanalyze;
    autovac_table *tab = NULL;
    bool        wraparound;
    AutoVacOpts *avopts;
    static bool reuse_stats = false;

    /* fetch the relation's relcache entry */
    classTup = SearchSysCacheCopy1(RELOID, ObjectIdGetDatum(relid));
    if (!HeapTupleIsValid(classTup))
        return NULL;
    classForm = (Form_pg_class) GETSTRUCT(classTup);

    /*
     * Get the applicable reloptions.  If it is a TOAST table, try to get the
     * main table reloptions if the toast table itself doesn't have.
     */
    avopts = extract_autovac_opts(classTup, pg_class_desc);
    if (classForm->relkind == RELKIND_TOASTVALUE &&
        avopts == NULL && table_toast_map != NULL)
    {
        av_relation *hentry;
        bool        found;

        hentry = hash_search(table_toast_map, &relid, HASH_FIND, &found);
        if (found && hentry->ar_hasrelopts)
            avopts = &hentry->ar_reloptions;
    }

    /*
     * Reuse the stats to recheck whether a relation needs to be vacuumed or
     * analyzed if it was reloaded before and has not been cleared yet. This
     * is necessary to avoid frequent refresh of stats, especially when there
     * are very large number of relations and the refresh can cause lots of
     * overhead.
     *
     * If we determined that a relation needs to be vacuumed or analyzed,
     * based on the old stats, we refresh stats and recheck the necessity
     * again. Because a relation may have already been vacuumed or analyzed by
     * someone since the last reload of stats.
     */
    if (reuse_stats)
    {
        recheck_relation_needs_vacanalyze(relid, avopts, classForm,
                                          effective_multixact_freeze_max_age,
                                          &dovacuum, &doanalyze, &wraparound);

        /* Quick exit if a relation doesn't need to be vacuumed or analyzed */
        if (!doanalyze && !dovacuum)
        {
            heap_freetuple(classTup);
            return NULL;
        }
    }

    /* Use fresh stats and recheck again */
    autovac_refresh_stats();

    recheck_relation_needs_vacanalyze(relid, avopts, classForm,
                                      effective_multixact_freeze_max_age,
                                      &dovacuum, &doanalyze, &wraparound);

    /* OK, it needs something done */
    if (doanalyze || dovacuum)
    {
        int         freeze_min_age;
        int         freeze_table_age;
        int         multixact_freeze_min_age;
        int         multixact_freeze_table_age;
        int         vac_cost_limit;
        double      vac_cost_delay;
        int         log_min_duration;

        /*
         * Calculate the vacuum cost parameters and the freeze ages.  If there
         * are options set in pg_class.reloptions, use them; in the case of a
         * toast table, try the main table too.  Otherwise use the GUC
         * defaults, autovacuum's own first and plain vacuum second.
         */

        /* -1 in autovac setting means use plain vacuum_cost_delay */
        vac_cost_delay = (avopts && avopts->vacuum_cost_delay >= 0)
            ? avopts->vacuum_cost_delay
            : (autovacuum_vac_cost_delay >= 0)
            ? autovacuum_vac_cost_delay
            : VacuumCostDelay;

        /* 0 or -1 in autovac setting means use plain vacuum_cost_limit */
        vac_cost_limit = (avopts && avopts->vacuum_cost_limit > 0)
            ? avopts->vacuum_cost_limit
            : (autovacuum_vac_cost_limit > 0)
            ? autovacuum_vac_cost_limit
            : VacuumCostLimit;

        /* -1 in autovac setting means use log_autovacuum_min_duration */
        log_min_duration = (avopts && avopts->log_min_duration >= 0)
            ? avopts->log_min_duration
            : Log_autovacuum_min_duration;

        /* these do not have autovacuum-specific settings */
        freeze_min_age = (avopts && avopts->freeze_min_age >= 0)
            ? avopts->freeze_min_age
            : default_freeze_min_age;

        freeze_table_age = (avopts && avopts->freeze_table_age >= 0)
            ? avopts->freeze_table_age
            : default_freeze_table_age;

        multixact_freeze_min_age = (avopts &&
                                    avopts->multixact_freeze_min_age >= 0)
            ? avopts->multixact_freeze_min_age
            : default_multixact_freeze_min_age;

        multixact_freeze_table_age = (avopts &&
                                      avopts->multixact_freeze_table_age >= 0)
            ? avopts->multixact_freeze_table_age
            : default_multixact_freeze_table_age;

        tab = palloc(sizeof(autovac_table));
        tab->at_relid = relid;
        tab->at_sharedrel = classForm->relisshared;

        /* Note that this skips toast relations */
        tab->at_params.options = (dovacuum ? VACOPT_VACUUM : 0) |
            (doanalyze ? VACOPT_ANALYZE : 0) |
            (!wraparound ? VACOPT_SKIP_LOCKED : 0);

        /*
         * index_cleanup and truncate are unspecified at first in autovacuum.
         * They will be filled in with usable values using their reloptions
         * (or reloption defaults) later.
         */
        tab->at_params.index_cleanup = VACOPTVALUE_UNSPECIFIED;
        tab->at_params.truncate = VACOPTVALUE_UNSPECIFIED;
        /* As of now, we don't support parallel vacuum for autovacuum */
        tab->at_params.nworkers = -1;
        tab->at_params.freeze_min_age = freeze_min_age;
        tab->at_params.freeze_table_age = freeze_table_age;
        tab->at_params.multixact_freeze_min_age = multixact_freeze_min_age;
        tab->at_params.multixact_freeze_table_age = multixact_freeze_table_age;
        tab->at_params.is_wraparound = wraparound;
        tab->at_params.log_min_duration = log_min_duration;
        tab->at_vacuum_cost_limit = vac_cost_limit;
        tab->at_vacuum_cost_delay = vac_cost_delay;
        tab->at_relname = NULL;
        tab->at_nspname = NULL;
        tab->at_datname = NULL;

        /*
         * If any of the cost delay parameters has been set individually for
         * this table, disable the balancing algorithm.
         */
        tab->at_dobalance =
            !(avopts && (avopts->vacuum_cost_limit > 0 ||
                         avopts->vacuum_cost_delay > 0));

        /*
         * When we decide to do vacuum or analyze, the existing stats cannot
         * be reused in the next cycle because it's cleared at the end of
         * vacuum or analyze (by AtEOXact_PgStat()).
         */
        reuse_stats = false;
    }
    else
    {
        /*
         * If neither vacuum nor analyze is necessary, the existing stats is
         * not cleared and can be reused in the next cycle.
         */
        reuse_stats = true;
    }

    heap_freetuple(classTup);
    return tab;
}

/*
 * recheck_relation_needs_vacanalyze
 *
 * Subroutine for table_recheck_autovac.
 *
 * Fetch the pgstat of a relation and recheck whether a relation
 * needs to be vacuumed or analyzed.
 */
static void
recheck_relation_needs_vacanalyze(Oid relid,
                                  AutoVacOpts *avopts,
                                  Form_pg_class classForm,
                                  int effective_multixact_freeze_max_age,
                                  bool *dovacuum,
                                  bool *doanalyze,
                                  bool *wraparound)
{
    PgStat_StatTabEntry *tabentry;
    PgStat_StatDBEntry *shared = NULL;
    PgStat_StatDBEntry *dbentry = NULL;

    if (classForm->relisshared)
        shared = pgstat_fetch_stat_dbentry(InvalidOid);
    else
        dbentry = pgstat_fetch_stat_dbentry(MyDatabaseId);

    /* fetch the pgstat table entry */
    tabentry = get_pgstat_tabentry_relid(relid, classForm->relisshared,
                                         shared, dbentry);

    relation_needs_vacanalyze(relid, avopts, classForm, tabentry,
                              effective_multixact_freeze_max_age,
                              dovacuum, doanalyze, wraparound);

    /* ignore ANALYZE for toast tables */
    if (classForm->relkind == RELKIND_TOASTVALUE)
        *doanalyze = false;
}

/*
 * relation_needs_vacanalyze
 *
 * Check whether a relation needs to be vacuumed or analyzed; return each into
 * "dovacuum" and "doanalyze", respectively.  Also return whether the vacuum is
 * being forced because of Xid or multixact wraparound.
 *
 * relopts is a pointer to the AutoVacOpts options (either for itself in the
 * case of a plain table, or for either itself or its parent table in the case
 * of a TOAST table), NULL if none; tabentry is the pgstats entry, which can be
 * NULL.
 *
 * A table needs to be vacuumed if the number of dead tuples exceeds a
 * threshold.  This threshold is calculated as
 *
 * threshold = vac_base_thresh + vac_scale_factor * reltuples
 *
 * For analyze, the analysis done is that the number of tuples inserted,
 * deleted and updated since the last analyze exceeds a threshold calculated
 * in the same fashion as above.  Note that the collector actually stores
 * the number of tuples (both live and dead) that there were as of the last
 * analyze.  This is asymmetric to the VACUUM case.
 *
 * We also force vacuum if the table's relfrozenxid is more than freeze_max_age
 * transactions back, and if its relminmxid is more than
 * multixact_freeze_max_age multixacts back.
 *
 * A table whose autovacuum_enabled option is false is
 * automatically skipped (unless we have to vacuum it due to freeze_max_age).
 * Thus autovacuum can be disabled for specific tables. Also, when the stats
 * collector does not have data about a table, it will be skipped.
 *
 * A table whose vac_base_thresh value is < 0 takes the base value from the
 * autovacuum_vacuum_threshold GUC variable.  Similarly, a vac_scale_factor
 * value < 0 is substituted with the value of
 * autovacuum_vacuum_scale_factor GUC variable.  Ditto for analyze.
 */
static void
relation_needs_vacanalyze(Oid relid,
                          AutoVacOpts *relopts,
                          Form_pg_class classForm,
                          PgStat_StatTabEntry *tabentry,
                          int effective_multixact_freeze_max_age,
 /* output params below */
                          bool *dovacuum,
                          bool *doanalyze,
                          bool *wraparound)
{
    bool        force_vacuum;
    bool        av_enabled;
    float4      reltuples;      /* pg_class.reltuples */

    /* constants from reloptions or GUC variables */
    int         vac_base_thresh,
                vac_ins_base_thresh,
                anl_base_thresh;
    float4      vac_scale_factor,
                vac_ins_scale_factor,
                anl_scale_factor;

    /* thresholds calculated from above constants */
    float4      vacthresh,
                vacinsthresh,
                anlthresh;

    /* number of vacuum (resp. analyze) tuples at this time */
    float4      vactuples,
                instuples,
                anltuples;

    /* freeze parameters */
    int         freeze_max_age;
    int         multixact_freeze_max_age;
    TransactionId xidForceLimit;
    MultiXactId multiForceLimit;

    AssertArg(classForm != NULL);
    AssertArg(OidIsValid(relid));

    /*
     * Determine vacuum/analyze equation parameters.  We have two possible
     * sources: the passed reloptions (which could be a main table or a toast
     * table), or the autovacuum GUC variables.
     */

    /* -1 in autovac setting means use plain vacuum_scale_factor */
    vac_scale_factor = (relopts && relopts->vacuum_scale_factor >= 0)
        ? relopts->vacuum_scale_factor
        : autovacuum_vac_scale;

    vac_base_thresh = (relopts && relopts->vacuum_threshold >= 0)
        ? relopts->vacuum_threshold
        : autovacuum_vac_thresh;

    vac_ins_scale_factor = (relopts && relopts->vacuum_ins_scale_factor >= 0)
        ? relopts->vacuum_ins_scale_factor
        : autovacuum_vac_ins_scale;

    /* -1 is used to disable insert vacuums */
    vac_ins_base_thresh = (relopts && relopts->vacuum_ins_threshold >= -1)
        ? relopts->vacuum_ins_threshold
        : autovacuum_vac_ins_thresh;

    anl_scale_factor = (relopts && relopts->analyze_scale_factor >= 0)
        ? relopts->analyze_scale_factor
        : autovacuum_anl_scale;

    anl_base_thresh = (relopts && relopts->analyze_threshold >= 0)
        ? relopts->analyze_threshold
        : autovacuum_anl_thresh;

    freeze_max_age = (relopts && relopts->freeze_max_age >= 0)
        ? Min(relopts->freeze_max_age, autovacuum_freeze_max_age)
        : autovacuum_freeze_max_age;

    multixact_freeze_max_age = (relopts && relopts->multixact_freeze_max_age >= 0)
        ? Min(relopts->multixact_freeze_max_age, effective_multixact_freeze_max_age)
        : effective_multixact_freeze_max_age;

    av_enabled = (relopts ? relopts->enabled : true);

    /* Force vacuum if table is at risk of wraparound */
    xidForceLimit = recentXid - freeze_max_age;
    if (xidForceLimit < FirstNormalTransactionId)
        xidForceLimit -= FirstNormalTransactionId;
    force_vacuum = (TransactionIdIsNormal(classForm->relfrozenxid) &&
                    TransactionIdPrecedes(classForm->relfrozenxid,
                                          xidForceLimit));
    if (!force_vacuum)
    {
        multiForceLimit = recentMulti - multixact_freeze_max_age;
        if (multiForceLimit < FirstMultiXactId)
            multiForceLimit -= FirstMultiXactId;
        force_vacuum = MultiXactIdIsValid(classForm->relminmxid) &&
            MultiXactIdPrecedes(classForm->relminmxid, multiForceLimit);
    }
    *wraparound = force_vacuum;

    /* User disabled it in pg_class.reloptions?  (But ignore if at risk) */
    if (!av_enabled && !force_vacuum)
    {
        *doanalyze = false;
        *dovacuum = false;
        return;
    }

    /*
     * If we found the table in the stats hash, and autovacuum is currently
     * enabled, make a threshold-based decision whether to vacuum and/or
     * analyze.  If autovacuum is currently disabled, we must be here for
     * anti-wraparound vacuuming only, so don't vacuum (or analyze) anything
     * that's not being forced.
     */
    if (PointerIsValid(tabentry) && AutoVacuumingActive())
    {
        reltuples = classForm->reltuples;
        vactuples = tabentry->n_dead_tuples;
        instuples = tabentry->inserts_since_vacuum;
        anltuples = tabentry->changes_since_analyze;

        /* If the table hasn't yet been vacuumed, take reltuples as zero */
        if (reltuples < 0)
            reltuples = 0;

        vacthresh = (float4) vac_base_thresh + vac_scale_factor * reltuples;
        vacinsthresh = (float4) vac_ins_base_thresh + vac_ins_scale_factor * reltuples;
        anlthresh = (float4) anl_base_thresh + anl_scale_factor * reltuples;

        /*
         * Note that we don't need to take special consideration for stat
         * reset, because if that happens, the last vacuum and analyze counts
         * will be reset too.
         */
        if (vac_ins_base_thresh >= 0)
            elog(DEBUG3, "%s: vac: %.0f (threshold %.0f), ins: %.0f (threshold %.0f), anl: %.0f (threshold %.0f)",
                 NameStr(classForm->relname),
                 vactuples, vacthresh, instuples, vacinsthresh, anltuples, anlthresh);
        else
            elog(DEBUG3, "%s: vac: %.0f (threshold %.0f), ins: (disabled), anl: %.0f (threshold %.0f)",
                 NameStr(classForm->relname),
                 vactuples, vacthresh, anltuples, anlthresh);

        /* Determine if this table needs vacuum or analyze. */
        *dovacuum = force_vacuum || (vactuples > vacthresh) ||
            (vac_ins_base_thresh >= 0 && instuples > vacinsthresh);
        *doanalyze = (anltuples > anlthresh);
    }
    else
    {
        /*
         * Skip a table not found in stat hash, unless we have to force vacuum
         * for anti-wrap purposes.  If it's not acted upon, there's no need to
         * vacuum it.
         */
        *dovacuum = force_vacuum;
        *doanalyze = false;
    }

    /* ANALYZE refuses to work with pg_statistic */
    if (relid == StatisticRelationId)
        *doanalyze = false;
}

/*
 * autovacuum_do_vac_analyze
 *      Vacuum and/or analyze the specified table
 */
static void
autovacuum_do_vac_analyze(autovac_table *tab, BufferAccessStrategy bstrategy)
{
    RangeVar   *rangevar;
    VacuumRelation *rel;
    List       *rel_list;

    /* Let pgstat know what we're doing */
    autovac_report_activity(tab);

    /* Set up one VacuumRelation target, identified by OID, for vacuum() */
    rangevar = makeRangeVar(tab->at_nspname, tab->at_relname, -1);
    rel = makeVacuumRelation(rangevar, tab->at_relid, NIL);
    rel_list = list_make1(rel);

    vacuum(rel_list, &tab->at_params, bstrategy, true);
}

/*
 * autovac_report_activity
 *      Report to pgstat what autovacuum is doing
 *
 * We send a SQL string corresponding to what the user would see if the
 * equivalent command was to be issued manually.
 *
 * Note we assume that we are going to report the next command as soon as we're
 * done with the current one, and exit right after the last one, so we don't
 * bother to report "<IDLE>" or some such.
 */
static void
autovac_report_activity(autovac_table *tab)
{
#define MAX_AUTOVAC_ACTIV_LEN (NAMEDATALEN * 2 + 56)
    char        activity[MAX_AUTOVAC_ACTIV_LEN];
    int         len;

    /* Report the command and possible options */
    if (tab->at_params.options & VACOPT_VACUUM)
        snprintf(activity, MAX_AUTOVAC_ACTIV_LEN,
                 "autovacuum: VACUUM%s",
                 tab->at_params.options & VACOPT_ANALYZE ? " ANALYZE" : "");
    else
        snprintf(activity, MAX_AUTOVAC_ACTIV_LEN,
                 "autovacuum: ANALYZE");

    /*
     * Report the qualified name of the relation.
     */
    len = strlen(activity);

    snprintf(activity + len, MAX_AUTOVAC_ACTIV_LEN - len,
             " %s.%s%s", tab->at_nspname, tab->at_relname,
             tab->at_params.is_wraparound ? " (to prevent wraparound)" : "");

    /* Set statement_timestamp() to current time for pg_stat_activity */
    SetCurrentStatementStartTimestamp();

    pgstat_report_activity(STATE_RUNNING, activity);
}

/*
 * autovac_report_workitem
 *      Report to pgstat that autovacuum is processing a work item
 */
static void
autovac_report_workitem(AutoVacuumWorkItem *workitem,
                        const char *nspname, const char *relname)
{
    char        activity[MAX_AUTOVAC_ACTIV_LEN + 12 + 2];
    char        blk[12 + 2];
    int         len;

    switch (workitem->avw_type)
    {
        case AVW_BRINSummarizeRange:
            snprintf(activity, MAX_AUTOVAC_ACTIV_LEN,
                     "autovacuum: BRIN summarize");
            break;
    }

    /*
     * Report the qualified name of the relation, and the block number if any
     */
    len = strlen(activity);

    if (BlockNumberIsValid(workitem->avw_blockNumber))
        snprintf(blk, sizeof(blk), " %u", workitem->avw_blockNumber);
    else
        blk[0] = '\0';

    snprintf(activity + len, MAX_AUTOVAC_ACTIV_LEN - len,
             " %s.%s%s", nspname, relname, blk);

    /* Set statement_timestamp() to current time for pg_stat_activity */
    SetCurrentStatementStartTimestamp();

    pgstat_report_activity(STATE_RUNNING, activity);
}

/*
 * AutoVacuumingActive
 *      Check GUC vars and report whether the autovacuum process should be
 *      running.
 */
bool
AutoVacuumingActive(void)
{
    if (!autovacuum_start_daemon || !pgstat_track_counts)
        return false;
    return true;
}

/*
 * Request one work item to the next autovacuum run processing our database.
 * Return false if the request can't be recorded.
 */
bool
AutoVacuumRequestWork(AutoVacuumWorkItemType type, Oid relationId,
                      BlockNumber blkno)
{
    int         i;
    bool        result = false;

    LWLockAcquire(AutovacuumLock, LW_EXCLUSIVE);

    /*
     * Locate an unused work item and fill it with the given data.
     */
    for (i = 0; i < NUM_WORKITEMS; i++)
    {
        AutoVacuumWorkItem *workitem = &AutoVacuumShmem->av_workItems[i];

        if (workitem->avw_used)
            continue;

        workitem->avw_used = true;
        workitem->avw_active = false;
        workitem->avw_type = type;
        workitem->avw_database = MyDatabaseId;
        workitem->avw_relation = relationId;
        workitem->avw_blockNumber = blkno;
        result = true;

        /* done */
        break;
    }

    LWLockRelease(AutovacuumLock);

    return result;
}

/*
 * autovac_init
 *      This is called at postmaster initialization.
 *
 * All we do here is annoy the user if he got it wrong.
 */
void
autovac_init(void)
{
    if (autovacuum_start_daemon && !pgstat_track_counts)
        ereport(WARNING,
                (errmsg("autovacuum not started because of misconfiguration"),
                 errhint("Enable the \"track_counts\" option.")));
}

/*
 * IsAutoVacuum functions
 *      Return whether this is either a launcher autovacuum process or a worker
 *      process.
 */
bool
IsAutoVacuumLauncherProcess(void)
{
    return am_autovacuum_launcher;
}

bool
IsAutoVacuumWorkerProcess(void)
{
    return am_autovacuum_worker;
}


/*
 * AutoVacuumShmemSize
 *      Compute space needed for autovacuum-related shared memory
 */
Size
AutoVacuumShmemSize(void)
{
    Size        size;

    /*
     * Need the fixed struct and the array of WorkerInfoData.
     */
    size = sizeof(AutoVacuumShmemStruct);
    size = MAXALIGN(size);
    size = add_size(size, mul_size(autovacuum_max_workers,
                                   sizeof(WorkerInfoData)));
    return size;
}

/*
 * AutoVacuumShmemInit
 *      Allocate and initialize autovacuum-related shared memory
 */
void
AutoVacuumShmemInit(void)
{
    bool        found;

    AutoVacuumShmem = (AutoVacuumShmemStruct *)
        ShmemInitStruct("AutoVacuum Data",
                        AutoVacuumShmemSize(),
                        &found);

    if (!IsUnderPostmaster)
    {
        WorkerInfo  worker;
        int         i;

        Assert(!found);

        AutoVacuumShmem->av_launcherpid = 0;
        dlist_init(&AutoVacuumShmem->av_freeWorkers);
        dlist_init(&AutoVacuumShmem->av_runningWorkers);
        AutoVacuumShmem->av_startingWorker = NULL;
        memset(AutoVacuumShmem->av_workItems, 0,
               sizeof(AutoVacuumWorkItem) * NUM_WORKITEMS);

        worker = (WorkerInfo) ((char *) AutoVacuumShmem +
                               MAXALIGN(sizeof(AutoVacuumShmemStruct)));

        /* initialize the WorkerInfo free list */
        for (i = 0; i < autovacuum_max_workers; i++)
            dlist_push_head(&AutoVacuumShmem->av_freeWorkers,
                            &worker[i].wi_links);
    }
    else
        Assert(found);
}

/*
 * autovac_refresh_stats
 *      Refresh pgstats data for an autovacuum process
 *
 * Cause the next pgstats read operation to obtain fresh data, but throttle
 * such refreshing in the autovacuum launcher.  This is mostly to avoid
 * rereading the pgstats files too many times in quick succession when there
 * are many databases.
 *
 * Note: we avoid throttling in the autovac worker, as it would be
 * counterproductive in the recheck logic.
 */
static void
autovac_refresh_stats(void)
{
    if (IsAutoVacuumLauncherProcess())
    {
        static TimestampTz last_read = 0;
        TimestampTz current_time;

        current_time = GetCurrentTimestamp();

        if (!TimestampDifferenceExceeds(last_read, current_time,
                                        STATS_READ_DELAY))
            return;

        last_read = current_time;
    }

    pgstat_clear_snapshot();
}