Separate multixact freezing parameters from xid's

[postgresql] / src / backend / postmaster / autovacuum.c

/*-------------------------------------------------------------------------
 *
 * 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) 1996-2014, 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/types.h>
#include <sys/time.h>
#include <time.h>
#include <unistd.h>

#include "access/heapam.h"
#include "access/htup_details.h"
#include "access/multixact.h"
#include "access/reloptions.h"
#include "access/transam.h"
#include "access/xact.h"
#include "catalog/dependency.h"
#include "catalog/namespace.h"
#include "catalog/pg_database.h"
#include "commands/dbcommands.h"
#include "commands/vacuum.h"
#include "lib/ilist.h"
#include "libpq/pqsignal.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "postmaster/autovacuum.h"
#include "postmaster/fork_process.h"
#include "postmaster/postmaster.h"
#include "storage/bufmgr.h"
#include "storage/ipc.h"
#include "storage/latch.h"
#include "storage/pmsignal.h"
#include "storage/proc.h"
#include "storage/procsignal.h"
#include "storage/sinvaladt.h"
#include "tcop/tcopprot.h"
#include "utils/fmgroids.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"
#include "utils/tqual.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_anl_thresh;
double          autovacuum_anl_scale;
int                     autovacuum_freeze_max_age;
int                     autovacuum_multixact_freeze_max_age;

int                     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 */

/* 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_SIGHUP = false;
static volatile sig_atomic_t got_SIGUSR2 = false;
static volatile sig_atomic_t got_SIGTERM = 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;
        bool            at_dovacuum;
        bool            at_doanalyze;
        int                     at_freeze_min_age;
        int                     at_freeze_table_age;
        int                     at_multixact_freeze_min_age;
        int                     at_multixact_freeze_table_age;
        int                     at_vacuum_cost_delay;
        int                     at_vacuum_cost_limit;
        bool            at_wraparound;
        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_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 which is
 * protected by AutovacuumScheduleLock (which is 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;
        int                     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;

/*-------------
 * 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)
 *
 * 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;
} 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[]) __attribute__((noreturn));
NON_EXEC_STATIC void AutoVacLauncherMain(int argc, char *argv[]) __attribute__((noreturn));

static Oid      do_start_worker(void);
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);
static void relation_needs_vacanalyze(Oid relid, AutoVacOpts *relopts,
                                                  Form_pg_class classForm,
                                                  PgStat_StatTabEntry *tabentry,
                                                  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 autovac_report_activity(autovac_table *tab);
static void avl_sighup_handler(SIGNAL_ARGS);
static void avl_sigusr2_handler(SIGNAL_ARGS);
static void avl_sigterm_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 ... */
                        /* Close the postmaster's sockets */
                        ClosePostmasterPorts(false);

                        /* Lose the postmaster's on-exit routines */
                        on_exit_reset();

                        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;

        /* we are a postmaster subprocess now */
        IsUnderPostmaster = true;
        am_autovacuum_launcher = true;

        /* reset MyProcPid */
        MyProcPid = getpid();

        /* record Start Time for logging */
        MyStartTime = time(NULL);

        /* Identify myself via ps */
        init_ps_display("autovacuum launcher process", "", "", "");

        ereport(LOG,
                        (errmsg("autovacuum launcher started")));

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

        SetProcessingMode(InitProcessing);

        /*
         * If possible, make this process a group leader, so that the postmaster
         * can signal any child processes too.  (autovacuum probably never has any
         * child processes, but for consistency we make all postmaster child
         * processes do this.)
         */
#ifdef HAVE_SETSID
        if (setsid() < 0)
                elog(FATAL, "setsid() failed: %m");
#endif

        /*
         * 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, avl_sighup_handler);
        pqsignal(SIGINT, StatementCancelHandler);
        pqsignal(SIGTERM, avl_sigterm_handler);

        pqsignal(SIGQUIT, quickdie);
        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, NULL);

        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_MINSIZE,
                                                                                  ALLOCSET_DEFAULT_INITSIZE,
                                                                                  ALLOCSET_DEFAULT_MAXSIZE);
        MemoryContextSwitchTo(AutovacMemCxt);

        /*
         * If an exception is encountered, processing resumes here.
         *
         * This code is a stripped down version of PostgresMain error recovery.
         */
        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();

                /*
                 * 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();

                /*
                 * 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);

        /*
         * 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 statement_timeout and lock_timeout to zero 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);

        /*
         * 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 and go away */
        if (!AutoVacuumingActive())
        {
                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);

        for (;;)
        {
                struct timeval nap;
                TimestampTz current_time = 0;
                bool            can_launch;
                int                     rc;

                /*
                 * 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);

                /* Allow sinval catchup interrupts while sleeping */
                EnableCatchupInterrupt();

                /*
                 * Wait until naptime expires or we get some type of signal (all the
                 * signal handlers will wake us by calling SetLatch).
                 */
                rc = WaitLatch(&MyProc->procLatch,
                                           WL_LATCH_SET | WL_TIMEOUT | WL_POSTMASTER_DEATH,
                                           (nap.tv_sec * 1000L) + (nap.tv_usec / 1000L));

                ResetLatch(&MyProc->procLatch);

                DisableCatchupInterrupt();

                /*
                 * Emergency bailout if postmaster has died.  This is to avoid the
                 * necessity for manual cleanup of all postmaster children.
                 */
                if (rc & WL_POSTMASTER_DEATH)
                        proc_exit(1);

                /* the normal shutdown case */
                if (got_SIGTERM)
                        break;

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

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

                        /* 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);
                }

                /*
                 * a worker finished, or postmaster signalled 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 launcher.  First, we need to make sure that there is a
                 * launcher 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_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);
                }
        }

        /* Normal exit from the autovac launcher is here */
        ereport(LOG,
                        (errmsg("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;
        }
}

/*
 * 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_MINSIZE,
                                                                   ALLOCSET_DEFAULT_INITSIZE,
                                                                   ALLOCSET_DEFAULT_MAXSIZE);
        tmpcxt = AllocSetContextCreate(newcxt,
                                                                   "tmp AV dblist",
                                                                   ALLOCSET_DEFAULT_MINSIZE,
                                                                   ALLOCSET_DEFAULT_INITSIZE,
                                                                   ALLOCSET_DEFAULT_MAXSIZE);
        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.hash = oid_hash;
        hctl.hcxt = tmpcxt;
        dbhash = hash_create("db hash", 20, &hctl,      /* magic number here FIXME */
                                                 HASH_ELEM | HASH_FUNCTION | 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 dllist, 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_MINSIZE,
                                                                   ALLOCSET_DEFAULT_INITSIZE,
                                                                   ALLOCSET_DEFAULT_MAXSIZE);
        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 = ReadNewTransactionId();
        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 - autovacuum_multixact_freeze_max_age;
        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;
}

/* SIGHUP: set flag to re-read config file at next convenient time */
static void
avl_sighup_handler(SIGNAL_ARGS)
{
        int                     save_errno = errno;

        got_SIGHUP = true;
        if (MyProc)
                SetLatch(&MyProc->procLatch);

        errno = save_errno;
}

/* 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;
        if (MyProc)
                SetLatch(&MyProc->procLatch);

        errno = save_errno;
}

/* SIGTERM: time to die */
static void
avl_sigterm_handler(SIGNAL_ARGS)
{
        int                     save_errno = errno;

        got_SIGTERM = true;
        if (MyProc)
                SetLatch(&MyProc->procLatch);

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

                        /* Lose the postmaster's on-exit routines */
                        on_exit_reset();

                        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;

        /* we are a postmaster subprocess now */
        IsUnderPostmaster = true;
        am_autovacuum_worker = true;

        /* reset MyProcPid */
        MyProcPid = getpid();

        /* record Start Time for logging */
        MyStartTime = time(NULL);

        /* Identify myself via ps */
        init_ps_display("autovacuum worker process", "", "", "");

        SetProcessingMode(InitProcessing);

        /*
         * If possible, make this process a group leader, so that the postmaster
         * can signal any child processes too.  (autovacuum probably never has any
         * child processes, but for consistency we make all postmaster child
         * processes do this.)
         */
#ifdef HAVE_SETSID
        if (setsid() < 0)
                elog(FATAL, "setsid() failed: %m");
#endif

        /*
         * 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.
         *
         * Currently, we don't pay attention to postgresql.conf changes that
         * happen during a single daemon iteration, so we can ignore SIGHUP.
         */
        pqsignal(SIGHUP, SIG_IGN);

        /*
         * 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);
        pqsignal(SIGQUIT, quickdie);
        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.
         *
         * See notes in postgres.c about the design of this coding.
         */
        if (sigsetjmp(local_sigjmp_buf, 1) != 0)
        {
                /* 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);

        /*
         * 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 statement_timeout and lock_timeout to zero 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);

        /*
         * 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, dbname);
                SetProcessingMode(NormalProcessing);
                set_ps_display(dbname, false);
                ereport(DEBUG1,
                                (errmsg("autovacuum: processing database \"%s\"", dbname)));

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

                /* And do an appropriate amount of work */
                recentXid = ReadNewTransactionId();
                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_proc = NULL;
                MyWorkerInfo->wi_launchtime = 0;
                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);
        int                     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;

        /* caculate the total base cost limit of 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_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_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);

                        elog(DEBUG2, "autovac_balance_cost(pid=%u db=%u, rel=%u, cost_limit=%d, cost_limit_base=%d, cost_delay=%d)",
                                 worker->wi_proc->pid, worker->wi_dboid, worker->wi_tableoid,
                                 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;
        HeapScanDesc 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.)
         */
        StartTransactionCommand();
        (void) GetTransactionSnapshot();

        rel = heap_open(DatabaseRelationId, AccessShareLock);
        scan = heap_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 = HeapTupleGetOid(tup);
                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);
        }

        heap_endscan(scan);
        heap_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;
        HeapScanDesc relScan;
        Form_pg_database dbForm;
        List       *table_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;

        /*
         * 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_MINSIZE,
                                                                                  ALLOCSET_DEFAULT_INITSIZE,
                                                                                  ALLOCSET_DEFAULT_MAXSIZE);
        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();

        /*
         * 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 = heap_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 */
        MemSet(&ctl, 0, sizeof(ctl));
        ctl.keysize = sizeof(Oid);
        ctl.entrysize = sizeof(av_relation);
        ctl.hash = oid_hash;

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

        /*
         * Scan pg_class to determine which tables to vacuum.
         *
         * We do this in two passes: on the first one we collect the list of plain
         * relations and materialized views, and on the second one we collect
         * TOAST tables. The reason for doing the second 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 = heap_beginscan_catalog(classRel, 0, NULL);

        /*
         * On the first 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)
                        continue;

                relid = HeapTupleGetOid(tuple);

                /* 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,
                                                                  &dovacuum, &doanalyze, &wraparound);

                /*
                 * 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)
                {
                        int                     backendID;

                        backendID = GetTempNamespaceBackendId(classForm->relnamespace);

                        /* We just ignore it if the owning backend is still active */
                        if (backendID == MyBackendId || BackendIdGetProc(backendID) == NULL)
                        {
                                /*
                                 * We found an orphan temp table (which was probably left
                                 * behind by a crashed backend).  If it's so old as to need
                                 * vacuum for wraparound, forcibly drop it.  Otherwise just
                                 * log a complaint.
                                 */
                                if (wraparound)
                                {
                                        ObjectAddress object;

                                        ereport(LOG,
                                                        (errmsg("autovacuum: dropping orphan temp table \"%s\".\"%s\" in database \"%s\"",
                                                                 get_namespace_name(classForm->relnamespace),
                                                                        NameStr(classForm->relname),
                                                                        get_database_name(MyDatabaseId))));
                                        object.classId = RelationRelationId;
                                        object.objectId = relid;
                                        object.objectSubId = 0;
                                        performDeletion(&object, DROP_CASCADE, PERFORM_DELETION_INTERNAL);
                                }
                                else
                                {
                                        ereport(LOG,
                                                        (errmsg("autovacuum: found orphan temp table \"%s\".\"%s\" in database \"%s\"",
                                                                 get_namespace_name(classForm->relnamespace),
                                                                        NameStr(classForm->relname),
                                                                        get_database_name(MyDatabaseId))));
                                }
                        }
                }
                else
                {
                        /* relations that need work are added to table_oids */
                        if (dovacuum || doanalyze)
                                table_oids = lappend_oid(table_oids, relid);

                        /*
                         * Remember the association for the second pass.  Note: we must do
                         * this even if 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));
                                        }
                                }
                        }
                }
        }

        heap_endscan(relScan);

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

        relScan = heap_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 = HeapTupleGetOid(tuple);

                /*
                 * 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,
                                                                  &dovacuum, &doanalyze, &wraparound);

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

        heap_endscan(relScan);
        heap_close(classRel, AccessShareLock);

        /*
         * 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_INITSIZE,
                                                                                  ALLOCSET_DEFAULT_MINSIZE,
                                                                                  ALLOCSET_DEFAULT_MAXSIZE);

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

                CHECK_FOR_INTERRUPTS();

                /*
                 * hold schedule lock from here until we're sure that this table still
                 * needs vacuuming.  We also need the AutovacuumLock to walk the
                 * worker array, but we'll let go of that one quickly.
                 */
                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 */
                        if (worker->wi_dboid != MyDatabaseId)
                                continue;

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

                /*
                 * 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);
                if (tab == NULL)
                {
                        /* someone else vacuumed the table, or it went away */
                        LWLockRelease(AutovacuumScheduleLock);
                        continue;
                }

                /*
                 * Ok, good to go.      Store the table in shared memory before releasing
                 * the lock so that other workers don't vacuum it concurrently.
                 */
                MyWorkerInfo->wi_tableoid = relid;
                LWLockRelease(AutovacuumScheduleLock);

                /*
                 * 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_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();
                {
                        /* have at it */
                        MemoryContextSwitchTo(TopTransactionContext);
                        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_dovacuum)
                                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 the PGXACT flags too */
                        AbortOutOfAnyTransaction();
                        FlushErrorState();
                        MemoryContextResetAndDeleteChildren(PortalContext);

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

                /* the PGXACT flags are reset at the next end of transaction */

                /* 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(AutovacuumLock, LW_EXCLUSIVE);
                MyWorkerInfo->wi_tableoid = InvalidOid;
                LWLockRelease(AutovacuumLock);

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

        /*
         * 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_clog if possible. We
         * only need to do this once, not after each table.
         */
        vac_update_datfrozenxid();

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

/*
 * extract_autovac_opts
 *
 * Given a relation's pg_class tuple, return the AutoVacOpts portion of
 * reloptions, if set; otherwise, return NULL.
 */
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_TOASTVALUE);

        relopts = extractRelOptions(tup, pg_class_desc, InvalidOid);
        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)
{
        Form_pg_class classForm;
        HeapTuple       classTup;
        bool            dovacuum;
        bool            doanalyze;
        autovac_table *tab = NULL;
        PgStat_StatTabEntry *tabentry;
        PgStat_StatDBEntry *shared;
        PgStat_StatDBEntry *dbentry;
        bool            wraparound;
        AutoVacOpts *avopts;

        /* use fresh stats */
        autovac_refresh_stats();

        shared = pgstat_fetch_stat_dbentry(InvalidOid);
        dbentry = pgstat_fetch_stat_dbentry(MyDatabaseId);

        /* 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;
        }

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

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

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

        /* 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;
                int                     vac_cost_delay;

                /*
                 * 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;

                /* 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_dovacuum = dovacuum;
                tab->at_doanalyze = doanalyze;
                tab->at_freeze_min_age = freeze_min_age;
                tab->at_freeze_table_age = freeze_table_age;
                tab->at_multixact_freeze_min_age = multixact_freeze_min_age;
                tab->at_multixact_freeze_table_age = multixact_freeze_table_age;
                tab->at_vacuum_cost_limit = vac_cost_limit;
                tab->at_vacuum_cost_delay = vac_cost_delay;
                tab->at_wraparound = wraparound;
                tab->at_relname = NULL;
                tab->at_nspname = NULL;
                tab->at_datname = NULL;
        }

        heap_freetuple(classTup);

        return tab;
}

/*
 * 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,
 /* 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,
                                anl_base_thresh;
        float4          vac_scale_factor,
                                anl_scale_factor;

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

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

        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, autovacuum_multixact_freeze_max_age)
                : autovacuum_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 = MultiXactIdPrecedes(classForm->relminmxid,
                                                                                   multiForceLimit);
        }
        *wraparound = force_vacuum;

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

        if (PointerIsValid(tabentry))
        {
                reltuples = classForm->reltuples;
                vactuples = tabentry->n_dead_tuples;
                anltuples = tabentry->changes_since_analyze;

                vacthresh = (float4) vac_base_thresh + vac_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.
                 */
                elog(DEBUG3, "%s: vac: %.0f (threshold %.0f), anl: %.0f (threshold %.0f)",
                         NameStr(classForm->relname),
                         vactuples, vacthresh, anltuples, anlthresh);

                /* Determine if this table needs vacuum or analyze. */
                *dovacuum = force_vacuum || (vactuples > vacthresh);
                *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_statistics */
        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)
{
        VacuumStmt      vacstmt;
        RangeVar        rangevar;

        /* Set up command parameters --- use local variables instead of palloc */
        MemSet(&vacstmt, 0, sizeof(vacstmt));
        MemSet(&rangevar, 0, sizeof(rangevar));

        rangevar.schemaname = tab->at_nspname;
        rangevar.relname = tab->at_relname;
        rangevar.location = -1;

        vacstmt.type = T_VacuumStmt;
        if (!tab->at_wraparound)
                vacstmt.options = VACOPT_NOWAIT;
        if (tab->at_dovacuum)
                vacstmt.options |= VACOPT_VACUUM;
        if (tab->at_doanalyze)
                vacstmt.options |= VACOPT_ANALYZE;
        vacstmt.freeze_min_age = tab->at_freeze_min_age;
        vacstmt.freeze_table_age = tab->at_freeze_table_age;
        vacstmt.multixact_freeze_min_age = tab->at_multixact_freeze_min_age;
        vacstmt.multixact_freeze_table_age = tab->at_multixact_freeze_table_age;
        /* we pass the OID, but might need this anyway for an error message */
        vacstmt.relation = &rangevar;
        vacstmt.va_cols = NIL;

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

        vacuum(&vacstmt, tab->at_relid, false, bstrategy, tab->at_wraparound, 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_dovacuum)
                snprintf(activity, MAX_AUTOVAC_ACTIV_LEN,
                                 "autovacuum: VACUUM%s",
                                 tab->at_doanalyze ? " 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_wraparound ? " (to prevent wraparound)" : "");

        /* 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;
}

/*
 * 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;

                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();
}