autovacuum.c

/*-------------------------------------------------------------------------
 *
 * autovacuum.c
 *
 * PostgreSQL Integrated Autovacuum Daemon
 *
 *
 * Portions Copyright (c) 1996-2007, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *	  $PostgreSQL: pgsql/src/backend/postmaster/autovacuum.c,v 1.40 2007/03/28 22:17:12 alvherre Exp $
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

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

#include "access/genam.h"
#include "access/heapam.h"
#include "access/transam.h"
#include "access/xact.h"
#include "catalog/indexing.h"
#include "catalog/namespace.h"
#include "catalog/pg_autovacuum.h"
#include "catalog/pg_database.h"
#include "commands/vacuum.h"
#include "libpq/hba.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/fd.h"
#include "storage/ipc.h"
#include "storage/pmsignal.h"
#include "storage/proc.h"
#include "storage/procarray.h"
#include "storage/sinval.h"
#include "tcop/tcopprot.h"
#include "utils/flatfiles.h"
#include "utils/fmgroids.h"
#include "utils/lsyscache.h"
#include "utils/memutils.h"
#include "utils/ps_status.h"
#include "utils/syscache.h"


static volatile sig_atomic_t got_SIGHUP = false;
static volatile sig_atomic_t avlauncher_shutdown_request = false;

/*
 * GUC parameters
 */
bool		autovacuum_start_daemon = false;
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_vac_cost_delay;
int			autovacuum_vac_cost_limit;

/* Flag to tell if we are in the autovacuum daemon process */
static bool am_autovacuum_launcher = false;
static bool am_autovacuum_worker = false;

/* Comparison point for determining whether freeze_max_age is exceeded */
static TransactionId recentXid;

/* Default freeze_min_age to use for autovacuum (varies by database) */
static int	default_freeze_min_age;

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

/* struct to keep list of candidate databases for vacuum */
typedef struct autovac_dbase
{
	Oid			ad_datid;
	char	   *ad_name;
	TransactionId ad_frozenxid;
	PgStat_StatDBEntry *ad_entry;
} autovac_dbase;

/* struct to keep track of tables to vacuum and/or analyze, in 1st pass */
typedef struct av_relation
{
	Oid		ar_relid;
	Oid		ar_toastrelid;
} av_relation;

/* struct to keep track of tables to vacuum and/or analyze, after rechecking */
typedef struct autovac_table
{
	Oid			at_relid;
	Oid			at_toastrelid;
	bool		at_dovacuum;
	bool		at_doanalyze;
	int			at_freeze_min_age;
	int			at_vacuum_cost_delay;
	int			at_vacuum_cost_limit;
} autovac_table;

typedef struct
{
	Oid		process_db;			/* OID of database to process */
	int		worker_pid;			/* PID of the worker process, if any */
} AutoVacuumShmemStruct;

static AutoVacuumShmemStruct *AutoVacuumShmem;

#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[]);
NON_EXEC_STATIC void AutoVacLauncherMain(int argc, char *argv[]);

static void do_start_worker(void);
static void do_autovacuum(void);
static List *autovac_get_database_list(void);

static void relation_check_autovac(Oid relid, Form_pg_class classForm,
					   Form_pg_autovacuum avForm, PgStat_StatTabEntry *tabentry,
					   List **table_oids, List **table_toast_list,
					   List **toast_oids);
static autovac_table *table_recheck_autovac(Oid relid);
static void relation_needs_vacanalyze(Oid relid, Form_pg_autovacuum avForm,
						  Form_pg_class classForm,
						  PgStat_StatTabEntry *tabentry, bool *dovacuum,
						  bool *doanalyze);

static void autovacuum_do_vac_analyze(Oid relid, bool dovacuum,
						  bool doanalyze, int freeze_min_age);
static HeapTuple get_pg_autovacuum_tuple_relid(Relation avRel, Oid relid);
static PgStat_StatTabEntry *get_pgstat_tabentry_relid(Oid relid, bool isshared,
						  PgStat_StatDBEntry *shared,
						  PgStat_StatDBEntry *dbentry);
static void autovac_report_activity(VacuumStmt *vacstmt, Oid relid);
static void avl_sighup_handler(SIGNAL_ARGS);
static void avlauncher_shutdown(SIGNAL_ARGS);
static void avl_quickdie(SIGNAL_ARGS);



/********************************************************************
 *                    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 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;
	MemoryContext	avlauncher_cxt;

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

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

	/* Identify myself via ps */
	init_ps_display("autovacuum launcher 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.	Since this is an auxiliary process, it has
	 * particular signal requirements -- no deadlock checker or sinval
	 * catchup, for example.
	 *
	 * XXX It may be a good idea to receive signals when an avworker process
	 * finishes.
	 */
	pqsignal(SIGHUP, avl_sighup_handler);

	pqsignal(SIGINT, SIG_IGN);
	pqsignal(SIGTERM, avlauncher_shutdown);
	pqsignal(SIGQUIT, avl_quickdie);
	pqsignal(SIGALRM, SIG_IGN);

	pqsignal(SIGPIPE, SIG_IGN);
	pqsignal(SIGUSR1, SIG_IGN);
	/* We don't listen for async notifies */
	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
	InitAuxiliaryProcess();
#endif

	/*
	 * 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.
	 */
	avlauncher_cxt = AllocSetContextCreate(TopMemoryContext,
										   "Autovacuum Launcher",
										   ALLOCSET_DEFAULT_MINSIZE,
										   ALLOCSET_DEFAULT_INITSIZE,
										   ALLOCSET_DEFAULT_MAXSIZE);
	MemoryContextSwitchTo(avlauncher_cxt);


	/*
	 * If an exception is encountered, processing resumes here.
	 *
	 * This code is heavily based on bgwriter.c, q.v.
	 */
	if (sigsetjmp(local_sigjmp_buf, 1) != 0)
	{
		/* since not using PG_TRY, must reset error stack by hand */
		error_context_stack = NULL;

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

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

		/*
		 * These operations are really just a minimal subset of
		 * AbortTransaction().  We don't have very many resources to worry
		 * about, but we do have LWLocks.
		 */
		LWLockReleaseAll();
		AtEOXact_Files();

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

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

		/* Make sure pgstat also considers our stat data as gone */
		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;

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

	PG_SETMASK(&UnBlockSig);

	/*
	 * take a nap before executing the first iteration, unless we were
	 * requested an emergency run.
	 */
	if (autovacuum_start_daemon)
		pg_usleep(autovacuum_naptime * 1000000L); 

	for (;;)
	{
		int		worker_pid;

		/*
		 * Emergency bailout if postmaster has died.  This is to avoid the
		 * necessity for manual cleanup of all postmaster children.
		 */
		if (!PostmasterIsAlive(true))
			exit(1);

		if (avlauncher_shutdown_request)
			break;

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

		/*
		 * if there's a worker already running, sleep until it
		 * disappears.
		 */
		LWLockAcquire(AutovacuumLock, LW_SHARED);
		worker_pid = AutoVacuumShmem->worker_pid;
		LWLockRelease(AutovacuumLock);

		if (worker_pid != 0)
		{
			PGPROC *proc = BackendPidGetProc(worker_pid);

			if (proc != NULL && proc->isAutovacuum)
				goto sleep;
			else
			{
				/*
				 * if the worker is not really running (or it's a process
				 * that's not an autovacuum worker), remove the PID from shmem.
				 * This should not happen, because either the worker exits
				 * cleanly, in which case it'll remove the PID, or it dies, in
				 * which case postmaster will cause a system reset cycle.
				 */
				LWLockAcquire(AutovacuumLock, LW_EXCLUSIVE);
				worker_pid = 0;
				LWLockRelease(AutovacuumLock);
			}
		}

		do_start_worker();

sleep:
		/*
		 * in emergency mode, exit immediately so that the postmaster can
		 * request another run right away if needed.
		 *
		 * XXX -- maybe it would be better to handle this inside the launcher
		 * itself.
		 */
		if (!autovacuum_start_daemon)
			break;

		/* have pgstat read the file again next time */
		pgstat_clear_snapshot();

		/* now sleep until the next autovac iteration */
		pg_usleep(autovacuum_naptime * 1000000L); 
	}

	/* Normal exit from the autovac launcher is here */
	ereport(LOG,
			(errmsg("autovacuum launcher shutting down")));

	proc_exit(0);		/* done */
}

/*
 * 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.
 */
static void
do_start_worker(void)
{
	List	   *dblist;
	bool		for_xid_wrap;
	autovac_dbase *db;
	ListCell *cell;
	TransactionId xidForceLimit;

	/* Get a list of databases */
	dblist = autovac_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 */
	if (xidForceLimit < FirstNormalTransactionId)
		xidForceLimit -= FirstNormalTransactionId;

	/*
	 * 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 wraparound is
	 * found, we pick the one with oldest datfrozenxid, independently of
	 * autovacuum times.
	 *
	 * 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.
	 */
	db = NULL;
	for_xid_wrap = false;
	foreach(cell, dblist)
	{
		autovac_dbase *tmp = lfirst(cell);

		/* Find pgstat entry if any */
		tmp->ad_entry = pgstat_fetch_stat_dbentry(tmp->ad_datid);

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

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

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

	/* Found a database -- process it */
	if (db != NULL)
	{
		LWLockAcquire(AutovacuumLock, LW_EXCLUSIVE);
		AutoVacuumShmem->process_db = db->ad_datid;
		LWLockRelease(AutovacuumLock);

		SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_WORKER);
	}
}

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

static void
avlauncher_shutdown(SIGNAL_ARGS)
{
	avlauncher_shutdown_request = true;
}

/*
 * avl_quickdie occurs when signalled SIGQUIT from postmaster.
 *
 * Some backend has bought the farm, so we need to stop what we're doing
 * and exit.
 */
static void
avl_quickdie(SIGNAL_ARGS)
{
	PG_SETMASK(&BlockSig);

	/*
	 * DO NOT proc_exit() -- we're here because shared memory may be
	 * corrupted, so we don't want to try to clean up our transaction. Just
	 * nail the windows shut and get out of town.
	 *
	 * Note we do exit(2) not exit(0).	This is to force the postmaster into a
	 * system reset cycle if some idiot DBA sends a manual SIGQUIT to a random
	 * backend.  This is necessary precisely because we don't clean up our
	 * shared memory state.
	 */
	exit(2);
}


/********************************************************************
 *                    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 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();

	/* 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);

	/*
	 * Presently, SIGINT will lead to autovacuum shutdown, because that's how
	 * we handle ereport(ERROR).  It could be improved however.
	 */
	pqsignal(SIGINT, StatementCancelHandler);
	pqsignal(SIGTERM, die);
	pqsignal(SIGQUIT, quickdie);
	pqsignal(SIGALRM, handle_sig_alarm);

	pqsignal(SIGPIPE, SIG_IGN);
	pqsignal(SIGUSR1, CatchupInterruptHandler);
	/* We don't listen for async notifies */
	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);

	/*
	 * Get the database Id we're going to work on, and announce our PID
	 * in the shared memory area.  We remove the database OID immediately
	 * from the shared memory area.
	 */
	LWLockAcquire(AutovacuumLock, LW_EXCLUSIVE);

	dbid = AutoVacuumShmem->process_db;
	AutoVacuumShmem->process_db = InvalidOid;
	AutoVacuumShmem->worker_pid = MyProcPid;

	LWLockRelease(AutovacuumLock);

	if (OidIsValid(dbid))
	{
		char	*dbname;

		/*
		 * 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)));

		/* Create the memory context where cross-transaction state is stored */
		AutovacMemCxt = AllocSetContextCreate(TopMemoryContext,
											  "Autovacuum context",
											  ALLOCSET_DEFAULT_MINSIZE,
											  ALLOCSET_DEFAULT_INITSIZE,
											  ALLOCSET_DEFAULT_MAXSIZE);

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

	/*
	 * Now remove our PID from shared memory, so that the launcher can start
	 * another worker as soon as appropriate.
	 */
	LWLockAcquire(AutovacuumLock, LW_EXCLUSIVE);
	AutoVacuumShmem->worker_pid = 0;
	LWLockRelease(AutovacuumLock);

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

/*
 * autovac_get_database_list
 *
 *		Return a list of all databases.  Note we cannot use pg_database,
 *		because we aren't connected; we use the flat database file.
 */
static List *
autovac_get_database_list(void)
{
	char	   *filename;
	List	   *dblist = NIL;
	char		thisname[NAMEDATALEN];
	FILE	   *db_file;
	Oid			db_id;
	Oid			db_tablespace;
	TransactionId db_frozenxid;

	filename = database_getflatfilename();
	db_file = AllocateFile(filename, "r");
	if (db_file == NULL)
		ereport(FATAL,
				(errcode_for_file_access(),
				 errmsg("could not open file \"%s\": %m", filename)));

	while (read_pg_database_line(db_file, thisname, &db_id,
								 &db_tablespace, &db_frozenxid))
	{
		autovac_dbase *avdb;

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

		avdb->ad_datid = db_id;
		avdb->ad_name = pstrdup(thisname);
		avdb->ad_frozenxid = db_frozenxid;
		/* this gets set later: */
		avdb->ad_entry = NULL;

		dblist = lappend(dblist, avdb);
	}

	FreeFile(db_file);
	pfree(filename);

	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,
				avRel;
	HeapTuple	tuple;
	HeapScanDesc relScan;
	Form_pg_database dbForm;
	List	   *table_oids = NIL;
	List	   *toast_oids = NIL;
	List	   *table_toast_list = NIL;
	ListCell   *cell;
	PgStat_StatDBEntry *shared;
	PgStat_StatDBEntry *dbentry;

	/*
	 * 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();

	/* functions in indexes may want a snapshot set */
	ActiveSnapshot = CopySnapshot(GetTransactionSnapshot());

	/*
	 * 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_tabstat();

	/*
	 * Find the pg_database entry and select the default freeze_min_age.
	 * We use zero in template and nonconnectable databases,
	 * else the system-wide default.
	 */
	tuple = SearchSysCache(DATABASEOID,
						   ObjectIdGetDatum(MyDatabaseId),
						   0, 0, 0);
	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;
	else
		default_freeze_min_age = vacuum_freeze_min_age;

	ReleaseSysCache(tuple);

	/*
	 * StartTransactionCommand and CommitTransactionCommand will automatically
	 * switch to other contexts.  We need this one to keep the list of
	 * relations to vacuum/analyze across transactions.
	 */
	MemoryContextSwitchTo(AutovacMemCxt);

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

	classRel = heap_open(RelationRelationId, AccessShareLock);
	avRel = heap_open(AutovacuumRelationId, AccessShareLock);

	/*
	 * Scan pg_class and determine which tables to vacuum.
	 *
	 * The stats subsystem collects stats for toast tables independently of
	 * the stats for their parent tables.  We need to check those stats since
	 * in cases with short, wide tables there might be proportionally much
	 * more activity in the toast table than in its parent.
	 *
	 * Since we can only issue VACUUM against the parent table, we need to
	 * transpose a decision to vacuum a toast table into a decision to vacuum
	 * its parent.	There's no point in considering ANALYZE on a toast table,
	 * either.	To support this, we keep a list of OIDs of toast tables that
	 * need vacuuming alongside the list of regular tables.  Regular tables
	 * will be entered into the table list even if they appear not to need
	 * vacuuming; we go back and re-mark them after finding all the vacuumable
	 * toast tables.
	 */
	relScan = heap_beginscan(classRel, SnapshotNow, 0, NULL);

	while ((tuple = heap_getnext(relScan, ForwardScanDirection)) != NULL)
	{
		Form_pg_class classForm = (Form_pg_class) GETSTRUCT(tuple);
		Form_pg_autovacuum avForm = NULL;
		PgStat_StatTabEntry *tabentry;
		HeapTuple	avTup;
		Oid			relid;

		/* Consider only regular and toast tables. */
		if (classForm->relkind != RELKIND_RELATION &&
			classForm->relkind != RELKIND_TOASTVALUE)
			continue;

		/*
		 * Skip temp tables (i.e. those in temp namespaces).  We cannot safely
		 * process other backends' temp tables.
		 */
		if (isAnyTempNamespace(classForm->relnamespace))
			continue;

		relid = HeapTupleGetOid(tuple);

		/* Fetch the pg_autovacuum tuple for the relation, if any */
		avTup = get_pg_autovacuum_tuple_relid(avRel, relid);
		if (HeapTupleIsValid(avTup))
			avForm = (Form_pg_autovacuum) GETSTRUCT(avTup);

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

		relation_check_autovac(relid, classForm, avForm, tabentry,
							   &table_oids, &table_toast_list, &toast_oids);

		if (HeapTupleIsValid(avTup))
			heap_freetuple(avTup);