Revise parse tree representation for VACUUM and ANALYZE.

[postgresql] / src / backend / commands / vacuum.c

/*-------------------------------------------------------------------------
 *
 * vacuum.c
 *        The postgres vacuum cleaner.
 *
 * This file now includes only control and dispatch code for VACUUM and
 * ANALYZE commands.  Regular VACUUM is implemented in vacuumlazy.c,
 * ANALYZE in analyze.c, and VACUUM FULL is a variant of CLUSTER, handled
 * in cluster.c.
 *
 *
 * Portions Copyright (c) 1996-2019, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *        src/backend/commands/vacuum.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include <math.h>

#include "access/clog.h"
#include "access/commit_ts.h"
#include "access/genam.h"
#include "access/heapam.h"
#include "access/htup_details.h"
#include "access/multixact.h"
#include "access/tableam.h"
#include "access/transam.h"
#include "access/xact.h"
#include "catalog/namespace.h"
#include "catalog/pg_database.h"
#include "catalog/pg_inherits.h"
#include "catalog/pg_namespace.h"
#include "commands/cluster.h"
#include "commands/vacuum.h"
#include "miscadmin.h"
#include "nodes/makefuncs.h"
#include "pgstat.h"
#include "postmaster/autovacuum.h"
#include "storage/bufmgr.h"
#include "storage/lmgr.h"
#include "storage/proc.h"
#include "storage/procarray.h"
#include "utils/acl.h"
#include "utils/fmgroids.h"
#include "utils/guc.h"
#include "utils/memutils.h"
#include "utils/snapmgr.h"
#include "utils/syscache.h"


/*
 * GUC parameters
 */
int                     vacuum_freeze_min_age;
int                     vacuum_freeze_table_age;
int                     vacuum_multixact_freeze_min_age;
int                     vacuum_multixact_freeze_table_age;


/* A few variables that don't seem worth passing around as parameters */
static MemoryContext vac_context = NULL;
static BufferAccessStrategy vac_strategy;


/* non-export function prototypes */
static List *expand_vacuum_rel(VacuumRelation *vrel, int options);
static List *get_all_vacuum_rels(int options);
static void vac_truncate_clog(TransactionId frozenXID,
                                  MultiXactId minMulti,
                                  TransactionId lastSaneFrozenXid,
                                  MultiXactId lastSaneMinMulti);
static bool vacuum_rel(Oid relid, RangeVar *relation, VacuumParams *params);

/*
 * Primary entry point for manual VACUUM and ANALYZE commands
 *
 * This is mainly a preparation wrapper for the real operations that will
 * happen in vacuum().
 */
void
ExecVacuum(ParseState *pstate, VacuumStmt *vacstmt, bool isTopLevel)
{
        VacuumParams params;
        ListCell        *lc;

        params.options = vacstmt->is_vacuumcmd ? VACOPT_VACUUM : VACOPT_ANALYZE;

        /* Parse options list */
        foreach(lc, vacstmt->options)
        {
                DefElem *opt = (DefElem *) lfirst(lc);

                /* Parse common options for VACUUM and ANALYZE */
                if (strcmp(opt->defname, "verbose") == 0)
                        params.options |= VACOPT_VERBOSE;
                else if (strcmp(opt->defname, "skip_locked") == 0)
                        params.options |= VACOPT_SKIP_LOCKED;
                else if (!vacstmt->is_vacuumcmd)
                        ereport(ERROR,
                                        (errcode(ERRCODE_SYNTAX_ERROR),
                                         errmsg("unrecognized ANALYZE option \"%s\"", opt->defname),
                                         parser_errposition(pstate, opt->location)));

                /* Parse options available on VACUUM */
                else if (strcmp(opt->defname, "analyze") == 0)
                                params.options |= VACOPT_ANALYZE;
                else if (strcmp(opt->defname, "freeze") == 0)
                                params.options |= VACOPT_FREEZE;
                else if (strcmp(opt->defname, "full") == 0)
                        params.options |= VACOPT_FULL;
                else if (strcmp(opt->defname, "disable_page_skipping") == 0)
                        params.options |= VACOPT_DISABLE_PAGE_SKIPPING;
                else
                        ereport(ERROR,
                                        (errcode(ERRCODE_SYNTAX_ERROR),
                                         errmsg("unrecognized VACUUM option \"%s\"", opt->defname),
                                         parser_errposition(pstate, opt->location)));
        }

        /* sanity checks on options */
        Assert(params.options & (VACOPT_VACUUM | VACOPT_ANALYZE));
        Assert((params.options & VACOPT_VACUUM) ||
                   !(params.options & (VACOPT_FULL | VACOPT_FREEZE)));
        Assert(!(params.options & VACOPT_SKIPTOAST));

        /*
         * Make sure VACOPT_ANALYZE is specified if any column lists are present.
         */
        if (!(params.options & VACOPT_ANALYZE))
        {
                ListCell   *lc;

                foreach(lc, vacstmt->rels)
                {
                        VacuumRelation *vrel = lfirst_node(VacuumRelation, lc);

                        if (vrel->va_cols != NIL)
                                ereport(ERROR,
                                                (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                                                 errmsg("ANALYZE option must be specified when a column list is provided")));
                }
        }

        /*
         * All freeze ages are zero if the FREEZE option is given; otherwise pass
         * them as -1 which means to use the default values.
         */
        if (params.options & VACOPT_FREEZE)
        {
                params.freeze_min_age = 0;
                params.freeze_table_age = 0;
                params.multixact_freeze_min_age = 0;
                params.multixact_freeze_table_age = 0;
        }
        else
        {
                params.freeze_min_age = -1;
                params.freeze_table_age = -1;
                params.multixact_freeze_min_age = -1;
                params.multixact_freeze_table_age = -1;
        }

        /* user-invoked vacuum is never "for wraparound" */
        params.is_wraparound = false;

        /* user-invoked vacuum never uses this parameter */
        params.log_min_duration = -1;

        /* Now go through the common routine */
        vacuum(vacstmt->rels, &params, NULL, isTopLevel);
}

/*
 * Internal entry point for VACUUM and ANALYZE commands.
 *
 * relations, if not NIL, is a list of VacuumRelation to process; otherwise,
 * we process all relevant tables in the database.  For each VacuumRelation,
 * if a valid OID is supplied, the table with that OID is what to process;
 * otherwise, the VacuumRelation's RangeVar indicates what to process.
 *
 * params contains a set of parameters that can be used to customize the
 * behavior.
 *
 * bstrategy is normally given as NULL, but in autovacuum it can be passed
 * in to use the same buffer strategy object across multiple vacuum() calls.
 *
 * isTopLevel should be passed down from ProcessUtility.
 *
 * It is the caller's responsibility that all parameters are allocated in a
 * memory context that will not disappear at transaction commit.
 */
void
vacuum(List *relations, VacuumParams *params,
           BufferAccessStrategy bstrategy, bool isTopLevel)
{
        static bool in_vacuum = false;

        const char *stmttype;
        volatile bool in_outer_xact,
                                use_own_xacts;

        Assert(params != NULL);

        stmttype = (params->options & VACOPT_VACUUM) ? "VACUUM" : "ANALYZE";

        /*
         * We cannot run VACUUM inside a user transaction block; if we were inside
         * a transaction, then our commit- and start-transaction-command calls
         * would not have the intended effect!  There are numerous other subtle
         * dependencies on this, too.
         *
         * ANALYZE (without VACUUM) can run either way.
         */
        if (params->options & VACOPT_VACUUM)
        {
                PreventInTransactionBlock(isTopLevel, stmttype);
                in_outer_xact = false;
        }
        else
                in_outer_xact = IsInTransactionBlock(isTopLevel);

        /*
         * Due to static variables vac_context, anl_context and vac_strategy,
         * vacuum() is not reentrant.  This matters when VACUUM FULL or ANALYZE
         * calls a hostile index expression that itself calls ANALYZE.
         */
        if (in_vacuum)
                ereport(ERROR,
                                (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                                 errmsg("%s cannot be executed from VACUUM or ANALYZE",
                                                stmttype)));

        /*
         * Sanity check DISABLE_PAGE_SKIPPING option.
         */
        if ((params->options & VACOPT_FULL) != 0 &&
                (params->options & VACOPT_DISABLE_PAGE_SKIPPING) != 0)
                ereport(ERROR,
                                (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                                 errmsg("VACUUM option DISABLE_PAGE_SKIPPING cannot be used with FULL")));

        /*
         * Send info about dead objects to the statistics collector, unless we are
         * in autovacuum --- autovacuum.c does this for itself.
         */
        if ((params->options & VACOPT_VACUUM) && !IsAutoVacuumWorkerProcess())
                pgstat_vacuum_stat();

        /*
         * Create special memory context for cross-transaction storage.
         *
         * Since it is a child of PortalContext, it will go away eventually even
         * if we suffer an error; there's no need for special abort cleanup logic.
         */
        vac_context = AllocSetContextCreate(PortalContext,
                                                                                "Vacuum",
                                                                                ALLOCSET_DEFAULT_SIZES);

        /*
         * If caller didn't give us a buffer strategy object, make one in the
         * cross-transaction memory context.
         */
        if (bstrategy == NULL)
        {
                MemoryContext old_context = MemoryContextSwitchTo(vac_context);

                bstrategy = GetAccessStrategy(BAS_VACUUM);
                MemoryContextSwitchTo(old_context);
        }
        vac_strategy = bstrategy;

        /*
         * Build list of relation(s) to process, putting any new data in
         * vac_context for safekeeping.
         */
        if (relations != NIL)
        {
                List       *newrels = NIL;
                ListCell   *lc;

                foreach(lc, relations)
                {
                        VacuumRelation *vrel = lfirst_node(VacuumRelation, lc);
                        List       *sublist;
                        MemoryContext old_context;

                        sublist = expand_vacuum_rel(vrel, params->options);
                        old_context = MemoryContextSwitchTo(vac_context);
                        newrels = list_concat(newrels, sublist);
                        MemoryContextSwitchTo(old_context);
                }
                relations = newrels;
        }
        else
                relations = get_all_vacuum_rels(params->options);

        /*
         * Decide whether we need to start/commit our own transactions.
         *
         * For VACUUM (with or without ANALYZE): always do so, so that we can
         * release locks as soon as possible.  (We could possibly use the outer
         * transaction for a one-table VACUUM, but handling TOAST tables would be
         * problematic.)
         *
         * For ANALYZE (no VACUUM): if inside a transaction block, we cannot
         * start/commit our own transactions.  Also, there's no need to do so if
         * only processing one relation.  For multiple relations when not within a
         * transaction block, and also in an autovacuum worker, use own
         * transactions so we can release locks sooner.
         */
        if (params->options & VACOPT_VACUUM)
                use_own_xacts = true;
        else
        {
                Assert(params->options & VACOPT_ANALYZE);
                if (IsAutoVacuumWorkerProcess())
                        use_own_xacts = true;
                else if (in_outer_xact)
                        use_own_xacts = false;
                else if (list_length(relations) > 1)
                        use_own_xacts = true;
                else
                        use_own_xacts = false;
        }

        /*
         * vacuum_rel expects to be entered with no transaction active; it will
         * start and commit its own transaction.  But we are called by an SQL
         * command, and so we are executing inside a transaction already. We
         * commit the transaction started in PostgresMain() here, and start
         * another one before exiting to match the commit waiting for us back in
         * PostgresMain().
         */
        if (use_own_xacts)
        {
                Assert(!in_outer_xact);

                /* ActiveSnapshot is not set by autovacuum */
                if (ActiveSnapshotSet())
                        PopActiveSnapshot();

                /* matches the StartTransaction in PostgresMain() */
                CommitTransactionCommand();
        }

        /* Turn vacuum cost accounting on or off, and set/clear in_vacuum */
        PG_TRY();
        {
                ListCell   *cur;

                in_vacuum = true;
                VacuumCostActive = (VacuumCostDelay > 0);
                VacuumCostBalance = 0;
                VacuumPageHit = 0;
                VacuumPageMiss = 0;
                VacuumPageDirty = 0;

                /*
                 * Loop to process each selected relation.
                 */
                foreach(cur, relations)
                {
                        VacuumRelation *vrel = lfirst_node(VacuumRelation, cur);

                        if (params->options & VACOPT_VACUUM)
                        {
                                if (!vacuum_rel(vrel->oid, vrel->relation, params))
                                        continue;
                        }

                        if (params->options & VACOPT_ANALYZE)
                        {
                                /*
                                 * If using separate xacts, start one for analyze. Otherwise,
                                 * we can use the outer transaction.
                                 */
                                if (use_own_xacts)
                                {
                                        StartTransactionCommand();
                                        /* functions in indexes may want a snapshot set */
                                        PushActiveSnapshot(GetTransactionSnapshot());
                                }

                                analyze_rel(vrel->oid, vrel->relation, params,
                                                        vrel->va_cols, in_outer_xact, vac_strategy);

                                if (use_own_xacts)
                                {
                                        PopActiveSnapshot();
                                        CommitTransactionCommand();
                                }
                        }
                }
        }
        PG_CATCH();
        {
                in_vacuum = false;
                VacuumCostActive = false;
                PG_RE_THROW();
        }
        PG_END_TRY();

        in_vacuum = false;
        VacuumCostActive = false;

        /*
         * Finish up processing.
         */
        if (use_own_xacts)
        {
                /* here, we are not in a transaction */

                /*
                 * This matches the CommitTransaction waiting for us in
                 * PostgresMain().
                 */
                StartTransactionCommand();
        }

        if ((params->options & VACOPT_VACUUM) && !IsAutoVacuumWorkerProcess())
        {
                /*
                 * Update pg_database.datfrozenxid, and truncate pg_xact if possible.
                 * (autovacuum.c does this for itself.)
                 */
                vac_update_datfrozenxid();
        }

        /*
         * Clean up working storage --- note we must do this after
         * StartTransactionCommand, else we might be trying to delete the active
         * context!
         */
        MemoryContextDelete(vac_context);
        vac_context = NULL;
}

/*
 * Check if a given relation can be safely vacuumed or analyzed.  If the
 * user is not the relation owner, issue a WARNING log message and return
 * false to let the caller decide what to do with this relation.  This
 * routine is used to decide if a relation can be processed for VACUUM or
 * ANALYZE.
 */
bool
vacuum_is_relation_owner(Oid relid, Form_pg_class reltuple, int options)
{
        char       *relname;

        Assert((options & (VACOPT_VACUUM | VACOPT_ANALYZE)) != 0);

        /*
         * Check permissions.
         *
         * We allow the user to vacuum or analyze a table if he is superuser, the
         * table owner, or the database owner (but in the latter case, only if
         * it's not a shared relation).  pg_class_ownercheck includes the
         * superuser case.
         *
         * Note we choose to treat permissions failure as a WARNING and keep
         * trying to vacuum or analyze the rest of the DB --- is this appropriate?
         */
        if (pg_class_ownercheck(relid, GetUserId()) ||
                (pg_database_ownercheck(MyDatabaseId, GetUserId()) && !reltuple->relisshared))
                return true;

        relname = NameStr(reltuple->relname);

        if ((options & VACOPT_VACUUM) != 0)
        {
                if (reltuple->relisshared)
                        ereport(WARNING,
                                        (errmsg("skipping \"%s\" --- only superuser can vacuum it",
                                                        relname)));
                else if (reltuple->relnamespace == PG_CATALOG_NAMESPACE)
                        ereport(WARNING,
                                        (errmsg("skipping \"%s\" --- only superuser or database owner can vacuum it",
                                                        relname)));
                else
                        ereport(WARNING,
                                        (errmsg("skipping \"%s\" --- only table or database owner can vacuum it",
                                                        relname)));

                /*
                 * For VACUUM ANALYZE, both logs could show up, but just generate
                 * information for VACUUM as that would be the first one to be
                 * processed.
                 */
                return false;
        }

        if ((options & VACOPT_ANALYZE) != 0)
        {
                if (reltuple->relisshared)
                        ereport(WARNING,
                                        (errmsg("skipping \"%s\" --- only superuser can analyze it",
                                                        relname)));
                else if (reltuple->relnamespace == PG_CATALOG_NAMESPACE)
                        ereport(WARNING,
                                        (errmsg("skipping \"%s\" --- only superuser or database owner can analyze it",
                                                        relname)));
                else
                        ereport(WARNING,
                                        (errmsg("skipping \"%s\" --- only table or database owner can analyze it",
                                                        relname)));
        }

        return false;
}


/*
 * vacuum_open_relation
 *
 * This routine is used for attempting to open and lock a relation which
 * is going to be vacuumed or analyzed.  If the relation cannot be opened
 * or locked, a log is emitted if possible.
 */
Relation
vacuum_open_relation(Oid relid, RangeVar *relation, int options,
                                         bool verbose, LOCKMODE lmode)
{
        Relation        onerel;
        bool            rel_lock = true;
        int                     elevel;

        Assert((options & (VACOPT_VACUUM | VACOPT_ANALYZE)) != 0);

        /*
         * Open the relation and get the appropriate lock on it.
         *
         * There's a race condition here: the relation may have gone away since
         * the last time we saw it.  If so, we don't need to vacuum or analyze it.
         *
         * If we've been asked not to wait for the relation lock, acquire it first
         * in non-blocking mode, before calling try_relation_open().
         */
        if (!(options & VACOPT_SKIP_LOCKED))
                onerel = try_relation_open(relid, lmode);
        else if (ConditionalLockRelationOid(relid, lmode))
                onerel = try_relation_open(relid, NoLock);
        else
        {
                onerel = NULL;
                rel_lock = false;
        }

        /* if relation is opened, leave */
        if (onerel)
                return onerel;

        /*
         * Relation could not be opened, hence generate if possible a log
         * informing on the situation.
         *
         * If the RangeVar is not defined, we do not have enough information to
         * provide a meaningful log statement.  Chances are that the caller has
         * intentionally not provided this information so that this logging is
         * skipped, anyway.
         */
        if (relation == NULL)
                return NULL;

        /*
         * Determine the log level.
         *
         * For manual VACUUM or ANALYZE, we emit a WARNING to match the log statements
         * in the permission checks; otherwise, only log if the caller so requested.
         */
        if (!IsAutoVacuumWorkerProcess())
                elevel = WARNING;
        else if (verbose)
                elevel = LOG;
        else
                return NULL;

        if ((options & VACOPT_VACUUM) != 0)
        {
                if (!rel_lock)
                        ereport(elevel,
                                        (errcode(ERRCODE_LOCK_NOT_AVAILABLE),
                                         errmsg("skipping vacuum of \"%s\" --- lock not available",
                                                        relation->relname)));
                else
                        ereport(elevel,
                                        (errcode(ERRCODE_UNDEFINED_TABLE),
                                         errmsg("skipping vacuum of \"%s\" --- relation no longer exists",
                                                        relation->relname)));

                /*
                 * For VACUUM ANALYZE, both logs could show up, but just generate
                 * information for VACUUM as that would be the first one to be
                 * processed.
                 */
                return NULL;
        }

        if ((options & VACOPT_ANALYZE) != 0)
        {
                if (!rel_lock)
                        ereport(elevel,
                                        (errcode(ERRCODE_LOCK_NOT_AVAILABLE),
                                         errmsg("skipping analyze of \"%s\" --- lock not available",
                                                        relation->relname)));
                else
                        ereport(elevel,
                                        (errcode(ERRCODE_UNDEFINED_TABLE),
                                         errmsg("skipping analyze of \"%s\" --- relation no longer exists",
                                                        relation->relname)));
        }

        return NULL;
}


/*
 * Given a VacuumRelation, fill in the table OID if it wasn't specified,
 * and optionally add VacuumRelations for partitions of the table.
 *
 * If a VacuumRelation does not have an OID supplied and is a partitioned
 * table, an extra entry will be added to the output for each partition.
 * Presently, only autovacuum supplies OIDs when calling vacuum(), and
 * it does not want us to expand partitioned tables.
 *
 * We take care not to modify the input data structure, but instead build
 * new VacuumRelation(s) to return.  (But note that they will reference
 * unmodified parts of the input, eg column lists.)  New data structures
 * are made in vac_context.
 */
static List *
expand_vacuum_rel(VacuumRelation *vrel, int options)
{
        List       *vacrels = NIL;
        MemoryContext oldcontext;

        /* If caller supplied OID, there's nothing we need do here. */
        if (OidIsValid(vrel->oid))
        {
                oldcontext = MemoryContextSwitchTo(vac_context);
                vacrels = lappend(vacrels, vrel);
                MemoryContextSwitchTo(oldcontext);
        }
        else
        {
                /* Process a specific relation, and possibly partitions thereof */
                Oid                     relid;
                HeapTuple       tuple;
                Form_pg_class classForm;
                bool            include_parts;
                int                     rvr_opts;

                /*
                 * Since autovacuum workers supply OIDs when calling vacuum(), no
                 * autovacuum worker should reach this code.
                 */
                Assert(!IsAutoVacuumWorkerProcess());

                /*
                 * We transiently take AccessShareLock to protect the syscache lookup
                 * below, as well as find_all_inheritors's expectation that the caller
                 * holds some lock on the starting relation.
                 */
                rvr_opts = (options & VACOPT_SKIP_LOCKED) ? RVR_SKIP_LOCKED : 0;
                relid = RangeVarGetRelidExtended(vrel->relation,
                                                                                 AccessShareLock,
                                                                                 rvr_opts,
                                                                                 NULL, NULL);

                /*
                 * If the lock is unavailable, emit the same log statement that
                 * vacuum_rel() and analyze_rel() would.
                 */
                if (!OidIsValid(relid))
                {
                        if (options & VACOPT_VACUUM)
                                ereport(WARNING,
                                                (errcode(ERRCODE_LOCK_NOT_AVAILABLE),
                                                 errmsg("skipping vacuum of \"%s\" --- lock not available",
                                                                vrel->relation->relname)));
                        else
                                ereport(WARNING,
                                                (errcode(ERRCODE_LOCK_NOT_AVAILABLE),
                                                 errmsg("skipping analyze of \"%s\" --- lock not available",
                                                                vrel->relation->relname)));
                        return vacrels;
                }

                /*
                 * To check whether the relation is a partitioned table and its
                 * ownership, fetch its syscache entry.
                 */
                tuple = SearchSysCache1(RELOID, ObjectIdGetDatum(relid));
                if (!HeapTupleIsValid(tuple))
                        elog(ERROR, "cache lookup failed for relation %u", relid);
                classForm = (Form_pg_class) GETSTRUCT(tuple);

                /*
                 * Make a returnable VacuumRelation for this rel if user is a proper
                 * owner.
                 */
                if (vacuum_is_relation_owner(relid, classForm, options))
                {
                        oldcontext = MemoryContextSwitchTo(vac_context);
                        vacrels = lappend(vacrels, makeVacuumRelation(vrel->relation,
                                                                                                                  relid,
                                                                                                                  vrel->va_cols));
                        MemoryContextSwitchTo(oldcontext);
                }


                include_parts = (classForm->relkind == RELKIND_PARTITIONED_TABLE);
                ReleaseSysCache(tuple);

                /*
                 * If it is, make relation list entries for its partitions.  Note that
                 * the list returned by find_all_inheritors() includes the passed-in
                 * OID, so we have to skip that.  There's no point in taking locks on
                 * the individual partitions yet, and doing so would just add
                 * unnecessary deadlock risk.  For this last reason we do not check
                 * yet the ownership of the partitions, which get added to the list to
                 * process.  Ownership will be checked later on anyway.
                 */
                if (include_parts)
                {
                        List       *part_oids = find_all_inheritors(relid, NoLock, NULL);
                        ListCell   *part_lc;

                        foreach(part_lc, part_oids)
                        {
                                Oid                     part_oid = lfirst_oid(part_lc);

                                if (part_oid == relid)
                                        continue;       /* ignore original table */

                                /*
                                 * We omit a RangeVar since it wouldn't be appropriate to
                                 * complain about failure to open one of these relations
                                 * later.
                                 */
                                oldcontext = MemoryContextSwitchTo(vac_context);
                                vacrels = lappend(vacrels, makeVacuumRelation(NULL,
                                                                                                                          part_oid,
                                                                                                                          vrel->va_cols));
                                MemoryContextSwitchTo(oldcontext);
                        }
                }

                /*
                 * Release lock again.  This means that by the time we actually try to
                 * process the table, it might be gone or renamed.  In the former case
                 * we'll silently ignore it; in the latter case we'll process it
                 * anyway, but we must beware that the RangeVar doesn't necessarily
                 * identify it anymore.  This isn't ideal, perhaps, but there's little
                 * practical alternative, since we're typically going to commit this
                 * transaction and begin a new one between now and then.  Moreover,
                 * holding locks on multiple relations would create significant risk
                 * of deadlock.
                 */
                UnlockRelationOid(relid, AccessShareLock);
        }

        return vacrels;
}

/*
 * Construct a list of VacuumRelations for all vacuumable rels in
 * the current database.  The list is built in vac_context.
 */
static List *
get_all_vacuum_rels(int options)
{
        List       *vacrels = NIL;
        Relation        pgclass;
        TableScanDesc scan;
        HeapTuple       tuple;

        pgclass = table_open(RelationRelationId, AccessShareLock);

        scan = table_beginscan_catalog(pgclass, 0, NULL);

        while ((tuple = heap_getnext(scan, ForwardScanDirection)) != NULL)
        {
                Form_pg_class classForm = (Form_pg_class) GETSTRUCT(tuple);
                MemoryContext oldcontext;
                Oid                     relid = classForm->oid;

                /* check permissions of relation */
                if (!vacuum_is_relation_owner(relid, classForm, options))
                        continue;

                /*
                 * We include partitioned tables here; depending on which operation is
                 * to be performed, caller will decide whether to process or ignore
                 * them.
                 */
                if (classForm->relkind != RELKIND_RELATION &&
                        classForm->relkind != RELKIND_MATVIEW &&
                        classForm->relkind != RELKIND_PARTITIONED_TABLE)
                        continue;

                /*
                 * Build VacuumRelation(s) specifying the table OIDs to be processed.
                 * We omit a RangeVar since it wouldn't be appropriate to complain
                 * about failure to open one of these relations later.
                 */
                oldcontext = MemoryContextSwitchTo(vac_context);
                vacrels = lappend(vacrels, makeVacuumRelation(NULL,
                                                                                                          relid,
                                                                                                          NIL));
                MemoryContextSwitchTo(oldcontext);
        }

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

        return vacrels;
}

/*
 * vacuum_set_xid_limits() -- compute oldest-Xmin and freeze cutoff points
 *
 * The output parameters are:
 * - oldestXmin is the cutoff value used to distinguish whether tuples are
 *       DEAD or RECENTLY_DEAD (see HeapTupleSatisfiesVacuum).
 * - freezeLimit is the Xid below which all Xids are replaced by
 *       FrozenTransactionId during vacuum.
 * - xidFullScanLimit (computed from table_freeze_age parameter)
 *       represents a minimum Xid value; a table whose relfrozenxid is older than
 *       this will have a full-table vacuum applied to it, to freeze tuples across
 *       the whole table.  Vacuuming a table younger than this value can use a
 *       partial scan.
 * - multiXactCutoff is the value below which all MultiXactIds are removed from
 *       Xmax.
 * - mxactFullScanLimit is a value against which a table's relminmxid value is
 *       compared to produce a full-table vacuum, as with xidFullScanLimit.
 *
 * xidFullScanLimit and mxactFullScanLimit can be passed as NULL if caller is
 * not interested.
 */
void
vacuum_set_xid_limits(Relation rel,
                                          int freeze_min_age,
                                          int freeze_table_age,
                                          int multixact_freeze_min_age,
                                          int multixact_freeze_table_age,
                                          TransactionId *oldestXmin,
                                          TransactionId *freezeLimit,
                                          TransactionId *xidFullScanLimit,
                                          MultiXactId *multiXactCutoff,
                                          MultiXactId *mxactFullScanLimit)
{
        int                     freezemin;
        int                     mxid_freezemin;
        int                     effective_multixact_freeze_max_age;
        TransactionId limit;
        TransactionId safeLimit;
        MultiXactId mxactLimit;
        MultiXactId safeMxactLimit;

        /*
         * We can always ignore processes running lazy vacuum.  This is because we
         * use these values only for deciding which tuples we must keep in the
         * tables.  Since lazy vacuum doesn't write its XID anywhere, it's safe to
         * ignore it.  In theory it could be problematic to ignore lazy vacuums in
         * a full vacuum, but keep in mind that only one vacuum process can be
         * working on a particular table at any time, and that each vacuum is
         * always an independent transaction.
         */
        *oldestXmin =
                TransactionIdLimitedForOldSnapshots(GetOldestXmin(rel, PROCARRAY_FLAGS_VACUUM), rel);

        Assert(TransactionIdIsNormal(*oldestXmin));

        /*
         * Determine the minimum freeze age to use: as specified by the caller, or
         * vacuum_freeze_min_age, but in any case not more than half
         * autovacuum_freeze_max_age, so that autovacuums to prevent XID
         * wraparound won't occur too frequently.
         */
        freezemin = freeze_min_age;
        if (freezemin < 0)
                freezemin = vacuum_freeze_min_age;
        freezemin = Min(freezemin, autovacuum_freeze_max_age / 2);
        Assert(freezemin >= 0);

        /*
         * Compute the cutoff XID, being careful not to generate a "permanent" XID
         */
        limit = *oldestXmin - freezemin;
        if (!TransactionIdIsNormal(limit))
                limit = FirstNormalTransactionId;

        /*
         * If oldestXmin is very far back (in practice, more than
         * autovacuum_freeze_max_age / 2 XIDs old), complain and force a minimum
         * freeze age of zero.
         */
        safeLimit = ReadNewTransactionId() - autovacuum_freeze_max_age;
        if (!TransactionIdIsNormal(safeLimit))
                safeLimit = FirstNormalTransactionId;

        if (TransactionIdPrecedes(limit, safeLimit))
        {
                ereport(WARNING,
                                (errmsg("oldest xmin is far in the past"),
                                 errhint("Close open transactions soon to avoid wraparound problems.\n"
                                                 "You might also need to commit or roll back old prepared transactions, or drop stale replication slots.")));
                limit = *oldestXmin;
        }

        *freezeLimit = limit;

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

        /*
         * Determine the minimum multixact freeze age to use: as specified by
         * caller, or vacuum_multixact_freeze_min_age, but in any case not more
         * than half effective_multixact_freeze_max_age, so that autovacuums to
         * prevent MultiXact wraparound won't occur too frequently.
         */
        mxid_freezemin = multixact_freeze_min_age;
        if (mxid_freezemin < 0)
                mxid_freezemin = vacuum_multixact_freeze_min_age;
        mxid_freezemin = Min(mxid_freezemin,
                                                 effective_multixact_freeze_max_age / 2);
        Assert(mxid_freezemin >= 0);

        /* compute the cutoff multi, being careful to generate a valid value */
        mxactLimit = GetOldestMultiXactId() - mxid_freezemin;
        if (mxactLimit < FirstMultiXactId)
                mxactLimit = FirstMultiXactId;

        safeMxactLimit =
                ReadNextMultiXactId() - effective_multixact_freeze_max_age;
        if (safeMxactLimit < FirstMultiXactId)
                safeMxactLimit = FirstMultiXactId;

        if (MultiXactIdPrecedes(mxactLimit, safeMxactLimit))
        {
                ereport(WARNING,
                                (errmsg("oldest multixact is far in the past"),
                                 errhint("Close open transactions with multixacts soon to avoid wraparound problems.")));
                mxactLimit = safeMxactLimit;
        }

        *multiXactCutoff = mxactLimit;

        if (xidFullScanLimit != NULL)
        {
                int                     freezetable;

                Assert(mxactFullScanLimit != NULL);

                /*
                 * Determine the table freeze age to use: as specified by the caller,
                 * or vacuum_freeze_table_age, but in any case not more than
                 * autovacuum_freeze_max_age * 0.95, so that if you have e.g nightly
                 * VACUUM schedule, the nightly VACUUM gets a chance to freeze tuples
                 * before anti-wraparound autovacuum is launched.
                 */
                freezetable = freeze_table_age;
                if (freezetable < 0)
                        freezetable = vacuum_freeze_table_age;
                freezetable = Min(freezetable, autovacuum_freeze_max_age * 0.95);
                Assert(freezetable >= 0);

                /*
                 * Compute XID limit causing a full-table vacuum, being careful not to
                 * generate a "permanent" XID.
                 */
                limit = ReadNewTransactionId() - freezetable;
                if (!TransactionIdIsNormal(limit))
                        limit = FirstNormalTransactionId;

                *xidFullScanLimit = limit;

                /*
                 * Similar to the above, determine the table freeze age to use for
                 * multixacts: as specified by the caller, or
                 * vacuum_multixact_freeze_table_age, but in any case not more than
                 * autovacuum_multixact_freeze_table_age * 0.95, so that if you have
                 * e.g. nightly VACUUM schedule, the nightly VACUUM gets a chance to
                 * freeze multixacts before anti-wraparound autovacuum is launched.
                 */
                freezetable = multixact_freeze_table_age;
                if (freezetable < 0)
                        freezetable = vacuum_multixact_freeze_table_age;
                freezetable = Min(freezetable,
                                                  effective_multixact_freeze_max_age * 0.95);
                Assert(freezetable >= 0);

                /*
                 * Compute MultiXact limit causing a full-table vacuum, being careful
                 * to generate a valid MultiXact value.
                 */
                mxactLimit = ReadNextMultiXactId() - freezetable;
                if (mxactLimit < FirstMultiXactId)
                        mxactLimit = FirstMultiXactId;

                *mxactFullScanLimit = mxactLimit;
        }
        else
        {
                Assert(mxactFullScanLimit == NULL);
        }
}

/*
 * vac_estimate_reltuples() -- estimate the new value for pg_class.reltuples
 *
 *              If we scanned the whole relation then we should just use the count of
 *              live tuples seen; but if we did not, we should not blindly extrapolate
 *              from that number, since VACUUM may have scanned a quite nonrandom
 *              subset of the table.  When we have only partial information, we take
 *              the old value of pg_class.reltuples as a measurement of the
 *              tuple density in the unscanned pages.
 *
 *              Note: scanned_tuples should count only *live* tuples, since
 *              pg_class.reltuples is defined that way.
 */
double
vac_estimate_reltuples(Relation relation,
                                           BlockNumber total_pages,
                                           BlockNumber scanned_pages,
                                           double scanned_tuples)
{
        BlockNumber old_rel_pages = relation->rd_rel->relpages;
        double          old_rel_tuples = relation->rd_rel->reltuples;
        double          old_density;
        double          unscanned_pages;
        double          total_tuples;

        /* If we did scan the whole table, just use the count as-is */
        if (scanned_pages >= total_pages)
                return scanned_tuples;

        /*
         * If scanned_pages is zero but total_pages isn't, keep the existing value
         * of reltuples.  (Note: callers should avoid updating the pg_class
         * statistics in this situation, since no new information has been
         * provided.)
         */
        if (scanned_pages == 0)
                return old_rel_tuples;

        /*
         * If old value of relpages is zero, old density is indeterminate; we
         * can't do much except scale up scanned_tuples to match total_pages.
         */
        if (old_rel_pages == 0)
                return floor((scanned_tuples / scanned_pages) * total_pages + 0.5);

        /*
         * Okay, we've covered the corner cases.  The normal calculation is to
         * convert the old measurement to a density (tuples per page), then
         * estimate the number of tuples in the unscanned pages using that figure,
         * and finally add on the number of tuples in the scanned pages.
         */
        old_density = old_rel_tuples / old_rel_pages;
        unscanned_pages = (double) total_pages - (double) scanned_pages;
        total_tuples = old_density * unscanned_pages + scanned_tuples;
        return floor(total_tuples + 0.5);
}


/*
 *      vac_update_relstats() -- update statistics for one relation
 *
 *              Update the whole-relation statistics that are kept in its pg_class
 *              row.  There are additional stats that will be updated if we are
 *              doing ANALYZE, but we always update these stats.  This routine works
 *              for both index and heap relation entries in pg_class.
 *
 *              We violate transaction semantics here by overwriting the rel's
 *              existing pg_class tuple with the new values.  This is reasonably
 *              safe as long as we're sure that the new values are correct whether or
 *              not this transaction commits.  The reason for doing this is that if
 *              we updated these tuples in the usual way, vacuuming pg_class itself
 *              wouldn't work very well --- by the time we got done with a vacuum
 *              cycle, most of the tuples in pg_class would've been obsoleted.  Of
 *              course, this only works for fixed-size not-null columns, but these are.
 *
 *              Another reason for doing it this way is that when we are in a lazy
 *              VACUUM and have PROC_IN_VACUUM set, we mustn't do any regular updates.
 *              Somebody vacuuming pg_class might think they could delete a tuple
 *              marked with xmin = our xid.
 *
 *              In addition to fundamentally nontransactional statistics such as
 *              relpages and relallvisible, we try to maintain certain lazily-updated
 *              DDL flags such as relhasindex, by clearing them if no longer correct.
 *              It's safe to do this in VACUUM, which can't run in parallel with
 *              CREATE INDEX/RULE/TRIGGER and can't be part of a transaction block.
 *              However, it's *not* safe to do it in an ANALYZE that's within an
 *              outer transaction, because for example the current transaction might
 *              have dropped the last index; then we'd think relhasindex should be
 *              cleared, but if the transaction later rolls back this would be wrong.
 *              So we refrain from updating the DDL flags if we're inside an outer
 *              transaction.  This is OK since postponing the flag maintenance is
 *              always allowable.
 *
 *              Note: num_tuples should count only *live* tuples, since
 *              pg_class.reltuples is defined that way.
 *
 *              This routine is shared by VACUUM and ANALYZE.
 */
void
vac_update_relstats(Relation relation,
                                        BlockNumber num_pages, double num_tuples,
                                        BlockNumber num_all_visible_pages,
                                        bool hasindex, TransactionId frozenxid,
                                        MultiXactId minmulti,
                                        bool in_outer_xact)
{
        Oid                     relid = RelationGetRelid(relation);
        Relation        rd;
        HeapTuple       ctup;
        Form_pg_class pgcform;
        bool            dirty;

        rd = table_open(RelationRelationId, RowExclusiveLock);

        /* Fetch a copy of the tuple to scribble on */
        ctup = SearchSysCacheCopy1(RELOID, ObjectIdGetDatum(relid));
        if (!HeapTupleIsValid(ctup))
                elog(ERROR, "pg_class entry for relid %u vanished during vacuuming",
                         relid);
        pgcform = (Form_pg_class) GETSTRUCT(ctup);

        /* Apply statistical updates, if any, to copied tuple */

        dirty = false;
        if (pgcform->relpages != (int32) num_pages)
        {
                pgcform->relpages = (int32) num_pages;
                dirty = true;
        }
        if (pgcform->reltuples != (float4) num_tuples)
        {
                pgcform->reltuples = (float4) num_tuples;
                dirty = true;
        }
        if (pgcform->relallvisible != (int32) num_all_visible_pages)
        {
                pgcform->relallvisible = (int32) num_all_visible_pages;
                dirty = true;
        }

        /* Apply DDL updates, but not inside an outer transaction (see above) */

        if (!in_outer_xact)
        {
                /*
                 * If we didn't find any indexes, reset relhasindex.
                 */
                if (pgcform->relhasindex && !hasindex)
                {
                        pgcform->relhasindex = false;
                        dirty = true;
                }

                /* We also clear relhasrules and relhastriggers if needed */
                if (pgcform->relhasrules && relation->rd_rules == NULL)
                {
                        pgcform->relhasrules = false;
                        dirty = true;
                }
                if (pgcform->relhastriggers && relation->trigdesc == NULL)
                {
                        pgcform->relhastriggers = false;
                        dirty = true;
                }
        }

        /*
         * Update relfrozenxid, unless caller passed InvalidTransactionId
         * indicating it has no new data.
         *
         * Ordinarily, we don't let relfrozenxid go backwards: if things are
         * working correctly, the only way the new frozenxid could be older would
         * be if a previous VACUUM was done with a tighter freeze_min_age, in
         * which case we don't want to forget the work it already did.  However,
         * if the stored relfrozenxid is "in the future", then it must be corrupt
         * and it seems best to overwrite it with the cutoff we used this time.
         * This should match vac_update_datfrozenxid() concerning what we consider
         * to be "in the future".
         */
        if (TransactionIdIsNormal(frozenxid) &&
                pgcform->relfrozenxid != frozenxid &&
                (TransactionIdPrecedes(pgcform->relfrozenxid, frozenxid) ||
                 TransactionIdPrecedes(ReadNewTransactionId(),
                                                           pgcform->relfrozenxid)))
        {
                pgcform->relfrozenxid = frozenxid;
                dirty = true;
        }

        /* Similarly for relminmxid */
        if (MultiXactIdIsValid(minmulti) &&
                pgcform->relminmxid != minmulti &&
                (MultiXactIdPrecedes(pgcform->relminmxid, minmulti) ||
                 MultiXactIdPrecedes(ReadNextMultiXactId(), pgcform->relminmxid)))
        {
                pgcform->relminmxid = minmulti;
                dirty = true;
        }

        /* If anything changed, write out the tuple. */
        if (dirty)
                heap_inplace_update(rd, ctup);

        table_close(rd, RowExclusiveLock);
}


/*
 *      vac_update_datfrozenxid() -- update pg_database.datfrozenxid for our DB
 *
 *              Update pg_database's datfrozenxid entry for our database to be the
 *              minimum of the pg_class.relfrozenxid values.
 *
 *              Similarly, update our datminmxid to be the minimum of the
 *              pg_class.relminmxid values.
 *
 *              If we are able to advance either pg_database value, also try to
 *              truncate pg_xact and pg_multixact.
 *
 *              We violate transaction semantics here by overwriting the database's
 *              existing pg_database tuple with the new values.  This is reasonably
 *              safe since the new values are correct whether or not this transaction
 *              commits.  As with vac_update_relstats, this avoids leaving dead tuples
 *              behind after a VACUUM.
 */
void
vac_update_datfrozenxid(void)
{
        HeapTuple       tuple;
        Form_pg_database dbform;
        Relation        relation;
        SysScanDesc scan;
        HeapTuple       classTup;
        TransactionId newFrozenXid;
        MultiXactId newMinMulti;
        TransactionId lastSaneFrozenXid;
        MultiXactId lastSaneMinMulti;
        bool            bogus = false;
        bool            dirty = false;

        /*
         * Initialize the "min" calculation with GetOldestXmin, which is a
         * reasonable approximation to the minimum relfrozenxid for not-yet-
         * committed pg_class entries for new tables; see AddNewRelationTuple().
         * So we cannot produce a wrong minimum by starting with this.
         */
        newFrozenXid = GetOldestXmin(NULL, PROCARRAY_FLAGS_VACUUM);

        /*
         * Similarly, initialize the MultiXact "min" with the value that would be
         * used on pg_class for new tables.  See AddNewRelationTuple().
         */
        newMinMulti = GetOldestMultiXactId();

        /*
         * Identify the latest relfrozenxid and relminmxid values that we could
         * validly see during the scan.  These are conservative values, but it's
         * not really worth trying to be more exact.
         */
        lastSaneFrozenXid = ReadNewTransactionId();
        lastSaneMinMulti = ReadNextMultiXactId();

        /*
         * We must seqscan pg_class to find the minimum Xid, because there is no
         * index that can help us here.
         */
        relation = table_open(RelationRelationId, AccessShareLock);

        scan = systable_beginscan(relation, InvalidOid, false,
                                                          NULL, 0, NULL);

        while ((classTup = systable_getnext(scan)) != NULL)
        {
                Form_pg_class classForm = (Form_pg_class) GETSTRUCT(classTup);

                /*
                 * Only consider relations able to hold unfrozen XIDs (anything else
                 * should have InvalidTransactionId in relfrozenxid anyway.)
                 */
                if (classForm->relkind != RELKIND_RELATION &&
                        classForm->relkind != RELKIND_MATVIEW &&
                        classForm->relkind != RELKIND_TOASTVALUE)
                        continue;

                Assert(TransactionIdIsNormal(classForm->relfrozenxid));
                Assert(MultiXactIdIsValid(classForm->relminmxid));

                /*
                 * If things are working properly, no relation should have a
                 * relfrozenxid or relminmxid that is "in the future".  However, such
                 * cases have been known to arise due to bugs in pg_upgrade.  If we
                 * see any entries that are "in the future", chicken out and don't do
                 * anything.  This ensures we won't truncate clog before those
                 * relations have been scanned and cleaned up.
                 */
                if (TransactionIdPrecedes(lastSaneFrozenXid, classForm->relfrozenxid) ||
                        MultiXactIdPrecedes(lastSaneMinMulti, classForm->relminmxid))
                {
                        bogus = true;
                        break;
                }

                if (TransactionIdPrecedes(classForm->relfrozenxid, newFrozenXid))
                        newFrozenXid = classForm->relfrozenxid;

                if (MultiXactIdPrecedes(classForm->relminmxid, newMinMulti))
                        newMinMulti = classForm->relminmxid;
        }

        /* we're done with pg_class */
        systable_endscan(scan);
        table_close(relation, AccessShareLock);

        /* chicken out if bogus data found */
        if (bogus)
                return;

        Assert(TransactionIdIsNormal(newFrozenXid));
        Assert(MultiXactIdIsValid(newMinMulti));

        /* Now fetch the pg_database tuple we need to update. */
        relation = table_open(DatabaseRelationId, RowExclusiveLock);

        /* Fetch a copy of the tuple to scribble on */
        tuple = SearchSysCacheCopy1(DATABASEOID, ObjectIdGetDatum(MyDatabaseId));
        if (!HeapTupleIsValid(tuple))
                elog(ERROR, "could not find tuple for database %u", MyDatabaseId);
        dbform = (Form_pg_database) GETSTRUCT(tuple);

        /*
         * As in vac_update_relstats(), we ordinarily don't want to let
         * datfrozenxid go backward; but if it's "in the future" then it must be
         * corrupt and it seems best to overwrite it.
         */
        if (dbform->datfrozenxid != newFrozenXid &&
                (TransactionIdPrecedes(dbform->datfrozenxid, newFrozenXid) ||
                 TransactionIdPrecedes(lastSaneFrozenXid, dbform->datfrozenxid)))
        {
                dbform->datfrozenxid = newFrozenXid;
                dirty = true;
        }
        else
                newFrozenXid = dbform->datfrozenxid;

        /* Ditto for datminmxid */
        if (dbform->datminmxid != newMinMulti &&
                (MultiXactIdPrecedes(dbform->datminmxid, newMinMulti) ||
                 MultiXactIdPrecedes(lastSaneMinMulti, dbform->datminmxid)))
        {
                dbform->datminmxid = newMinMulti;
                dirty = true;
        }
        else
                newMinMulti = dbform->datminmxid;

        if (dirty)
                heap_inplace_update(relation, tuple);

        heap_freetuple(tuple);
        table_close(relation, RowExclusiveLock);

        /*
         * If we were able to advance datfrozenxid or datminmxid, see if we can
         * truncate pg_xact and/or pg_multixact.  Also do it if the shared
         * XID-wrap-limit info is stale, since this action will update that too.
         */
        if (dirty || ForceTransactionIdLimitUpdate())
                vac_truncate_clog(newFrozenXid, newMinMulti,
                                                  lastSaneFrozenXid, lastSaneMinMulti);
}


/*
 *      vac_truncate_clog() -- attempt to truncate the commit log
 *
 *              Scan pg_database to determine the system-wide oldest datfrozenxid,
 *              and use it to truncate the transaction commit log (pg_xact).
 *              Also update the XID wrap limit info maintained by varsup.c.
 *              Likewise for datminmxid.
 *
 *              The passed frozenXID and minMulti are the updated values for my own
 *              pg_database entry. They're used to initialize the "min" calculations.
 *              The caller also passes the "last sane" XID and MXID, since it has
 *              those at hand already.
 *
 *              This routine is only invoked when we've managed to change our
 *              DB's datfrozenxid/datminmxid values, or we found that the shared
 *              XID-wrap-limit info is stale.
 */
static void
vac_truncate_clog(TransactionId frozenXID,
                                  MultiXactId minMulti,
                                  TransactionId lastSaneFrozenXid,
                                  MultiXactId lastSaneMinMulti)
{
        TransactionId nextXID = ReadNewTransactionId();
        Relation        relation;
        TableScanDesc scan;
        HeapTuple       tuple;
        Oid                     oldestxid_datoid;
        Oid                     minmulti_datoid;
        bool            bogus = false;
        bool            frozenAlreadyWrapped = false;

        /* init oldest datoids to sync with my frozenXID/minMulti values */
        oldestxid_datoid = MyDatabaseId;
        minmulti_datoid = MyDatabaseId;

        /*
         * Scan pg_database to compute the minimum datfrozenxid/datminmxid
         *
         * Since vac_update_datfrozenxid updates datfrozenxid/datminmxid in-place,
         * the values could change while we look at them.  Fetch each one just
         * once to ensure sane behavior of the comparison logic.  (Here, as in
         * many other places, we assume that fetching or updating an XID in shared
         * storage is atomic.)
         *
         * Note: we need not worry about a race condition with new entries being
         * inserted by CREATE DATABASE.  Any such entry will have a copy of some
         * existing DB's datfrozenxid, and that source DB cannot be ours because
         * of the interlock against copying a DB containing an active backend.
         * Hence the new entry will not reduce the minimum.  Also, if two VACUUMs
         * concurrently modify the datfrozenxid's of different databases, the
         * worst possible outcome is that pg_xact is not truncated as aggressively
         * as it could be.
         */
        relation = table_open(DatabaseRelationId, AccessShareLock);

        scan = table_beginscan_catalog(relation, 0, NULL);

        while ((tuple = heap_getnext(scan, ForwardScanDirection)) != NULL)
        {
                volatile FormData_pg_database *dbform = (Form_pg_database) GETSTRUCT(tuple);
                TransactionId datfrozenxid = dbform->datfrozenxid;
                TransactionId datminmxid = dbform->datminmxid;

                Assert(TransactionIdIsNormal(datfrozenxid));
                Assert(MultiXactIdIsValid(datminmxid));

                /*
                 * If things are working properly, no database should have a
                 * datfrozenxid or datminmxid that is "in the future".  However, such
                 * cases have been known to arise due to bugs in pg_upgrade.  If we
                 * see any entries that are "in the future", chicken out and don't do
                 * anything.  This ensures we won't truncate clog before those
                 * databases have been scanned and cleaned up.  (We will issue the
                 * "already wrapped" warning if appropriate, though.)
                 */
                if (TransactionIdPrecedes(lastSaneFrozenXid, datfrozenxid) ||
                        MultiXactIdPrecedes(lastSaneMinMulti, datminmxid))
                        bogus = true;

                if (TransactionIdPrecedes(nextXID, datfrozenxid))
                        frozenAlreadyWrapped = true;
                else if (TransactionIdPrecedes(datfrozenxid, frozenXID))
                {
                        frozenXID = datfrozenxid;
                        oldestxid_datoid = dbform->oid;
                }

                if (MultiXactIdPrecedes(datminmxid, minMulti))
                {
                        minMulti = datminmxid;
                        minmulti_datoid = dbform->oid;
                }
        }

        table_endscan(scan);

        table_close(relation, AccessShareLock);

        /*
         * Do not truncate CLOG if we seem to have suffered wraparound already;
         * the computed minimum XID might be bogus.  This case should now be
         * impossible due to the defenses in GetNewTransactionId, but we keep the
         * test anyway.
         */
        if (frozenAlreadyWrapped)
        {
                ereport(WARNING,
                                (errmsg("some databases have not been vacuumed in over 2 billion transactions"),
                                 errdetail("You might have already suffered transaction-wraparound data loss.")));
                return;
        }

        /* chicken out if data is bogus in any other way */
        if (bogus)
                return;

        /*
         * Advance the oldest value for commit timestamps before truncating, so
         * that if a user requests a timestamp for a transaction we're truncating
         * away right after this point, they get NULL instead of an ugly "file not
         * found" error from slru.c.  This doesn't matter for xact/multixact
         * because they are not subject to arbitrary lookups from users.
         */
        AdvanceOldestCommitTsXid(frozenXID);

        /*
         * Truncate CLOG, multixact and CommitTs to the oldest computed value.
         */
        TruncateCLOG(frozenXID, oldestxid_datoid);
        TruncateCommitTs(frozenXID);
        TruncateMultiXact(minMulti, minmulti_datoid);

        /*
         * Update the wrap limit for GetNewTransactionId and creation of new
         * MultiXactIds.  Note: these functions will also signal the postmaster
         * for an(other) autovac cycle if needed.   XXX should we avoid possibly
         * signalling twice?
         */
        SetTransactionIdLimit(frozenXID, oldestxid_datoid);
        SetMultiXactIdLimit(minMulti, minmulti_datoid, false);
}


/*
 *      vacuum_rel() -- vacuum one heap relation
 *
 *              relid identifies the relation to vacuum.  If relation is supplied,
 *              use the name therein for reporting any failure to open/lock the rel;
 *              do not use it once we've successfully opened the rel, since it might
 *              be stale.
 *
 *              Returns true if it's okay to proceed with a requested ANALYZE
 *              operation on this table.
 *
 *              Doing one heap at a time incurs extra overhead, since we need to
 *              check that the heap exists again just before we vacuum it.  The
 *              reason that we do this is so that vacuuming can be spread across
 *              many small transactions.  Otherwise, two-phase locking would require
 *              us to lock the entire database during one pass of the vacuum cleaner.
 *
 *              At entry and exit, we are not inside a transaction.
 */
static bool
vacuum_rel(Oid relid, RangeVar *relation, VacuumParams *params)
{
        LOCKMODE        lmode;
        Relation        onerel;
        LockRelId       onerelid;
        Oid                     toast_relid;
        Oid                     save_userid;
        int                     save_sec_context;
        int                     save_nestlevel;

        Assert(params != NULL);

        /* Begin a transaction for vacuuming this relation */
        StartTransactionCommand();

        /*
         * Functions in indexes may want a snapshot set.  Also, setting a snapshot
         * ensures that RecentGlobalXmin is kept truly recent.
         */
        PushActiveSnapshot(GetTransactionSnapshot());

        if (!(params->options & VACOPT_FULL))
        {
                /*
                 * In lazy vacuum, we can set the PROC_IN_VACUUM flag, which lets
                 * other concurrent VACUUMs know that they can ignore this one while
                 * determining their OldestXmin.  (The reason we don't set it during a
                 * full VACUUM is exactly that we may have to run user-defined
                 * functions for functional indexes, and we want to make sure that if
                 * they use the snapshot set above, any tuples it requires can't get
                 * removed from other tables.  An index function that depends on the
                 * contents of other tables is arguably broken, but we won't break it
                 * here by violating transaction semantics.)
                 *
                 * We also set the VACUUM_FOR_WRAPAROUND flag, which is passed down by
                 * autovacuum; it's used to avoid canceling a vacuum that was invoked
                 * in an emergency.
                 *
                 * Note: these flags remain set until CommitTransaction or
                 * AbortTransaction.  We don't want to clear them until we reset
                 * MyPgXact->xid/xmin, else OldestXmin might appear to go backwards,
                 * which is probably Not Good.
                 */
                LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
                MyPgXact->vacuumFlags |= PROC_IN_VACUUM;
                if (params->is_wraparound)
                        MyPgXact->vacuumFlags |= PROC_VACUUM_FOR_WRAPAROUND;
                LWLockRelease(ProcArrayLock);
        }

        /*
         * Check for user-requested abort.  Note we want this to be inside a
         * transaction, so xact.c doesn't issue useless WARNING.
         */
        CHECK_FOR_INTERRUPTS();

        /*
         * Determine the type of lock we want --- hard exclusive lock for a FULL
         * vacuum, but just ShareUpdateExclusiveLock for concurrent vacuum. Either
         * way, we can be sure that no other backend is vacuuming the same table.
         */
        lmode = (params->options & VACOPT_FULL) ?
                AccessExclusiveLock : ShareUpdateExclusiveLock;

        /* open the relation and get the appropriate lock on it */
        onerel = vacuum_open_relation(relid, relation, params->options,
                                                                  params->log_min_duration >= 0, lmode);

        /* leave if relation could not be opened or locked */
        if (!onerel)
        {
                PopActiveSnapshot();
                CommitTransactionCommand();
                return false;
        }

        /*
         * Check if relation needs to be skipped based on ownership.  This check
         * happens also when building the relation list to vacuum for a manual
         * operation, and needs to be done additionally here as VACUUM could
         * happen across multiple transactions where relation ownership could have
         * changed in-between.  Make sure to only generate logs for VACUUM in this
         * case.
         */
        if (!vacuum_is_relation_owner(RelationGetRelid(onerel),
                                                                  onerel->rd_rel,
                                                                  params->options & VACOPT_VACUUM))
        {
                relation_close(onerel, lmode);
                PopActiveSnapshot();
                CommitTransactionCommand();
                return false;
        }

        /*
         * Check that it's of a vacuumable relkind.
         */
        if (onerel->rd_rel->relkind != RELKIND_RELATION &&
                onerel->rd_rel->relkind != RELKIND_MATVIEW &&
                onerel->rd_rel->relkind != RELKIND_TOASTVALUE &&
                onerel->rd_rel->relkind != RELKIND_PARTITIONED_TABLE)
        {
                ereport(WARNING,
                                (errmsg("skipping \"%s\" --- cannot vacuum non-tables or special system tables",
                                                RelationGetRelationName(onerel))));
                relation_close(onerel, lmode);
                PopActiveSnapshot();
                CommitTransactionCommand();
                return false;
        }

        /*
         * Silently ignore tables that are temp tables of other backends ---
         * trying to vacuum these will lead to great unhappiness, since their
         * contents are probably not up-to-date on disk.  (We don't throw a
         * warning here; it would just lead to chatter during a database-wide
         * VACUUM.)
         */
        if (RELATION_IS_OTHER_TEMP(onerel))
        {
                relation_close(onerel, lmode);
                PopActiveSnapshot();
                CommitTransactionCommand();
                return false;
        }

        /*
         * Silently ignore partitioned tables as there is no work to be done.  The
         * useful work is on their child partitions, which have been queued up for
         * us separately.
         */
        if (onerel->rd_rel->relkind == RELKIND_PARTITIONED_TABLE)
        {
                relation_close(onerel, lmode);
                PopActiveSnapshot();
                CommitTransactionCommand();
                /* It's OK to proceed with ANALYZE on this table */
                return true;
        }

        /*
         * Get a session-level lock too. This will protect our access to the
         * relation across multiple transactions, so that we can vacuum the
         * relation's TOAST table (if any) secure in the knowledge that no one is
         * deleting the parent relation.
         *
         * NOTE: this cannot block, even if someone else is waiting for access,
         * because the lock manager knows that both lock requests are from the
         * same process.
         */
        onerelid = onerel->rd_lockInfo.lockRelId;
        LockRelationIdForSession(&onerelid, lmode);

        /*
         * Remember the relation's TOAST relation for later, if the caller asked
         * us to process it.  In VACUUM FULL, though, the toast table is
         * automatically rebuilt by cluster_rel so we shouldn't recurse to it.
         */
        if (!(params->options & VACOPT_SKIPTOAST) && !(params->options & VACOPT_FULL))
                toast_relid = onerel->rd_rel->reltoastrelid;
        else
                toast_relid = InvalidOid;

        /*
         * Switch to the table owner's userid, so that any index functions are run
         * as that user.  Also lock down security-restricted operations and
         * arrange to make GUC variable changes local to this command. (This is
         * unnecessary, but harmless, for lazy VACUUM.)
         */
        GetUserIdAndSecContext(&save_userid, &save_sec_context);
        SetUserIdAndSecContext(onerel->rd_rel->relowner,
                                                   save_sec_context | SECURITY_RESTRICTED_OPERATION);
        save_nestlevel = NewGUCNestLevel();

        /*
         * Do the actual work --- either FULL or "lazy" vacuum
         */
        if (params->options & VACOPT_FULL)
        {
                int                     cluster_options = 0;

                /* close relation before vacuuming, but hold lock until commit */
                relation_close(onerel, NoLock);
                onerel = NULL;

                if ((params->options & VACOPT_VERBOSE) != 0)
                        cluster_options |= CLUOPT_VERBOSE;

                /* VACUUM FULL is now a variant of CLUSTER; see cluster.c */
                cluster_rel(relid, InvalidOid, cluster_options);
        }
        else
                heap_vacuum_rel(onerel, params, vac_strategy);

        /* Roll back any GUC changes executed by index functions */
        AtEOXact_GUC(false, save_nestlevel);

        /* Restore userid and security context */
        SetUserIdAndSecContext(save_userid, save_sec_context);

        /* all done with this class, but hold lock until commit */
        if (onerel)
                relation_close(onerel, NoLock);

        /*
         * Complete the transaction and free all temporary memory used.
         */
        PopActiveSnapshot();
        CommitTransactionCommand();

        /*
         * If the relation has a secondary toast rel, vacuum that too while we
         * still hold the session lock on the master table.  Note however that
         * "analyze" will not get done on the toast table.  This is good, because
         * the toaster always uses hardcoded index access and statistics are
         * totally unimportant for toast relations.
         */
        if (toast_relid != InvalidOid)
                vacuum_rel(toast_relid, NULL, params);

        /*
         * Now release the session-level lock on the master table.
         */
        UnlockRelationIdForSession(&onerelid, lmode);

        /* Report that we really did it. */
        return true;
}


/*
 * Open all the vacuumable indexes of the given relation, obtaining the
 * specified kind of lock on each.  Return an array of Relation pointers for
 * the indexes into *Irel, and the number of indexes into *nindexes.
 *
 * We consider an index vacuumable if it is marked insertable (indisready).
 * If it isn't, probably a CREATE INDEX CONCURRENTLY command failed early in
 * execution, and what we have is too corrupt to be processable.  We will
 * vacuum even if the index isn't indisvalid; this is important because in a
 * unique index, uniqueness checks will be performed anyway and had better not
 * hit dangling index pointers.
 */
void
vac_open_indexes(Relation relation, LOCKMODE lockmode,
                                 int *nindexes, Relation **Irel)
{
        List       *indexoidlist;
        ListCell   *indexoidscan;
        int                     i;

        Assert(lockmode != NoLock);

        indexoidlist = RelationGetIndexList(relation);

        /* allocate enough memory for all indexes */
        i = list_length(indexoidlist);

        if (i > 0)
                *Irel = (Relation *) palloc(i * sizeof(Relation));
        else
                *Irel = NULL;

        /* collect just the ready indexes */
        i = 0;
        foreach(indexoidscan, indexoidlist)
        {
                Oid                     indexoid = lfirst_oid(indexoidscan);
                Relation        indrel;

                indrel = index_open(indexoid, lockmode);
                if (indrel->rd_index->indisready)
                        (*Irel)[i++] = indrel;
                else
                        index_close(indrel, lockmode);
        }

        *nindexes = i;

        list_free(indexoidlist);
}

/*
 * Release the resources acquired by vac_open_indexes.  Optionally release
 * the locks (say NoLock to keep 'em).
 */
void
vac_close_indexes(int nindexes, Relation *Irel, LOCKMODE lockmode)
{
        if (Irel == NULL)
                return;

        while (nindexes--)
        {
                Relation        ind = Irel[nindexes];

                index_close(ind, lockmode);
        }
        pfree(Irel);
}

/*
 * vacuum_delay_point --- check for interrupts and cost-based delay.
 *
 * This should be called in each major loop of VACUUM processing,
 * typically once per page processed.
 */
void
vacuum_delay_point(void)
{
        /* Always check for interrupts */
        CHECK_FOR_INTERRUPTS();

        /* Nap if appropriate */
        if (VacuumCostActive && !InterruptPending &&
                VacuumCostBalance >= VacuumCostLimit)
        {
                double          msec;

                msec = VacuumCostDelay * VacuumCostBalance / VacuumCostLimit;
                if (msec > VacuumCostDelay * 4)
                        msec = VacuumCostDelay * 4;

                pg_usleep((long) (msec * 1000));

                VacuumCostBalance = 0;

                /* update balance values for workers */
                AutoVacuumUpdateDelay();

                /* Might have gotten an interrupt while sleeping */
                CHECK_FOR_INTERRUPTS();
        }
}