Fix more instances of "the the" in comments.

[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) 1996-2013, 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/genam.h"
#include "access/heapam.h"
#include "access/htup_details.h"
#include "access/multixact.h"
#include "access/transam.h"
#include "access/xact.h"
#include "catalog/namespace.h"
#include "catalog/pg_database.h"
#include "catalog/pg_namespace.h"
#include "commands/cluster.h"
#include "commands/vacuum.h"
#include "miscadmin.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"
#include "utils/tqual.h"


/*
 * GUC parameters
 */
int                     vacuum_freeze_min_age;
int                     vacuum_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 *get_rel_oids(Oid relid, const RangeVar *vacrel);
static void vac_truncate_clog(TransactionId frozenXID, MultiXactId minMulti);
static bool vacuum_rel(Oid relid, VacuumStmt *vacstmt, bool do_toast,
                   bool for_wraparound);


/*
 * Primary entry point for VACUUM and ANALYZE commands.
 *
 * relid is normally InvalidOid; if it is not, then it provides the relation
 * OID to be processed, and vacstmt->relation is ignored.  (The non-invalid
 * case is currently only used by autovacuum.)
 *
 * do_toast is passed as FALSE by autovacuum, because it processes TOAST
 * tables separately.
 *
 * for_wraparound is used by autovacuum to let us know when it's forcing
 * a vacuum for wraparound, which should not be auto-canceled.
 *
 * 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 vacstmt and bstrategy
 * (if given) be allocated in a memory context that won't disappear
 * at transaction commit.
 */
void
vacuum(VacuumStmt *vacstmt, Oid relid, bool do_toast,
           BufferAccessStrategy bstrategy, bool for_wraparound, bool isTopLevel)
{
        const char *stmttype;
        volatile bool in_outer_xact,
                                use_own_xacts;
        List       *relations;

        /* sanity checks on options */
        Assert(vacstmt->options & (VACOPT_VACUUM | VACOPT_ANALYZE));
        Assert((vacstmt->options & VACOPT_VACUUM) ||
                   !(vacstmt->options & (VACOPT_FULL | VACOPT_FREEZE)));
        Assert((vacstmt->options & VACOPT_ANALYZE) || vacstmt->va_cols == NIL);

        stmttype = (vacstmt->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 (vacstmt->options & VACOPT_VACUUM)
        {
                PreventTransactionChain(isTopLevel, stmttype);
                in_outer_xact = false;
        }
        else
                in_outer_xact = IsInTransactionChain(isTopLevel);

        /*
         * Send info about dead objects to the statistics collector, unless we are
         * in autovacuum --- autovacuum.c does this for itself.
         */
        if ((vacstmt->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_MINSIZE,
                                                                                ALLOCSET_DEFAULT_INITSIZE,
                                                                                ALLOCSET_DEFAULT_MAXSIZE);

        /*
         * 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 relations to process, unless caller gave us one. (If we
         * build one, we put it in vac_context for safekeeping.)
         */
        relations = get_rel_oids(relid, vacstmt->relation);

        /*
         * 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 (vacstmt->options & VACOPT_VACUUM)
                use_own_xacts = true;
        else
        {
                Assert(vacstmt->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)
        {
                /* ActiveSnapshot is not set by autovacuum */
                if (ActiveSnapshotSet())
                        PopActiveSnapshot();

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

        /* Turn vacuum cost accounting on or off */
        PG_TRY();
        {
                ListCell   *cur;

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

                /*
                 * Loop to process each selected relation.
                 */
                foreach(cur, relations)
                {
                        Oid                     relid = lfirst_oid(cur);

                        if (vacstmt->options & VACOPT_VACUUM)
                        {
                                if (!vacuum_rel(relid, vacstmt, do_toast, for_wraparound))
                                        continue;
                        }

                        if (vacstmt->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(relid, vacstmt, vac_strategy);

                                if (use_own_xacts)
                                {
                                        PopActiveSnapshot();
                                        CommitTransactionCommand();
                                }
                        }
                }
        }
        PG_CATCH();
        {
                /* Make sure cost accounting is turned off after error */
                VacuumCostActive = false;
                PG_RE_THROW();
        }
        PG_END_TRY();

        /* Turn off vacuum cost accounting */
        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 ((vacstmt->options & VACOPT_VACUUM) && !IsAutoVacuumWorkerProcess())
        {
                /*
                 * Update pg_database.datfrozenxid, and truncate pg_clog 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;
}

/*
 * Build a list of Oids for each relation to be processed
 *
 * The list is built in vac_context so that it will survive across our
 * per-relation transactions.
 */
static List *
get_rel_oids(Oid relid, const RangeVar *vacrel)
{
        List       *oid_list = NIL;
        MemoryContext oldcontext;

        /* OID supplied by VACUUM's caller? */
        if (OidIsValid(relid))
        {
                oldcontext = MemoryContextSwitchTo(vac_context);
                oid_list = lappend_oid(oid_list, relid);
                MemoryContextSwitchTo(oldcontext);
        }
        else if (vacrel)
        {
                /* Process a specific relation */
                Oid                     relid;

                /*
                 * Since we don't take a lock here, the relation might be gone, or the
                 * RangeVar might no longer refer to the OID we look up here.  In the
                 * former case, VACUUM will do nothing; in the latter case, it will
                 * process the OID we looked up here, rather than the new one. Neither
                 * is ideal, but there's little practical alternative, since we're
                 * going to commit this transaction and begin a new one between now
                 * and then.
                 */
                relid = RangeVarGetRelid(vacrel, NoLock, false);

                /* Make a relation list entry for this guy */
                oldcontext = MemoryContextSwitchTo(vac_context);
                oid_list = lappend_oid(oid_list, relid);
                MemoryContextSwitchTo(oldcontext);
        }
        else
        {
                /*
                 * Process all plain relations and materialized views listed in
                 * pg_class
                 */
                Relation        pgclass;
                HeapScanDesc scan;
                HeapTuple       tuple;

                pgclass = heap_open(RelationRelationId, AccessShareLock);

                scan = heap_beginscan_catalog(pgclass, 0, NULL);

                while ((tuple = heap_getnext(scan, ForwardScanDirection)) != NULL)
                {
                        Form_pg_class classForm = (Form_pg_class) GETSTRUCT(tuple);

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

                        /* Make a relation list entry for this guy */
                        oldcontext = MemoryContextSwitchTo(vac_context);
                        oid_list = lappend_oid(oid_list, HeapTupleGetOid(tuple));
                        MemoryContextSwitchTo(oldcontext);
                }

                heap_endscan(scan);
                heap_close(pgclass, AccessShareLock);
        }

        return oid_list;
}

/*
 * 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(int freeze_min_age,
                                          int freeze_table_age,
                                          bool sharedRel,
                                          TransactionId *oldestXmin,
                                          TransactionId *freezeLimit,
                                          TransactionId *xidFullScanLimit,
                                          MultiXactId *multiXactCutoff,
                                          MultiXactId *mxactFullScanLimit)
{
        int                     freezemin;
        TransactionId limit;
        TransactionId safeLimit;
        MultiXactId     mxactLimit;

        /*
         * 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 = GetOldestXmin(sharedRel, true);

        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.")));
                limit = *oldestXmin;
        }

        *freezeLimit = limit;

        /*
         * simplistic MultiXactId removal limit: use the same policy as for
         * freezing Xids (except we use the oldest known mxact instead of the
         * current next value).
         */
        mxactLimit = GetOldestMultiXactId() - freezemin;
        if (mxactLimit < FirstMultiXactId)
                mxactLimit = FirstMultiXactId;
        *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;

                /*
                 * Compute MultiXactId limit to cause a full-table vacuum, being
                 * careful not to generate an invalid multi. We just copy the logic
                 * (and limits) from plain XIDs here.
                 */
                mxactLimit = ReadNextMultiXactId() - freezetable;
                if (mxactLimit < FirstMultiXactId)
                        mxactLimit = FirstMultiXactId;

                *mxactFullScanLimit = mxactLimit;
        }
}

/*
 * 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 trust the count
 *              unreservedly, especially not in VACUUM, which 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.
 *
 *              This routine is shared by VACUUM and ANALYZE.
 */
double
vac_estimate_reltuples(Relation relation, bool is_analyze,
                                           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          new_density;
        double          multiplier;
        double          updated_density;

        /* 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 update
         * the density using an exponential-moving-average approach, and finally
         * compute reltuples as updated_density * total_pages.
         *
         * For ANALYZE, the moving average multiplier is just the fraction of the
         * table's pages we scanned.  This is equivalent to assuming that the
         * tuple density in the unscanned pages didn't change.  Of course, it
         * probably did, if the new density measurement is different. But over
         * repeated cycles, the value of reltuples will converge towards the
         * correct value, if repeated measurements show the same new density.
         *
         * For VACUUM, the situation is a bit different: we have looked at a
         * nonrandom sample of pages, but we know for certain that the pages we
         * didn't look at are precisely the ones that haven't changed lately.
         * Thus, there is a reasonable argument for doing exactly the same thing
         * as for the ANALYZE case, that is use the old density measurement as the
         * value for the unscanned pages.
         *
         * This logic could probably use further refinement.
         */
        old_density = old_rel_tuples / old_rel_pages;
        new_density = scanned_tuples / scanned_pages;
        multiplier = (double) scanned_pages / (double) total_pages;
        updated_density = old_density + (new_density - old_density) * multiplier;
        return floor(updated_density * total_pages + 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 since the new values are correct whether or not this transaction
 *              commits.  The reason for 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 never-null columns, but these are.
 *
 *              Note another assumption: that two VACUUMs/ANALYZEs on a table can't
 *              run in parallel, nor can VACUUM/ANALYZE run in parallel with a
 *              schema alteration such as adding an index, rule, or trigger.  Otherwise
 *              our updates of relhasindex etc might overwrite uncommitted updates.
 *
 *              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 updates ---
 *              somebody vacuuming pg_class might think they could delete a tuple
 *              marked with xmin = our xid.
 *
 *              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)
{
        Oid                     relid = RelationGetRelid(relation);
        Relation        rd;
        HeapTuple       ctup;
        Form_pg_class pgcform;
        bool            dirty;

        rd = heap_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 required 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;
        }
        if (pgcform->relhasindex != hasindex)
        {
                pgcform->relhasindex = hasindex;
                dirty = true;
        }

        /*
         * If we have discovered that there are no indexes, then there's no
         * primary key either.  This could be done more thoroughly...
         */
        if (pgcform->relhaspkey && !hasindex)
        {
                pgcform->relhaspkey = 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;
        }

        /*
         * relfrozenxid should never go backward.  Caller can pass
         * InvalidTransactionId if it has no new data.
         */
        if (TransactionIdIsNormal(frozenxid) &&
                TransactionIdPrecedes(pgcform->relfrozenxid, frozenxid))
        {
                pgcform->relfrozenxid = frozenxid;
                dirty = true;
        }

        /* relminmxid must never go backward, either */
        if (MultiXactIdIsValid(minmulti) &&
                MultiXactIdPrecedes(pgcform->relminmxid, minmulti))
        {
                pgcform->relminmxid = minmulti;
                dirty = true;
        }

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

        heap_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_clog and pg_multixact.
 *
 *              We violate transaction semantics here by overwriting the database's
 *              existing pg_database tuple with the new value.  This is reasonably
 *              safe since the new value is 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;
        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(true, true);

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

        /*
         * We must seqscan pg_class to find the minimum Xid, because there is no
         * index that can help us here.
         */
        relation = heap_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 (TransactionIdPrecedes(classForm->relfrozenxid, newFrozenXid))
                        newFrozenXid = classForm->relfrozenxid;

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

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

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

        /* Now fetch the pg_database tuple we need to update. */
        relation = heap_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);

        /*
         * Don't allow datfrozenxid to go backward (probably can't happen anyway);
         * and detect the common case where it doesn't go forward either.
         */
        if (TransactionIdPrecedes(dbform->datfrozenxid, newFrozenXid))
        {
                dbform->datfrozenxid = newFrozenXid;
                dirty = true;
        }

        /* ditto */
        if (MultiXactIdPrecedes(dbform->datminmxid, newMinMulti))
        {
                dbform->datminmxid = newMinMulti;
                dirty = true;
        }

        if (dirty)
                heap_inplace_update(relation, tuple);

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

        /*
         * If we were able to advance datfrozenxid, see if we can truncate
         * pg_clog. 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);
}


/*
 *      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_clog).
 *              Also update the XID wrap limit info maintained by varsup.c.
 *
 *              The passed XID is simply the one I just wrote into my pg_database
 *              entry.  It's used to initialize the "min" calculation.
 *
 *              This routine is only invoked when we've managed to change our
 *              DB's datfrozenxid entry, or we found that the shared XID-wrap-limit
 *              info is stale.
 */
static void
vac_truncate_clog(TransactionId frozenXID, MultiXactId minMulti)
{
        TransactionId myXID = GetCurrentTransactionId();
        Relation        relation;
        HeapScanDesc scan;
        HeapTuple       tuple;
        Oid                     oldestxid_datoid;
        Oid                     minmulti_datoid;
        bool            frozenAlreadyWrapped = false;

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

        /*
         * Scan pg_database to compute the minimum datfrozenxid
         *
         * 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_clog is not truncated as aggressively
         * as it could be.
         */
        relation = heap_open(DatabaseRelationId, AccessShareLock);

        scan = heap_beginscan_catalog(relation, 0, NULL);

        while ((tuple = heap_getnext(scan, ForwardScanDirection)) != NULL)
        {
                Form_pg_database dbform = (Form_pg_database) GETSTRUCT(tuple);

                Assert(TransactionIdIsNormal(dbform->datfrozenxid));
                Assert(MultiXactIdIsValid(dbform->datminmxid));

                if (TransactionIdPrecedes(myXID, dbform->datfrozenxid))
                        frozenAlreadyWrapped = true;
                else if (TransactionIdPrecedes(dbform->datfrozenxid, frozenXID))
                {
                        frozenXID = dbform->datfrozenxid;
                        oldestxid_datoid = HeapTupleGetOid(tuple);
                }

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

        heap_endscan(scan);

        heap_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;
        }

        /* Truncate CLOG and Multi to the oldest computed value */
        TruncateCLOG(frozenXID);
        TruncateMultiXact(minMulti);

        /*
         * 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);
        MultiXactAdvanceOldest(minMulti, minmulti_datoid);
}


/*
 *      vacuum_rel() -- vacuum one heap relation
 *
 *              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, VacuumStmt *vacstmt, bool do_toast, bool for_wraparound)
{
        LOCKMODE        lmode;
        Relation        onerel;
        LockRelId       onerelid;
        Oid                     toast_relid;
        Oid                     save_userid;
        int                     save_sec_context;
        int                     save_nestlevel;

        /* 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 (!(vacstmt->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 (for_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 = (vacstmt->options & VACOPT_FULL) ? AccessExclusiveLock : ShareUpdateExclusiveLock;

        /*
         * Open the relation and get the appropriate lock on it.
         *
         * There's a race condition here: the rel may have gone away since the
         * last time we saw it.  If so, we don't need to vacuum 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 (!(vacstmt->options & VACOPT_NOWAIT))
                onerel = try_relation_open(relid, lmode);
        else if (ConditionalLockRelationOid(relid, lmode))
                onerel = try_relation_open(relid, NoLock);
        else
        {
                onerel = NULL;
                if (IsAutoVacuumWorkerProcess() && Log_autovacuum_min_duration >= 0)
                        ereport(LOG,
                                        (errcode(ERRCODE_LOCK_NOT_AVAILABLE),
                                   errmsg("skipping vacuum of \"%s\" --- lock not available",
                                                  vacstmt->relation->relname)));
        }

        if (!onerel)
        {
                PopActiveSnapshot();
                CommitTransactionCommand();
                return false;
        }

        /*
         * Check permissions.
         *
         * We allow the user to vacuum 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 the rest of the DB --- is this appropriate?
         */
        if (!(pg_class_ownercheck(RelationGetRelid(onerel), GetUserId()) ||
                  (pg_database_ownercheck(MyDatabaseId, GetUserId()) && !onerel->rd_rel->relisshared)))
        {
                if (onerel->rd_rel->relisshared)
                        ereport(WARNING,
                                  (errmsg("skipping \"%s\" --- only superuser can vacuum it",
                                                  RelationGetRelationName(onerel))));
                else if (onerel->rd_rel->relnamespace == PG_CATALOG_NAMESPACE)
                        ereport(WARNING,
                                        (errmsg("skipping \"%s\" --- only superuser or database owner can vacuum it",
                                                        RelationGetRelationName(onerel))));
                else
                        ereport(WARNING,
                                        (errmsg("skipping \"%s\" --- only table or database owner can vacuum it",
                                                        RelationGetRelationName(onerel))));
                relation_close(onerel, lmode);
                PopActiveSnapshot();
                CommitTransactionCommand();
                return false;
        }

        /*
         * Check that it's a vacuumable relation; we used to do this in
         * get_rel_oids() but seems safer to check after we've locked the
         * relation.
         */
        if (onerel->rd_rel->relkind != RELKIND_RELATION &&
                onerel->rd_rel->relkind != RELKIND_MATVIEW &&
                onerel->rd_rel->relkind != RELKIND_TOASTVALUE)
        {
                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;
        }

        /*
         * 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 (do_toast && !(vacstmt->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 (vacstmt->options & VACOPT_FULL)
        {
                /* close relation before vacuuming, but hold lock until commit */
                relation_close(onerel, NoLock);
                onerel = NULL;

                /* VACUUM FULL is now a variant of CLUSTER; see cluster.c */
                cluster_rel(relid, InvalidOid, false,
                                        (vacstmt->options & VACOPT_VERBOSE) != 0,
                                        vacstmt->freeze_min_age, vacstmt->freeze_table_age);
        }
        else
                lazy_vacuum_rel(onerel, vacstmt, 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, vacstmt, false, for_wraparound);

        /*
         * 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 (IndexIsReady).
 * 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 (IndexIsReady(indrel->rd_index))
                        (*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)
        {
                int                     msec;

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

                pg_usleep(msec * 1000L);

                VacuumCostBalance = 0;

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

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