pgstatapprox.c

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/*-------------------------------------------------------------------------
 *
 * pgstatapprox.c
 *        Bloat estimation functions
 *
 * Copyright (c) 2014-2026, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *        contrib/pgstattuple/pgstatapprox.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"
 
#include "access/heapam.h"
#include "access/htup_details.h"
#include "access/relation.h"
#include "access/visibilitymap.h"
#include "catalog/pg_am_d.h"
#include "commands/vacuum.h"
#include "funcapi.h"
#include "miscadmin.h"
#include "storage/bufmgr.h"
#include "storage/freespace.h"
#include "storage/procarray.h"
#include "storage/read_stream.h"
 
PG_FUNCTION_INFO_V1(pgstattuple_approx);
PG_FUNCTION_INFO_V1(pgstattuple_approx_v1_5);
 
Datum       pgstattuple_approx_internal(Oid relid, FunctionCallInfo fcinfo);
 
typedef struct output_type
{
    uint64      table_len;
    double      scanned_percent;
    uint64      tuple_count;
    uint64      tuple_len;
    double      tuple_percent;
    uint64      dead_tuple_count;
    uint64      dead_tuple_len;
    double      dead_tuple_percent;
    uint64      free_space;
    double      free_percent;
} output_type;
 
#define NUM_OUTPUT_COLUMNS 10
 
/*
 * Struct for statapprox_heap read stream callback.
 */
typedef struct StatApproxReadStreamPrivate
{
    Relation    rel;
    output_type *stat;
    BlockNumber current_blocknum;
    BlockNumber nblocks;
    BlockNumber scanned;        /* count of pages actually read */
    Buffer      vmbuffer;       /* for VM lookups */
} StatApproxReadStreamPrivate;
 
/*
 * Read stream callback for statapprox_heap.
 *
 * This callback checks the visibility map for each block.  If the block is
 * all-visible, we can get the free space from the FSM without reading the
 * actual page, and skip to the next block.  Only the blocks that are not
 * all-visible are returned for actual reading after being locked.
 */
static BlockNumber
statapprox_heap_read_stream_next(ReadStream *stream,
                                 void *callback_private_data,
                                 void *per_buffer_data)
{
    StatApproxReadStreamPrivate *p =
        (StatApproxReadStreamPrivate *) callback_private_data;
 
    while (p->current_blocknum < p->nblocks)
    {
        BlockNumber blkno = p->current_blocknum++;
        Size        freespace;
 
        CHECK_FOR_INTERRUPTS();
 
        /*
         * If the page has only visible tuples, then we can find out the free
         * space from the FSM and move on without reading the page.
         */
        if (VM_ALL_VISIBLE(p->rel, blkno, &p->vmbuffer))
        {
            freespace = GetRecordedFreeSpace(p->rel, blkno);
            p->stat->tuple_len += BLCKSZ - freespace;
            p->stat->free_space += freespace;
            continue;
        }
 
        /* This block needs to be read */
        p->scanned++;
        return blkno;
    }
 
    return InvalidBlockNumber;
}
 
/*
 * This function takes an already open relation and scans its pages,
 * skipping those that have the corresponding visibility map bit set.
 * For pages we skip, we find the free space from the free space map
 * and approximate tuple_len on that basis. For the others, we count
 * the exact number of dead tuples etc.
 *
 * This scan is loosely based on vacuumlazy.c:lazy_scan_heap(), but
 * we do not try to avoid skipping single pages.
 */
static void
statapprox_heap(Relation rel, output_type *stat)
{
    BlockNumber nblocks;
    BufferAccessStrategy bstrategy;
    TransactionId OldestXmin;
    StatApproxReadStreamPrivate p;
    ReadStream *stream;
 
    OldestXmin = GetOldestNonRemovableTransactionId(rel);
    bstrategy = GetAccessStrategy(BAS_BULKREAD);
 
    nblocks = RelationGetNumberOfBlocks(rel);
 
    /* Initialize read stream private data */
    p.rel = rel;
    p.stat = stat;
    p.current_blocknum = 0;
    p.nblocks = nblocks;
    p.scanned = 0;
    p.vmbuffer = InvalidBuffer;
 
    /*
     * Create the read stream. We don't use READ_STREAM_USE_BATCHING because
     * the callback accesses the visibility map which may need to read VM
     * pages. While this shouldn't cause deadlocks, we err on the side of
     * caution.
     */
    stream = read_stream_begin_relation(READ_STREAM_FULL,
                                        bstrategy,
                                        rel,
                                        MAIN_FORKNUM,
                                        statapprox_heap_read_stream_next,
                                        &p,
                                        0);
 
    for (;;)
    {
        Buffer      buf;
        Page        page;
        OffsetNumber offnum,
                    maxoff;
        BlockNumber blkno;
 
        buf = read_stream_next_buffer(stream, NULL);
        if (buf == InvalidBuffer)
            break;
 
        LockBuffer(buf, BUFFER_LOCK_SHARE);
 
        page = BufferGetPage(buf);
        blkno = BufferGetBlockNumber(buf);
 
        stat->free_space += PageGetExactFreeSpace(page);
 
        if (PageIsNew(page) || PageIsEmpty(page))
        {
            UnlockReleaseBuffer(buf);
            continue;
        }
 
        /*
         * Look at each tuple on the page and decide whether it's live or
         * dead, then count it and its size. Unlike lazy_scan_heap, we can
         * afford to ignore problems and special cases.
         */
        maxoff = PageGetMaxOffsetNumber(page);
 
        for (offnum = FirstOffsetNumber;
             offnum <= maxoff;
             offnum = OffsetNumberNext(offnum))
        {
            ItemId      itemid;
            HeapTupleData tuple;
 
            itemid = PageGetItemId(page, offnum);
 
            if (!ItemIdIsUsed(itemid) || ItemIdIsRedirected(itemid) ||
                ItemIdIsDead(itemid))
            {
                continue;
            }
 
            Assert(ItemIdIsNormal(itemid));
 
            ItemPointerSet(&(tuple.t_self), blkno, offnum);
 
            tuple.t_data = (HeapTupleHeader) PageGetItem(page, itemid);
            tuple.t_len = ItemIdGetLength(itemid);
            tuple.t_tableOid = RelationGetRelid(rel);
 
            /*
             * We follow VACUUM's lead in counting INSERT_IN_PROGRESS tuples
             * as "dead" while DELETE_IN_PROGRESS tuples are "live".  We don't
             * bother distinguishing tuples inserted/deleted by our own
             * transaction.
             */
            switch (HeapTupleSatisfiesVacuum(&tuple, OldestXmin, buf))
            {
                case HEAPTUPLE_LIVE:
                case HEAPTUPLE_DELETE_IN_PROGRESS:
                    stat->tuple_len += tuple.t_len;
                    stat->tuple_count++;
                    break;
                case HEAPTUPLE_DEAD:
                case HEAPTUPLE_RECENTLY_DEAD:
                case HEAPTUPLE_INSERT_IN_PROGRESS:
                    stat->dead_tuple_len += tuple.t_len;
                    stat->dead_tuple_count++;
                    break;
                default:
                    elog(ERROR, "unexpected HeapTupleSatisfiesVacuum result");
                    break;
            }
        }
 
        UnlockReleaseBuffer(buf);
    }
 
    Assert(p.current_blocknum == nblocks);
    read_stream_end(stream);
 
    stat->table_len = (uint64) nblocks * BLCKSZ;
 
    /*
     * We don't know how many tuples are in the pages we didn't scan, so
     * extrapolate the live-tuple count to the whole table in the same way
     * that VACUUM does.  (Like VACUUM, we're not taking a random sample, so
     * just extrapolating linearly seems unsafe.)  There should be no dead
     * tuples in all-visible pages, so no correction is needed for that, and
     * we already accounted for the space in those pages, too.
     */
    stat->tuple_count = vac_estimate_reltuples(rel, nblocks, p.scanned,
                                               stat->tuple_count);
 
    /* It's not clear if we could get -1 here, but be safe. */
    stat->tuple_count = Max(stat->tuple_count, 0);
 
    /*
     * Calculate percentages if the relation has one or more pages.
     */
    if (nblocks != 0)
    {
        stat->scanned_percent = 100.0 * p.scanned / nblocks;
        stat->tuple_percent = 100.0 * stat->tuple_len / stat->table_len;
        stat->dead_tuple_percent = 100.0 * stat->dead_tuple_len / stat->table_len;
        stat->free_percent = 100.0 * stat->free_space / stat->table_len;
    }
 
    if (BufferIsValid(p.vmbuffer))
    {
        ReleaseBuffer(p.vmbuffer);
        p.vmbuffer = InvalidBuffer;
    }
}
 
/*
 * Returns estimated live/dead tuple statistics for the given relid.
 *
 * The superuser() check here must be kept as the library might be upgraded
 * without the extension being upgraded, meaning that in pre-1.5 installations
 * these functions could be called by any user.
 */
Datum
pgstattuple_approx(PG_FUNCTION_ARGS)
{
    Oid         relid = PG_GETARG_OID(0);
 
    if (!superuser())
        ereport(ERROR,
                (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
                 errmsg("must be superuser to use pgstattuple functions")));
 
    PG_RETURN_DATUM(pgstattuple_approx_internal(relid, fcinfo));
}
 
/*
 * As of pgstattuple version 1.5, we no longer need to check if the user
 * is a superuser because we REVOKE EXECUTE on the SQL function from PUBLIC.
 * Users can then grant access to it based on their policies.
 *
 * Otherwise identical to pgstattuple_approx (above).
 */
Datum
pgstattuple_approx_v1_5(PG_FUNCTION_ARGS)
{
    Oid         relid = PG_GETARG_OID(0);
 
    PG_RETURN_DATUM(pgstattuple_approx_internal(relid, fcinfo));
}
 
Datum
pgstattuple_approx_internal(Oid relid, FunctionCallInfo fcinfo)
{
    Relation    rel;
    output_type stat = {0};
    TupleDesc   tupdesc;
    bool        nulls[NUM_OUTPUT_COLUMNS];
    Datum       values[NUM_OUTPUT_COLUMNS];
    HeapTuple   ret;
    int         i = 0;
 
    if (get_call_result_type(fcinfo, NULL, &tupdesc) != TYPEFUNC_COMPOSITE)
        elog(ERROR, "return type must be a row type");
 
    if (tupdesc->natts != NUM_OUTPUT_COLUMNS)
        elog(ERROR, "incorrect number of output arguments");
 
    rel = relation_open(relid, AccessShareLock);
 
    /*
     * Reject attempts to read non-local temporary relations; we would be
     * likely to get wrong data since we have no visibility into the owning
     * session's local buffers.
     */
    if (RELATION_IS_OTHER_TEMP(rel))
        ereport(ERROR,
                (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                 errmsg("cannot access temporary tables of other sessions")));
 
    /*
     * We support only relation kinds with a visibility map and a free space
     * map.
     */
    if (!(rel->rd_rel->relkind == RELKIND_RELATION ||
          rel->rd_rel->relkind == RELKIND_MATVIEW ||
          rel->rd_rel->relkind == RELKIND_TOASTVALUE))
        ereport(ERROR,
                (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                 errmsg("relation \"%s\" is of wrong relation kind",
                        RelationGetRelationName(rel)),
                 errdetail_relkind_not_supported(rel->rd_rel->relkind)));
 
    if (rel->rd_rel->relam != HEAP_TABLE_AM_OID)
        ereport(ERROR, (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                        errmsg("only heap AM is supported")));
 
    statapprox_heap(rel, &stat);
 
    relation_close(rel, AccessShareLock);
 
    memset(nulls, 0, sizeof(nulls));
 
    values[i++] = Int64GetDatum(stat.table_len);
    values[i++] = Float8GetDatum(stat.scanned_percent);
    values[i++] = Int64GetDatum(stat.tuple_count);
    values[i++] = Int64GetDatum(stat.tuple_len);
    values[i++] = Float8GetDatum(stat.tuple_percent);
    values[i++] = Int64GetDatum(stat.dead_tuple_count);
    values[i++] = Int64GetDatum(stat.dead_tuple_len);
    values[i++] = Float8GetDatum(stat.dead_tuple_percent);
    values[i++] = Int64GetDatum(stat.free_space);
    values[i++] = Float8GetDatum(stat.free_percent);
 
    ret = heap_form_tuple(tupdesc, values, nulls);
    return HeapTupleGetDatum(ret);
}