btreefuncs.c

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/*
 * contrib/pageinspect/btreefuncs.c
 *
 *
 * btreefuncs.c
 *
 * Copyright (c) 2006 Satoshi Nagayasu <nagayasus@nttdata.co.jp>
 *
 * Permission to use, copy, modify, and distribute this software and
 * its documentation for any purpose, without fee, and without a
 * written agreement is hereby granted, provided that the above
 * copyright notice and this paragraph and the following two
 * paragraphs appear in all copies.
 *
 * IN NO EVENT SHALL THE AUTHOR BE LIABLE TO ANY PARTY FOR DIRECT,
 * INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES, INCLUDING
 * LOST PROFITS, ARISING OUT OF THE USE OF THIS SOFTWARE AND ITS
 * DOCUMENTATION, EVEN IF THE UNIVERSITY OF CALIFORNIA HAS BEEN ADVISED
 * OF THE POSSIBILITY OF SUCH DAMAGE.
 *
 * THE AUTHOR SPECIFICALLY DISCLAIMS ANY WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
 * A PARTICULAR PURPOSE.  THE SOFTWARE PROVIDED HEREUNDER IS ON AN "AS
 * IS" BASIS, AND THE AUTHOR HAS NO OBLIGATIONS TO PROVIDE MAINTENANCE,
 * SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS.
 */

#include "postgres.h"

#include "access/nbtree.h"
#include "access/relation.h"
#include "catalog/namespace.h"
#include "catalog/pg_am.h"
#include "catalog/pg_type.h"
#include "funcapi.h"
#include "miscadmin.h"
#include "pageinspect.h"
#include "utils/array.h"
#include "utils/builtins.h"
#include "utils/rel.h"
#include "utils/varlena.h"

PG_FUNCTION_INFO_V1(bt_metap);
PG_FUNCTION_INFO_V1(bt_page_items_1_9);
PG_FUNCTION_INFO_V1(bt_page_items);
PG_FUNCTION_INFO_V1(bt_page_items_bytea);
PG_FUNCTION_INFO_V1(bt_page_stats_1_9);
PG_FUNCTION_INFO_V1(bt_page_stats);

#define IS_INDEX(r) ((r)->rd_rel->relkind == RELKIND_INDEX)
#define IS_BTREE(r) ((r)->rd_rel->relam == BTREE_AM_OID)
#define DatumGetItemPointer(X)   ((ItemPointer) DatumGetPointer(X))
#define ItemPointerGetDatum(X)   PointerGetDatum(X)

/* note: BlockNumber is unsigned, hence can't be negative */
#define CHECK_RELATION_BLOCK_RANGE(rel, blkno) { \
        if ( RelationGetNumberOfBlocks(rel) <= (BlockNumber) (blkno) ) \
             elog(ERROR, "block number out of range"); }

/* ------------------------------------------------
 * structure for single btree page statistics
 * ------------------------------------------------
 */
typedef struct BTPageStat
{
    uint32      blkno;
    uint32      live_items;
    uint32      dead_items;
    uint32      page_size;
    uint32      max_avail;
    uint32      free_size;
    uint32      avg_item_size;
    char        type;

    /* opaque data */
    BlockNumber btpo_prev;
    BlockNumber btpo_next;
    uint32      btpo_level;
    uint16      btpo_flags;
    BTCycleId   btpo_cycleid;
} BTPageStat;


/* -------------------------------------------------
 * GetBTPageStatistics()
 *
 * Collect statistics of single b-tree page
 * -------------------------------------------------
 */
static void
GetBTPageStatistics(BlockNumber blkno, Buffer buffer, BTPageStat *stat)
{
    Page        page = BufferGetPage(buffer);
    PageHeader  phdr = (PageHeader) page;
    OffsetNumber maxoff = PageGetMaxOffsetNumber(page);
    BTPageOpaque opaque = (BTPageOpaque) PageGetSpecialPointer(page);
    int         item_size = 0;
    int         off;

    stat->blkno = blkno;

    stat->max_avail = BLCKSZ - (BLCKSZ - phdr->pd_special + SizeOfPageHeaderData);

    stat->dead_items = stat->live_items = 0;

    stat->page_size = PageGetPageSize(page);

    /* page type (flags) */
    if (P_ISDELETED(opaque))
    {
        /* We divide deleted pages into leaf ('d') or internal ('D') */
        if (P_ISLEAF(opaque) || !P_HAS_FULLXID(opaque))
            stat->type = 'd';
        else
            stat->type = 'D';

        /*
         * Report safexid in a deleted page.
         *
         * Handle pg_upgrade'd deleted pages that used the previous safexid
         * representation in btpo_level field (this used to be a union type
         * called "bpto").
         */
        if (P_HAS_FULLXID(opaque))
        {
            FullTransactionId safexid = BTPageGetDeleteXid(page);

            elog(NOTICE, "deleted page from block %u has safexid %u:%u",
                 blkno, EpochFromFullTransactionId(safexid),
                 XidFromFullTransactionId(safexid));
        }
        else
            elog(NOTICE, "deleted page from block %u has safexid %u",
                 blkno, opaque->btpo_level);

        /* Don't interpret BTDeletedPageData as index tuples */
        maxoff = InvalidOffsetNumber;
    }
    else if (P_IGNORE(opaque))
        stat->type = 'e';
    else if (P_ISLEAF(opaque))
        stat->type = 'l';
    else if (P_ISROOT(opaque))
        stat->type = 'r';
    else
        stat->type = 'i';

    /* btpage opaque data */
    stat->btpo_prev = opaque->btpo_prev;
    stat->btpo_next = opaque->btpo_next;
    stat->btpo_level = opaque->btpo_level;
    stat->btpo_flags = opaque->btpo_flags;
    stat->btpo_cycleid = opaque->btpo_cycleid;

    /* count live and dead tuples, and free space */
    for (off = FirstOffsetNumber; off <= maxoff; off++)
    {
        IndexTuple  itup;

        ItemId      id = PageGetItemId(page, off);

        itup = (IndexTuple) PageGetItem(page, id);

        item_size += IndexTupleSize(itup);

        if (!ItemIdIsDead(id))
            stat->live_items++;
        else
            stat->dead_items++;
    }
    stat->free_size = PageGetFreeSpace(page);

    if ((stat->live_items + stat->dead_items) > 0)
        stat->avg_item_size = item_size / (stat->live_items + stat->dead_items);
    else
        stat->avg_item_size = 0;
}

/* -----------------------------------------------
 * bt_page_stats()
 *
 * Usage: SELECT * FROM bt_page_stats('t1_pkey', 1);
 * -----------------------------------------------
 */
static Datum
bt_page_stats_internal(PG_FUNCTION_ARGS, enum pageinspect_version ext_version)
{
    text       *relname = PG_GETARG_TEXT_PP(0);
    int64       blkno = (ext_version == PAGEINSPECT_V1_8 ? PG_GETARG_UINT32(1) : PG_GETARG_INT64(1));
    Buffer      buffer;
    Relation    rel;
    RangeVar   *relrv;
    Datum       result;
    HeapTuple   tuple;
    TupleDesc   tupleDesc;
    int         j;
    char       *values[11];
    BTPageStat  stat;

    if (!superuser())
        ereport(ERROR,
                (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
                 errmsg("must be superuser to use pageinspect functions")));

    relrv = makeRangeVarFromNameList(textToQualifiedNameList(relname));
    rel = relation_openrv(relrv, AccessShareLock);

    if (!IS_INDEX(rel) || !IS_BTREE(rel))
        elog(ERROR, "relation \"%s\" is not a btree index",
             RelationGetRelationName(rel));

    /*
     * 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")));

    if (blkno < 0 || blkno > MaxBlockNumber)
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                 errmsg("invalid block number")));

    if (blkno == 0)
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                 errmsg("block 0 is a meta page")));

    CHECK_RELATION_BLOCK_RANGE(rel, blkno);

    buffer = ReadBuffer(rel, blkno);
    LockBuffer(buffer, BUFFER_LOCK_SHARE);

    /* keep compiler quiet */
    stat.btpo_prev = stat.btpo_next = InvalidBlockNumber;
    stat.btpo_flags = stat.free_size = stat.avg_item_size = 0;

    GetBTPageStatistics(blkno, buffer, &stat);

    UnlockReleaseBuffer(buffer);
    relation_close(rel, AccessShareLock);

    /* Build a tuple descriptor for our result type */
    if (get_call_result_type(fcinfo, NULL, &tupleDesc) != TYPEFUNC_COMPOSITE)
        elog(ERROR, "return type must be a row type");

    j = 0;
    values[j++] = psprintf("%u", stat.blkno);
    values[j++] = psprintf("%c", stat.type);
    values[j++] = psprintf("%u", stat.live_items);
    values[j++] = psprintf("%u", stat.dead_items);
    values[j++] = psprintf("%u", stat.avg_item_size);
    values[j++] = psprintf("%u", stat.page_size);
    values[j++] = psprintf("%u", stat.free_size);
    values[j++] = psprintf("%u", stat.btpo_prev);
    values[j++] = psprintf("%u", stat.btpo_next);
    values[j++] = psprintf("%u", stat.btpo_level);
    values[j++] = psprintf("%d", stat.btpo_flags);

    tuple = BuildTupleFromCStrings(TupleDescGetAttInMetadata(tupleDesc),
                                   values);

    result = HeapTupleGetDatum(tuple);

    PG_RETURN_DATUM(result);
}

Datum
bt_page_stats_1_9(PG_FUNCTION_ARGS)
{
    return bt_page_stats_internal(fcinfo, PAGEINSPECT_V1_9);
}

/* entry point for old extension version */
Datum
bt_page_stats(PG_FUNCTION_ARGS)
{
    return bt_page_stats_internal(fcinfo, PAGEINSPECT_V1_8);
}


/*
 * cross-call data structure for SRF
 */
struct user_args
{
    Page        page;
    OffsetNumber offset;
    bool        leafpage;
    bool        rightmost;
    TupleDesc   tupd;
};

/*-------------------------------------------------------
 * bt_page_print_tuples()
 *
 * Form a tuple describing index tuple at a given offset
 * ------------------------------------------------------
 */
static Datum
bt_page_print_tuples(struct user_args *uargs)
{
    Page        page = uargs->page;
    OffsetNumber offset = uargs->offset;
    bool        leafpage = uargs->leafpage;
    bool        rightmost = uargs->rightmost;
    bool        ispivottuple;
    Datum       values[9];
    bool        nulls[9];
    HeapTuple   tuple;
    ItemId      id;
    IndexTuple  itup;
    int         j;
    int         off;
    int         dlen;
    char       *dump,
               *datacstring;
    char       *ptr;
    ItemPointer htid;

    id = PageGetItemId(page, offset);

    if (!ItemIdIsValid(id))
        elog(ERROR, "invalid ItemId");

    itup = (IndexTuple) PageGetItem(page, id);

    j = 0;
    memset(nulls, 0, sizeof(nulls));
    values[j++] = DatumGetInt16(offset);
    values[j++] = ItemPointerGetDatum(&itup->t_tid);
    values[j++] = Int32GetDatum((int) IndexTupleSize(itup));
    values[j++] = BoolGetDatum(IndexTupleHasNulls(itup));
    values[j++] = BoolGetDatum(IndexTupleHasVarwidths(itup));

    ptr = (char *) itup + IndexInfoFindDataOffset(itup->t_info);
    dlen = IndexTupleSize(itup) - IndexInfoFindDataOffset(itup->t_info);

    /*
     * Make sure that "data" column does not include posting list or pivot
     * tuple representation of heap TID(s).
     *
     * Note: BTreeTupleIsPivot() won't work reliably on !heapkeyspace indexes
     * (those built before BTREE_VERSION 4), but we have no way of determining
     * if this page came from a !heapkeyspace index.  We may only have a bytea
     * nbtree page image to go on, so in general there is no metapage that we
     * can check.
     *
     * That's okay here because BTreeTupleIsPivot() can only return false for
     * a !heapkeyspace pivot, never true for a !heapkeyspace non-pivot.  Since
     * heap TID isn't part of the keyspace in a !heapkeyspace index anyway,
     * there cannot possibly be a pivot tuple heap TID representation that we
     * fail to make an adjustment for.  A !heapkeyspace index can have
     * BTreeTupleIsPivot() return true (due to things like suffix truncation
     * for INCLUDE indexes in Postgres v11), but when that happens
     * BTreeTupleGetHeapTID() can be trusted to work reliably (i.e. return
     * NULL).
     *
     * Note: BTreeTupleIsPosting() always works reliably, even with
     * !heapkeyspace indexes.
     */
    if (BTreeTupleIsPosting(itup))
        dlen -= IndexTupleSize(itup) - BTreeTupleGetPostingOffset(itup);
    else if (BTreeTupleIsPivot(itup) && BTreeTupleGetHeapTID(itup) != NULL)
        dlen -= MAXALIGN(sizeof(ItemPointerData));

    if (dlen < 0 || dlen > INDEX_SIZE_MASK)
        elog(ERROR, "invalid tuple length %d for tuple at offset number %u",
             dlen, offset);
    dump = palloc0(dlen * 3 + 1);
    datacstring = dump;
    for (off = 0; off < dlen; off++)
    {
        if (off > 0)
            *dump++ = ' ';
        sprintf(dump, "%02x", *(ptr + off) & 0xff);
        dump += 2;
    }
    values[j++] = CStringGetTextDatum(datacstring);
    pfree(datacstring);

    /*
     * We need to work around the BTreeTupleIsPivot() !heapkeyspace limitation
     * again.  Deduce whether or not tuple must be a pivot tuple based on
     * whether or not the page is a leaf page, as well as the page offset
     * number of the tuple.
     */
    ispivottuple = (!leafpage || (!rightmost && offset == P_HIKEY));

    /* LP_DEAD bit can never be set for pivot tuples, so show a NULL there */
    if (!ispivottuple)
        values[j++] = BoolGetDatum(ItemIdIsDead(id));
    else
    {
        Assert(!ItemIdIsDead(id));
        nulls[j++] = true;
    }

    htid = BTreeTupleGetHeapTID(itup);
    if (ispivottuple && !BTreeTupleIsPivot(itup))
    {
        /* Don't show bogus heap TID in !heapkeyspace pivot tuple */
        htid = NULL;
    }

    if (htid)
        values[j++] = ItemPointerGetDatum(htid);
    else
        nulls[j++] = true;

    if (BTreeTupleIsPosting(itup))
    {
        /* Build an array of item pointers */
        ItemPointer tids;
        Datum      *tids_datum;
        int         nposting;

        tids = BTreeTupleGetPosting(itup);
        nposting = BTreeTupleGetNPosting(itup);
        tids_datum = (Datum *) palloc(nposting * sizeof(Datum));
        for (int i = 0; i < nposting; i++)
            tids_datum[i] = ItemPointerGetDatum(&tids[i]);
        values[j++] = PointerGetDatum(construct_array(tids_datum,
                                                      nposting,
                                                      TIDOID,
                                                      sizeof(ItemPointerData),
                                                      false, TYPALIGN_SHORT));
        pfree(tids_datum);
    }
    else
        nulls[j++] = true;

    /* Build and return the result tuple */
    tuple = heap_form_tuple(uargs->tupd, values, nulls);

    return HeapTupleGetDatum(tuple);
}

/*-------------------------------------------------------
 * bt_page_items()
 *
 * Get IndexTupleData set in a btree page
 *
 * Usage: SELECT * FROM bt_page_items('t1_pkey', 1);
 *-------------------------------------------------------
 */
static Datum
bt_page_items_internal(PG_FUNCTION_ARGS, enum pageinspect_version ext_version)
{
    text       *relname = PG_GETARG_TEXT_PP(0);
    int64       blkno = (ext_version == PAGEINSPECT_V1_8 ? PG_GETARG_UINT32(1) : PG_GETARG_INT64(1));
    Datum       result;
    FuncCallContext *fctx;
    MemoryContext mctx;
    struct user_args *uargs;

    if (!superuser())
        ereport(ERROR,
                (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
                 errmsg("must be superuser to use pageinspect functions")));

    if (SRF_IS_FIRSTCALL())
    {
        RangeVar   *relrv;
        Relation    rel;
        Buffer      buffer;
        BTPageOpaque opaque;
        TupleDesc   tupleDesc;

        fctx = SRF_FIRSTCALL_INIT();

        relrv = makeRangeVarFromNameList(textToQualifiedNameList(relname));
        rel = relation_openrv(relrv, AccessShareLock);

        if (!IS_INDEX(rel) || !IS_BTREE(rel))
            elog(ERROR, "relation \"%s\" is not a btree index",
                 RelationGetRelationName(rel));

        /*
         * 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")));

        if (blkno < 0 || blkno > MaxBlockNumber)
            ereport(ERROR,
                    (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                     errmsg("invalid block number")));

        if (blkno == 0)
            ereport(ERROR,
                    (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                     errmsg("block 0 is a meta page")));

        CHECK_RELATION_BLOCK_RANGE(rel, blkno);

        buffer = ReadBuffer(rel, blkno);
        LockBuffer(buffer, BUFFER_LOCK_SHARE);

        /*
         * We copy the page into local storage to avoid holding pin on the
         * buffer longer than we must, and possibly failing to release it at
         * all if the calling query doesn't fetch all rows.
         */
        mctx = MemoryContextSwitchTo(fctx->multi_call_memory_ctx);

        uargs = palloc(sizeof(struct user_args));

        uargs->page = palloc(BLCKSZ);
        memcpy(uargs->page, BufferGetPage(buffer), BLCKSZ);

        UnlockReleaseBuffer(buffer);
        relation_close(rel, AccessShareLock);

        uargs->offset = FirstOffsetNumber;

        opaque = (BTPageOpaque) PageGetSpecialPointer(uargs->page);

        if (!P_ISDELETED(opaque))
            fctx->max_calls = PageGetMaxOffsetNumber(uargs->page);
        else
        {
            /* Don't interpret BTDeletedPageData as index tuples */
            elog(NOTICE, "page from block " INT64_FORMAT " is deleted", blkno);
            fctx->max_calls = 0;
        }
        uargs->leafpage = P_ISLEAF(opaque);
        uargs->rightmost = P_RIGHTMOST(opaque);

        /* Build a tuple descriptor for our result type */
        if (get_call_result_type(fcinfo, NULL, &tupleDesc) != TYPEFUNC_COMPOSITE)
            elog(ERROR, "return type must be a row type");
        tupleDesc = BlessTupleDesc(tupleDesc);

        uargs->tupd = tupleDesc;

        fctx->user_fctx = uargs;

        MemoryContextSwitchTo(mctx);
    }

    fctx = SRF_PERCALL_SETUP();
    uargs = fctx->user_fctx;

    if (fctx->call_cntr < fctx->max_calls)
    {
        result = bt_page_print_tuples(uargs);
        uargs->offset++;
        SRF_RETURN_NEXT(fctx, result);
    }

    SRF_RETURN_DONE(fctx);
}

Datum
bt_page_items_1_9(PG_FUNCTION_ARGS)
{
    return bt_page_items_internal(fcinfo, PAGEINSPECT_V1_9);
}

/* entry point for old extension version */
Datum
bt_page_items(PG_FUNCTION_ARGS)
{
    return bt_page_items_internal(fcinfo, PAGEINSPECT_V1_8);
}

/*-------------------------------------------------------
 * bt_page_items_bytea()
 *
 * Get IndexTupleData set in a btree page
 *
 * Usage: SELECT * FROM bt_page_items(get_raw_page('t1_pkey', 1));
 *-------------------------------------------------------
 */

Datum
bt_page_items_bytea(PG_FUNCTION_ARGS)
{
    bytea      *raw_page = PG_GETARG_BYTEA_P(0);
    Datum       result;
    FuncCallContext *fctx;
    struct user_args *uargs;
    int         raw_page_size;

    if (!superuser())
        ereport(ERROR,
                (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
                 errmsg("must be superuser to use raw page functions")));

    if (SRF_IS_FIRSTCALL())
    {
        BTPageOpaque opaque;
        MemoryContext mctx;
        TupleDesc   tupleDesc;

        raw_page_size = VARSIZE(raw_page) - VARHDRSZ;

        if (raw_page_size < SizeOfPageHeaderData)
            ereport(ERROR,
                    (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                     errmsg("input page too small (%d bytes)", raw_page_size)));

        fctx = SRF_FIRSTCALL_INIT();
        mctx = MemoryContextSwitchTo(fctx->multi_call_memory_ctx);

        uargs = palloc(sizeof(struct user_args));

        uargs->page = VARDATA(raw_page);

        uargs->offset = FirstOffsetNumber;

        opaque = (BTPageOpaque) PageGetSpecialPointer(uargs->page);

        if (P_ISMETA(opaque))
            ereport(ERROR,
                    (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                     errmsg("block is a meta page")));

        if (P_ISDELETED(opaque))
            elog(NOTICE, "page is deleted");

        if (!P_ISDELETED(opaque))
            fctx->max_calls = PageGetMaxOffsetNumber(uargs->page);
        else
        {
            /* Don't interpret BTDeletedPageData as index tuples */
            elog(NOTICE, "page from block is deleted");
            fctx->max_calls = 0;
        }
        uargs->leafpage = P_ISLEAF(opaque);
        uargs->rightmost = P_RIGHTMOST(opaque);

        /* Build a tuple descriptor for our result type */
        if (get_call_result_type(fcinfo, NULL, &tupleDesc) != TYPEFUNC_COMPOSITE)
            elog(ERROR, "return type must be a row type");
        tupleDesc = BlessTupleDesc(tupleDesc);

        uargs->tupd = tupleDesc;

        fctx->user_fctx = uargs;

        MemoryContextSwitchTo(mctx);
    }

    fctx = SRF_PERCALL_SETUP();
    uargs = fctx->user_fctx;

    if (fctx->call_cntr < fctx->max_calls)
    {
        result = bt_page_print_tuples(uargs);
        uargs->offset++;
        SRF_RETURN_NEXT(fctx, result);
    }

    SRF_RETURN_DONE(fctx);
}

/* Number of output arguments (columns) for bt_metap() */
#define BT_METAP_COLS_V1_8      9

/* ------------------------------------------------
 * bt_metap()
 *
 * Get a btree's meta-page information
 *
 * Usage: SELECT * FROM bt_metap('t1_pkey')
 * ------------------------------------------------
 */
Datum
bt_metap(PG_FUNCTION_ARGS)
{
    text       *relname = PG_GETARG_TEXT_PP(0);
    Datum       result;
    Relation    rel;
    RangeVar   *relrv;
    BTMetaPageData *metad;
    TupleDesc   tupleDesc;
    int         j;
    char       *values[9];
    Buffer      buffer;
    Page        page;
    HeapTuple   tuple;

    if (!superuser())
        ereport(ERROR,
                (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
                 errmsg("must be superuser to use pageinspect functions")));

    relrv = makeRangeVarFromNameList(textToQualifiedNameList(relname));
    rel = relation_openrv(relrv, AccessShareLock);

    if (!IS_INDEX(rel) || !IS_BTREE(rel))
        elog(ERROR, "relation \"%s\" is not a btree index",
             RelationGetRelationName(rel));

    /*
     * 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")));

    buffer = ReadBuffer(rel, 0);
    LockBuffer(buffer, BUFFER_LOCK_SHARE);

    page = BufferGetPage(buffer);
    metad = BTPageGetMeta(page);

    /* Build a tuple descriptor for our result type */
    if (get_call_result_type(fcinfo, NULL, &tupleDesc) != TYPEFUNC_COMPOSITE)
        elog(ERROR, "return type must be a row type");

    /*
     * We need a kluge here to detect API versions prior to 1.8.  Earlier
     * versions incorrectly used int4 for certain columns.
     *
     * There is no way to reliably avoid the problems created by the old
     * function definition at this point, so insist that the user update the
     * extension.
     */
    if (tupleDesc->natts < BT_METAP_COLS_V1_8)
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_FUNCTION_DEFINITION),
                 errmsg("function has wrong number of declared columns"),
                 errhint("To resolve the problem, update the \"pageinspect\" extension to the latest version.")));

    j = 0;
    values[j++] = psprintf("%d", metad->btm_magic);
    values[j++] = psprintf("%d", metad->btm_version);
    values[j++] = psprintf(INT64_FORMAT, (int64) metad->btm_root);
    values[j++] = psprintf(INT64_FORMAT, (int64) metad->btm_level);
    values[j++] = psprintf(INT64_FORMAT, (int64) metad->btm_fastroot);
    values[j++] = psprintf(INT64_FORMAT, (int64) metad->btm_fastlevel);

    /*
     * Get values of extended metadata if available, use default values
     * otherwise.  Note that we rely on the assumption that btm_allequalimage
     * is initialized to zero with indexes that were built on versions prior
     * to Postgres 13 (just like _bt_metaversion()).
     */
    if (metad->btm_version >= BTREE_NOVAC_VERSION)
    {
        values[j++] = psprintf(INT64_FORMAT,
                               (int64) metad->btm_last_cleanup_num_delpages);
        values[j++] = psprintf("%f", metad->btm_last_cleanup_num_heap_tuples);
        values[j++] = metad->btm_allequalimage ? "t" : "f";
    }
    else
    {
        values[j++] = "0";
        values[j++] = "-1";
        values[j++] = "f";
    }

    tuple = BuildTupleFromCStrings(TupleDescGetAttInMetadata(tupleDesc),
                                   values);

    result = HeapTupleGetDatum(tuple);

    UnlockReleaseBuffer(buffer);
    relation_close(rel, AccessShareLock);

    PG_RETURN_DATUM(result);
}