pg_freespacemap.c

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/*-------------------------------------------------------------------------
 *
 * pg_freespacemap.c
 *    display contents of a free space map
 *
 *    contrib/pg_freespacemap/pg_freespacemap.c
 *-------------------------------------------------------------------------
 */
#include "postgres.h"
 
#include "access/relation.h"
#include "fmgr.h"
#include "storage/freespace.h"
#include "utils/rel.h"
 
PG_MODULE_MAGIC_EXT(
                    .name = "pg_freespacemap",
                    .version = PG_VERSION
);
 
/*
 * Returns the amount of free space on a given page, according to the
 * free space map.
 */
PG_FUNCTION_INFO_V1(pg_freespace);
 
Datum
pg_freespace(PG_FUNCTION_ARGS)
{
    Oid         relid = PG_GETARG_OID(0);
    int64       blkno = PG_GETARG_INT64(1);
    int16       freespace;
    Relation    rel;
 
    rel = relation_open(relid, AccessShareLock);
 
    if (!RELKIND_HAS_STORAGE(rel->rd_rel->relkind))
        ereport(ERROR,
                (errcode(ERRCODE_WRONG_OBJECT_TYPE),
                 errmsg("relation \"%s\" does not have storage",
                        RelationGetRelationName(rel)),
                 errdetail_relkind_not_supported(rel->rd_rel->relkind)));
 
    if (blkno < 0 || blkno > MaxBlockNumber)
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                 errmsg("invalid block number")));
 
    freespace = GetRecordedFreeSpace(rel, blkno);
 
    relation_close(rel, AccessShareLock);
    PG_RETURN_INT16(freespace);
}