pg_largeobject.c

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/*-------------------------------------------------------------------------
 *
 * pg_largeobject.c
 *    routines to support manipulation of the pg_largeobject relation
 *
 * Portions Copyright (c) 1996-2025, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/catalog/pg_largeobject.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"
 
#include "access/table.h"
#include "catalog/catalog.h"
#include "catalog/indexing.h"
#include "catalog/pg_largeobject.h"
#include "catalog/pg_largeobject_metadata.h"
#include "miscadmin.h"
#include "utils/fmgroids.h"
#include "utils/rel.h"
 
 
/*
 * Create a large object having the given LO identifier.
 *
 * We create a new large object by inserting an entry into
 * pg_largeobject_metadata without any data pages, so that the object
 * will appear to exist with size 0.
 */
Oid
LargeObjectCreate(Oid loid)
{
    Relation    pg_lo_meta;
    HeapTuple   ntup;
    Oid         loid_new;
    Datum       values[Natts_pg_largeobject_metadata];
    bool        nulls[Natts_pg_largeobject_metadata];
 
    pg_lo_meta = table_open(LargeObjectMetadataRelationId,
                            RowExclusiveLock);
 
    /*
     * Insert metadata of the largeobject
     */
    memset(values, 0, sizeof(values));
    memset(nulls, false, sizeof(nulls));
 
    if (OidIsValid(loid))
        loid_new = loid;
    else
        loid_new = GetNewOidWithIndex(pg_lo_meta,
                                      LargeObjectMetadataOidIndexId,
                                      Anum_pg_largeobject_metadata_oid);
 
    values[Anum_pg_largeobject_metadata_oid - 1] = ObjectIdGetDatum(loid_new);
    values[Anum_pg_largeobject_metadata_lomowner - 1]
        = ObjectIdGetDatum(GetUserId());
    nulls[Anum_pg_largeobject_metadata_lomacl - 1] = true;
 
    ntup = heap_form_tuple(RelationGetDescr(pg_lo_meta),
                           values, nulls);
 
    CatalogTupleInsert(pg_lo_meta, ntup);
 
    heap_freetuple(ntup);
 
    table_close(pg_lo_meta, RowExclusiveLock);
 
    return loid_new;
}
 
/*
 * Drop a large object having the given LO identifier.  Both the data pages
 * and metadata must be dropped.
 */
void
LargeObjectDrop(Oid loid)
{
    Relation    pg_lo_meta;
    Relation    pg_largeobject;
    ScanKeyData skey[1];
    SysScanDesc scan;
    HeapTuple   tuple;
 
    pg_lo_meta = table_open(LargeObjectMetadataRelationId,
                            RowExclusiveLock);
 
    pg_largeobject = table_open(LargeObjectRelationId,
                                RowExclusiveLock);
 
    /*
     * Delete an entry from pg_largeobject_metadata
     */
    ScanKeyInit(&skey[0],
                Anum_pg_largeobject_metadata_oid,
                BTEqualStrategyNumber, F_OIDEQ,
                ObjectIdGetDatum(loid));
 
    scan = systable_beginscan(pg_lo_meta,
                              LargeObjectMetadataOidIndexId, true,
                              NULL, 1, skey);
 
    tuple = systable_getnext(scan);
    if (!HeapTupleIsValid(tuple))
        ereport(ERROR,
                (errcode(ERRCODE_UNDEFINED_OBJECT),
                 errmsg("large object %u does not exist", loid)));
 
    CatalogTupleDelete(pg_lo_meta, &tuple->t_self);
 
    systable_endscan(scan);
 
    /*
     * Delete all the associated entries from pg_largeobject
     */
    ScanKeyInit(&skey[0],
                Anum_pg_largeobject_loid,
                BTEqualStrategyNumber, F_OIDEQ,
                ObjectIdGetDatum(loid));
 
    scan = systable_beginscan(pg_largeobject,
                              LargeObjectLOidPNIndexId, true,
                              NULL, 1, skey);
    while (HeapTupleIsValid(tuple = systable_getnext(scan)))
    {
        CatalogTupleDelete(pg_largeobject, &tuple->t_self);
    }
 
    systable_endscan(scan);
 
    table_close(pg_largeobject, RowExclusiveLock);
 
    table_close(pg_lo_meta, RowExclusiveLock);
}
 
/*
 * LargeObjectExists
 *
 * We don't use the system cache for large object metadata, for fear of
 * using too much local memory.
 *
 * This function always scans the system catalog using an up-to-date snapshot,
 * so it should not be used when a large object is opened in read-only mode
 * (because large objects opened in read only mode are supposed to be viewed
 * relative to the caller's snapshot, whereas in read-write mode they are
 * relative to a current snapshot).
 */
bool
LargeObjectExists(Oid loid)
{
    return LargeObjectExistsWithSnapshot(loid, NULL);
}
 
/*
 * Same as LargeObjectExists(), except snapshot to read with can be specified.
 */
bool
LargeObjectExistsWithSnapshot(Oid loid, Snapshot snapshot)
{
    Relation    pg_lo_meta;
    ScanKeyData skey[1];
    SysScanDesc sd;
    HeapTuple   tuple;
    bool        retval = false;
 
    ScanKeyInit(&skey[0],
                Anum_pg_largeobject_metadata_oid,
                BTEqualStrategyNumber, F_OIDEQ,
                ObjectIdGetDatum(loid));
 
    pg_lo_meta = table_open(LargeObjectMetadataRelationId,
                            AccessShareLock);
 
    sd = systable_beginscan(pg_lo_meta,
                            LargeObjectMetadataOidIndexId, true,
                            snapshot, 1, skey);
 
    tuple = systable_getnext(sd);
    if (HeapTupleIsValid(tuple))
        retval = true;
 
    systable_endscan(sd);
 
    table_close(pg_lo_meta, AccessShareLock);
 
    return retval;
}