hashscan.c

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/*-------------------------------------------------------------------------
 *
 * hashscan.c
 *    manage scans on hash tables
 *
 * Portions Copyright (c) 1996-2013, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/access/hash/hashscan.c
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include "access/hash.h"
#include "access/relscan.h"
#include "utils/memutils.h"
#include "utils/rel.h"
#include "utils/resowner.h"


/*
 * We track all of a backend's active scans on hash indexes using a list
 * of HashScanListData structs, which are allocated in TopMemoryContext.
 * It's okay to use a long-lived context because we rely on the ResourceOwner
 * mechanism to clean up unused entries after transaction or subtransaction
 * abort.  We can't safely keep the entries in the executor's per-query
 * context, because that might be already freed before we get a chance to
 * clean up the list.  (XXX seems like there should be a better way to
 * manage this...)
 */
typedef struct HashScanListData
{
    IndexScanDesc hashsl_scan;
    ResourceOwner hashsl_owner;
    struct HashScanListData *hashsl_next;
} HashScanListData;

typedef HashScanListData *HashScanList;

static HashScanList HashScans = NULL;


/*
 * ReleaseResources_hash() --- clean up hash subsystem resources.
 *
 * This is here because it needs to touch this module's static var HashScans.
 */
void
ReleaseResources_hash(void)
{
    HashScanList l;
    HashScanList prev;
    HashScanList next;

    /*
     * Release all HashScanList items belonging to the current ResourceOwner.
     * Note that we do not release the underlying IndexScanDesc; that's in
     * executor memory and will go away on its own (in fact quite possibly has
     * gone away already, so we mustn't try to touch it here).
     *
     * Note: this should be a no-op during normal query shutdown. However, in
     * an abort situation ExecutorEnd is not called and so there may be open
     * index scans to clean up.
     */
    prev = NULL;

    for (l = HashScans; l != NULL; l = next)
    {
        next = l->hashsl_next;
        if (l->hashsl_owner == CurrentResourceOwner)
        {
            if (prev == NULL)
                HashScans = next;
            else
                prev->hashsl_next = next;

            pfree(l);
            /* prev does not change */
        }
        else
            prev = l;
    }
}

/*
 *  _hash_regscan() -- register a new scan.
 */
void
_hash_regscan(IndexScanDesc scan)
{
    HashScanList new_el;

    new_el = (HashScanList) MemoryContextAlloc(TopMemoryContext,
                                               sizeof(HashScanListData));
    new_el->hashsl_scan = scan;
    new_el->hashsl_owner = CurrentResourceOwner;
    new_el->hashsl_next = HashScans;
    HashScans = new_el;
}

/*
 *  _hash_dropscan() -- drop a scan from the scan list
 */
void
_hash_dropscan(IndexScanDesc scan)
{
    HashScanList chk,
                last;

    last = NULL;
    for (chk = HashScans;
         chk != NULL && chk->hashsl_scan != scan;
         chk = chk->hashsl_next)
        last = chk;

    if (chk == NULL)
        elog(ERROR, "hash scan list trashed; cannot find 0x%p", (void *) scan);

    if (last == NULL)
        HashScans = chk->hashsl_next;
    else
        last->hashsl_next = chk->hashsl_next;

    pfree(chk);
}

/*
 * Is there an active scan in this bucket?
 */
bool
_hash_has_active_scan(Relation rel, Bucket bucket)
{
    Oid         relid = RelationGetRelid(rel);
    HashScanList l;

    for (l = HashScans; l != NULL; l = l->hashsl_next)
    {
        if (relid == l->hashsl_scan->indexRelation->rd_id)
        {
            HashScanOpaque so = (HashScanOpaque) l->hashsl_scan->opaque;

            if (so->hashso_bucket_valid &&
                so->hashso_bucket == bucket)
                return true;
        }
    }

    return false;
}