nbtscan.c

/*-------------------------------------------------------------------------
 *
 * btscan.c--
 *    manage scans on btrees.
 *
 * Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    $Header: /cvsroot/pgsql/src/backend/access/nbtree/Attic/nbtscan.c,v 1.7 1997/02/18 17:13:45 momjian Exp $
 *
 *
 * NOTES
 *   Because we can be doing an index scan on a relation while we update
 *   it, we need to avoid missing data that moves around in the index.
 *   The routines and global variables in this file guarantee that all
 *   scans in the local address space stay correctly positioned.  This
 *   is all we need to worry about, since write locking guarantees that
 *   no one else will be on the same page at the same time as we are.
 *
 *   The scheme is to manage a list of active scans in the current backend.
 *   Whenever we add or remove records from an index, or whenever we
 *   split a leaf page, we check the list of active scans to see if any
 *   has been affected.  A scan is affected only if it is on the same
 *   relation, and the same page, as the update.
 *
 *-------------------------------------------------------------------------
 */

#include <postgres.h>

#include <storage/bufpage.h>
#include <access/nbtree.h>

typedef struct BTScanListData {
    IndexScanDesc		btsl_scan;
    struct BTScanListData	*btsl_next;
} BTScanListData;

typedef BTScanListData	*BTScanList;

static BTScanList	BTScans = (BTScanList) NULL;

static void _bt_scandel(IndexScanDesc scan, int op, BlockNumber blkno, OffsetNumber offno);
static bool _bt_scantouched(IndexScanDesc scan, BlockNumber blkno, OffsetNumber offno);

/*
 *  _bt_regscan() -- register a new scan.
 */
void
_bt_regscan(IndexScanDesc scan)
{
    BTScanList new_el;
    
    new_el = (BTScanList) palloc(sizeof(BTScanListData));
    new_el->btsl_scan = scan;
    new_el->btsl_next = BTScans;
    BTScans = new_el;
}

/*
 *  _bt_dropscan() -- drop a scan from the scan list
 */
void
_bt_dropscan(IndexScanDesc scan)
{
    BTScanList chk, last;
    
    last = (BTScanList) NULL;
    for (chk = BTScans;
	 chk != (BTScanList) NULL && chk->btsl_scan != scan;
	 chk = chk->btsl_next) {
	last = chk;
    }
    
    if (chk == (BTScanList) NULL)
	elog(WARN, "btree scan list trashed; can't find 0x%lx", scan);
    
    if (last == (BTScanList) NULL)
	BTScans = chk->btsl_next;
    else
	last->btsl_next = chk->btsl_next;
    
    pfree (chk);
}

/*
 *  _bt_adjscans() -- adjust all scans in the scan list to compensate
 *		      for a given deletion or insertion
 */
void
_bt_adjscans(Relation rel, ItemPointer tid, int op)
{
    BTScanList l;
    Oid relid;
    
    relid = rel->rd_id;
    for (l = BTScans; l != (BTScanList) NULL; l = l->btsl_next) {
	if (relid == l->btsl_scan->relation->rd_id)
	    _bt_scandel(l->btsl_scan, op,
			ItemPointerGetBlockNumber(tid),
			ItemPointerGetOffsetNumber(tid));
    }
}

/*
 *  _bt_scandel() -- adjust a single scan
 *
 * because each index page is always maintained as an ordered array of
 * index tuples, the index tuples on a given page shift beneath any
 * given scan.  an index modification "behind" a scan position (i.e.,
 * same page, lower or equal offset number) will therefore force us to
 * adjust the scan in the following ways:
 *
 * - on insertion, we shift the scan forward by one item.
 * - on deletion, we shift the scan backward by one item.
 *
 * note that:
 *
 * - we need not worry about the actual ScanDirection of the scan
 * itself, since the problem is that the "current" scan position has
 * shifted.
 * - modifications "ahead" of our scan position do not change the
 * array index of the current scan position and so can be ignored.
 */
static void
_bt_scandel(IndexScanDesc scan, int op, BlockNumber blkno, OffsetNumber offno)
{
    ItemPointer current;
    Buffer buf;
    BTScanOpaque so;
    
    if (!_bt_scantouched(scan, blkno, offno))
	return;
    
    so = (BTScanOpaque) scan->opaque;
    buf = so->btso_curbuf;
    
    current = &(scan->currentItemData);
    if (ItemPointerIsValid(current)
	&& ItemPointerGetBlockNumber(current) == blkno
	&& ItemPointerGetOffsetNumber(current) >= offno) {
	switch (op) {
	case BT_INSERT:
	    _bt_step(scan, &buf, ForwardScanDirection);
	    break;
	case BT_DELETE:
	    _bt_step(scan, &buf, BackwardScanDirection);
	    break;
	default:
	    elog(WARN, "_bt_scandel: bad operation '%d'", op);
	    /*NOTREACHED*/
	}
	so->btso_curbuf = buf;
    }
    
    current = &(scan->currentMarkData);
    if (ItemPointerIsValid(current)
	&& ItemPointerGetBlockNumber(current) == blkno
	&& ItemPointerGetOffsetNumber(current) >= offno) {
	ItemPointerData tmp;
	tmp = *current;
	*current = scan->currentItemData;
	scan->currentItemData = tmp;
	switch (op) {
	case BT_INSERT:
	    _bt_step(scan, &buf, ForwardScanDirection);
	    break;
	case BT_DELETE:
	    _bt_step(scan, &buf, BackwardScanDirection);
	    break;
	default:
	    elog(WARN, "_bt_scandel: bad operation '%d'", op);
	    /*NOTREACHED*/
	}
	so->btso_mrkbuf = buf;
	tmp = *current;
	*current = scan->currentItemData;
	scan->currentItemData = tmp;
    }
}

/*
 *  _bt_scantouched() -- check to see if a scan is affected by a given
 *			 change to the index
 */
static bool
_bt_scantouched(IndexScanDesc scan, BlockNumber blkno, OffsetNumber offno)
{
    ItemPointer current;
    
    current = &(scan->currentItemData);
    if (ItemPointerIsValid(current)
	&& ItemPointerGetBlockNumber(current) == blkno
	&& ItemPointerGetOffsetNumber(current) >= offno)
	return (true);
    
    current = &(scan->currentMarkData);
    if (ItemPointerIsValid(current)
	&& ItemPointerGetBlockNumber(current) == blkno
	&& ItemPointerGetOffsetNumber(current) >= offno)
	return (true);
    
    return (false);
}