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/*-------------------------------------------------------------------------
*
* 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 1.13 1998/02/28 13:53:18 vadim 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(ERROR, "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(ERROR, "_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;
so->btso_curbuf = so->btso_mrkbuf;
so->btso_mrkbuf = buf;
buf = so->btso_curbuf;
switch (op)
{
case BT_INSERT:
_bt_step(scan, &buf, ForwardScanDirection);
break;
case BT_DELETE:
_bt_step(scan, &buf, BackwardScanDirection);
break;
default:
elog(ERROR, "_bt_scandel: bad operation '%d'", op);
/* NOTREACHED */
}
so->btso_curbuf = so->btso_mrkbuf;
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);
}
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