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Invent WAL timelines, as per recent discussion, to make point-in-time

Browse Source 
recovery more manageable.  Also, undo recent change to add FILE_HEADER
and WASTED_SPACE records to XLOG; instead make the XLOG page header
variable-size with extra fields in the first page of an XLOG file.
This should fix the boundary-case bugs observed by Mark Kirkwood.
initdb forced due to change of XLOG representation.
This commit is contained in:
Tom Lane
2004-07-21 22:31:26 +00:00
parent 8d3517dc1f
commit 2042b3428d
26 changed files with 1322 additions and 982 deletions
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24
src/backend/access/heap/heapam.c
View File

@@ -8,7 +8,7 @@
*
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/access/heap/heapam.c,v 1.170 2004/07/11 18:01:44 tgl Exp $
* $PostgreSQL: pgsql/src/backend/access/heap/heapam.c,v 1.171 2004/07/21 22:31:19 tgl Exp $
*
*
* INTERFACE ROUTINES
@@ -1214,7 +1214,7 @@ heap_insert(Relation relation, HeapTuple tup, CommandId cid)
recptr = XLogInsert(RM_HEAP_ID, info, rdata);
PageSetLSN(page, recptr);
PageSetSUI(page, ThisStartUpID);
PageSetTLI(page, ThisTimeLineID);
}
else
{
@@ -1390,7 +1390,7 @@ l1:
recptr = XLogInsert(RM_HEAP_ID, XLOG_HEAP_DELETE, rdata);
PageSetLSN(dp, recptr);
PageSetSUI(dp, ThisStartUpID);
PageSetTLI(dp, ThisTimeLineID);
}
else
{
@@ -1748,10 +1748,10 @@ l2:
if (newbuf != buffer)
{
PageSetLSN(BufferGetPage(newbuf), recptr);
PageSetSUI(BufferGetPage(newbuf), ThisStartUpID);
PageSetTLI(BufferGetPage(newbuf), ThisTimeLineID);
}
PageSetLSN(BufferGetPage(buffer), recptr);
PageSetSUI(BufferGetPage(buffer), ThisStartUpID);
PageSetTLI(BufferGetPage(buffer), ThisTimeLineID);
}
else
{
@@ -1902,7 +1902,7 @@ l3:
* XLOG stuff: no logging is required as long as we have no
* savepoints. For savepoints private log could be used...
*/
PageSetSUI(BufferGetPage(*buffer), ThisStartUpID);
PageSetTLI(BufferGetPage(*buffer), ThisTimeLineID);
/* store transaction information of xact marking the tuple */
tuple->t_data->t_infomask &= ~(HEAP_XMAX_COMMITTED |
@@ -2184,7 +2184,7 @@ heap_xlog_clean(bool redo, XLogRecPtr lsn, XLogRecord *record)
PageRepairFragmentation(page, NULL);
PageSetLSN(page, lsn);
PageSetSUI(page, ThisStartUpID); /* prev sui */
PageSetTLI(page, ThisTimeLineID);
LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
WriteBuffer(buffer);
}
@@ -2217,7 +2217,7 @@ heap_xlog_newpage(bool redo, XLogRecPtr lsn, XLogRecord *record)
memcpy(page, (char *) xlrec + SizeOfHeapNewpage, BLCKSZ);
PageSetLSN(page, lsn);
PageSetSUI(page, ThisStartUpID);
PageSetTLI(page, ThisTimeLineID);
LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
WriteBuffer(buffer);
}
@@ -2283,7 +2283,7 @@ heap_xlog_delete(bool redo, XLogRecPtr lsn, XLogRecord *record)
/* Make sure there is no forward chain link in t_ctid */
htup->t_ctid = xlrec->target.tid;
PageSetLSN(page, lsn);
PageSetSUI(page, ThisStartUpID);
PageSetTLI(page, ThisTimeLineID);
LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
WriteBuffer(buffer);
return;
@@ -2368,7 +2368,7 @@ heap_xlog_insert(bool redo, XLogRecPtr lsn, XLogRecord *record)
if (offnum == InvalidOffsetNumber)
elog(PANIC, "heap_insert_redo: failed to add tuple");
PageSetLSN(page, lsn);
PageSetSUI(page, ThisStartUpID); /* prev sui */
PageSetTLI(page, ThisTimeLineID);
LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
WriteBuffer(buffer);
return;
@@ -2466,7 +2466,7 @@ heap_xlog_update(bool redo, XLogRecPtr lsn, XLogRecord *record, bool move)
if (samepage)
goto newsame;
PageSetLSN(page, lsn);
PageSetSUI(page, ThisStartUpID);
PageSetTLI(page, ThisTimeLineID);
LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
WriteBuffer(buffer);
goto newt;
@@ -2564,7 +2564,7 @@ newsame:;
if (offnum == InvalidOffsetNumber)
elog(PANIC, "heap_update_redo: failed to add tuple");
PageSetLSN(page, lsn);
PageSetSUI(page, ThisStartUpID); /* prev sui */
PageSetTLI(page, ThisTimeLineID);
LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
WriteBuffer(buffer);
return;

20
src/backend/access/nbtree/nbtinsert.c
View File

@@ -8,7 +8,7 @@
*
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/access/nbtree/nbtinsert.c,v 1.112 2004/04/21 18:24:25 tgl Exp $
* $PostgreSQL: pgsql/src/backend/access/nbtree/nbtinsert.c,v 1.113 2004/07/21 22:31:19 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@@ -621,11 +621,11 @@ _bt_insertonpg(Relation rel,
if (BufferIsValid(metabuf))
{
PageSetLSN(metapg, recptr);
PageSetSUI(metapg, ThisStartUpID);
PageSetTLI(metapg, ThisTimeLineID);
}
PageSetLSN(page, recptr);
PageSetSUI(page, ThisStartUpID);
PageSetTLI(page, ThisTimeLineID);
}
END_CRIT_SECTION();
@@ -903,13 +903,13 @@ _bt_split(Relation rel, Buffer buf, OffsetNumber firstright,
recptr = XLogInsert(RM_BTREE_ID, xlinfo, rdata);
PageSetLSN(leftpage, recptr);
PageSetSUI(leftpage, ThisStartUpID);
PageSetTLI(leftpage, ThisTimeLineID);
PageSetLSN(rightpage, recptr);
PageSetSUI(rightpage, ThisStartUpID);
PageSetTLI(rightpage, ThisTimeLineID);
if (!P_RIGHTMOST(ropaque))
{
PageSetLSN(spage, recptr);
PageSetSUI(spage, ThisStartUpID);
PageSetTLI(spage, ThisTimeLineID);
}
}
@@ -1494,13 +1494,13 @@ _bt_newroot(Relation rel, Buffer lbuf, Buffer rbuf)
recptr = XLogInsert(RM_BTREE_ID, XLOG_BTREE_NEWROOT, rdata);
PageSetLSN(rootpage, recptr);
PageSetSUI(rootpage, ThisStartUpID);
PageSetTLI(rootpage, ThisTimeLineID);
PageSetLSN(metapg, recptr);
PageSetSUI(metapg, ThisStartUpID);
PageSetTLI(metapg, ThisTimeLineID);
PageSetLSN(lpage, recptr);
PageSetSUI(lpage, ThisStartUpID);
PageSetTLI(lpage, ThisTimeLineID);
PageSetLSN(rpage, recptr);
PageSetSUI(rpage, ThisStartUpID);
PageSetTLI(rpage, ThisTimeLineID);
}
END_CRIT_SECTION();

20
src/backend/access/nbtree/nbtpage.c
View File

@@ -9,7 +9,7 @@
*
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/access/nbtree/nbtpage.c,v 1.76 2004/06/02 17:28:17 tgl Exp $
* $PostgreSQL: pgsql/src/backend/access/nbtree/nbtpage.c,v 1.77 2004/07/21 22:31:20 tgl Exp $
*
* NOTES
* Postgres btree pages look like ordinary relation pages. The opaque
@@ -84,7 +84,7 @@ _bt_metapinit(Relation rel)
rdata);
PageSetLSN(pg, recptr);
PageSetSUI(pg, ThisStartUpID);
PageSetTLI(pg, ThisTimeLineID);
}
END_CRIT_SECTION();
@@ -249,9 +249,9 @@ _bt_getroot(Relation rel, int access)
recptr = XLogInsert(RM_BTREE_ID, XLOG_BTREE_NEWROOT, &rdata);
PageSetLSN(rootpage, recptr);
PageSetSUI(rootpage, ThisStartUpID);
PageSetTLI(rootpage, ThisTimeLineID);
PageSetLSN(metapg, recptr);
PageSetSUI(metapg, ThisStartUpID);
PageSetTLI(metapg, ThisTimeLineID);
}
END_CRIT_SECTION();
@@ -686,7 +686,7 @@ _bt_delitems(Relation rel, Buffer buf,
recptr = XLogInsert(RM_BTREE_ID, XLOG_BTREE_DELETE, rdata);
PageSetLSN(page, recptr);
PageSetSUI(page, ThisStartUpID);
PageSetTLI(page, ThisTimeLineID);
}
END_CRIT_SECTION();
@@ -1080,22 +1080,22 @@ _bt_pagedel(Relation rel, Buffer buf, bool vacuum_full)
if (BufferIsValid(metabuf))
{
PageSetLSN(metapg, recptr);
PageSetSUI(metapg, ThisStartUpID);
PageSetTLI(metapg, ThisTimeLineID);
}
page = BufferGetPage(pbuf);
PageSetLSN(page, recptr);
PageSetSUI(page, ThisStartUpID);
PageSetTLI(page, ThisTimeLineID);
page = BufferGetPage(rbuf);
PageSetLSN(page, recptr);
PageSetSUI(page, ThisStartUpID);
PageSetTLI(page, ThisTimeLineID);
page = BufferGetPage(buf);
PageSetLSN(page, recptr);
PageSetSUI(page, ThisStartUpID);
PageSetTLI(page, ThisTimeLineID);
if (BufferIsValid(lbuf))
{
page = BufferGetPage(lbuf);
PageSetLSN(page, recptr);
PageSetSUI(page, ThisStartUpID);
PageSetTLI(page, ThisTimeLineID);
}
}

8
src/backend/access/nbtree/nbtsort.c
View File

@@ -56,7 +56,7 @@
* Portions Copyright (c) 1994, Regents of the University of California
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/access/nbtree/nbtsort.c,v 1.84 2004/07/19 02:47:03 tgl Exp $
* $PostgreSQL: pgsql/src/backend/access/nbtree/nbtsort.c,v 1.85 2004/07/21 22:31:20 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@@ -299,14 +299,14 @@ _bt_blwritepage(BTWriteState *wstate, Page page, BlockNumber blkno)
recptr = XLogInsert(RM_HEAP_ID, XLOG_HEAP_NEWPAGE, rdata);
PageSetLSN(page, recptr);
PageSetSUI(page, ThisStartUpID);
PageSetTLI(page, ThisTimeLineID);
END_CRIT_SECTION();
}
else
{
/* Leave the page LSN zero if not WAL-logged, but set SUI anyway */
PageSetSUI(page, ThisStartUpID);
/* Leave the page LSN zero if not WAL-logged, but set TLI anyway */
PageSetTLI(page, ThisTimeLineID);
}
/*

24
src/backend/access/nbtree/nbtxlog.c
View File

@@ -8,7 +8,7 @@
* Portions Copyright (c) 1994, Regents of the University of California
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/access/nbtree/nbtxlog.c,v 1.15 2004/07/11 18:01:45 tgl Exp $
* $PostgreSQL: pgsql/src/backend/access/nbtree/nbtxlog.c,v 1.16 2004/07/21 22:31:20 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@@ -136,7 +136,7 @@ _bt_restore_meta(Relation reln, XLogRecPtr lsn,
pageop->btpo_flags = BTP_META;
PageSetLSN(metapg, lsn);
PageSetSUI(metapg, ThisStartUpID);
PageSetTLI(metapg, ThisTimeLineID);
LockBuffer(metabuf, BUFFER_LOCK_UNLOCK);
WriteBuffer(metabuf);
}
@@ -197,7 +197,7 @@ btree_xlog_insert(bool redo, bool isleaf, bool ismeta,
elog(PANIC, "btree_insert_redo: failed to add item");
PageSetLSN(page, lsn);
PageSetSUI(page, ThisStartUpID);
PageSetTLI(page, ThisTimeLineID);
LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
WriteBuffer(buffer);
}
@@ -281,7 +281,7 @@ btree_xlog_split(bool redo, bool onleft, bool isroot,
xlrec->leftlen);
PageSetLSN(page, lsn);
PageSetSUI(page, ThisStartUpID);
PageSetTLI(page, ThisTimeLineID);
LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
WriteBuffer(buffer);
}
@@ -317,7 +317,7 @@ btree_xlog_split(bool redo, bool onleft, bool isroot,
record->xl_len - SizeOfBtreeSplit - xlrec->leftlen);
PageSetLSN(page, lsn);
PageSetSUI(page, ThisStartUpID);
PageSetTLI(page, ThisTimeLineID);
LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
WriteBuffer(buffer);
}
@@ -353,7 +353,7 @@ btree_xlog_split(bool redo, bool onleft, bool isroot,
pageop->btpo_prev = rightsib;
PageSetLSN(page, lsn);
PageSetSUI(page, ThisStartUpID);
PageSetTLI(page, ThisTimeLineID);
LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
WriteBuffer(buffer);
}
@@ -420,7 +420,7 @@ btree_xlog_delete(bool redo, XLogRecPtr lsn, XLogRecord *record)
}
PageSetLSN(page, lsn);
PageSetSUI(page, ThisStartUpID);
PageSetTLI(page, ThisTimeLineID);
LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
WriteBuffer(buffer);
}
@@ -489,7 +489,7 @@ btree_xlog_delete_page(bool redo, bool ismeta,
}
PageSetLSN(page, lsn);
PageSetSUI(page, ThisStartUpID);
PageSetTLI(page, ThisTimeLineID);
LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
WriteBuffer(buffer);
}
@@ -515,7 +515,7 @@ btree_xlog_delete_page(bool redo, bool ismeta,
pageop->btpo_prev = leftsib;
PageSetLSN(page, lsn);
PageSetSUI(page, ThisStartUpID);
PageSetTLI(page, ThisTimeLineID);
LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
WriteBuffer(buffer);
}
@@ -543,7 +543,7 @@ btree_xlog_delete_page(bool redo, bool ismeta,
pageop->btpo_next = rightsib;
PageSetLSN(page, lsn);
PageSetSUI(page, ThisStartUpID);
PageSetTLI(page, ThisTimeLineID);
LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
WriteBuffer(buffer);
}
@@ -569,7 +569,7 @@ btree_xlog_delete_page(bool redo, bool ismeta,
pageop->btpo_flags = BTP_DELETED;
PageSetLSN(page, lsn);
PageSetSUI(page, ThisStartUpID);
PageSetTLI(page, ThisTimeLineID);
LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
WriteBuffer(buffer);
}
@@ -632,7 +632,7 @@ btree_xlog_newroot(bool redo, XLogRecPtr lsn, XLogRecord *record)
record->xl_len - SizeOfBtreeNewroot);
PageSetLSN(page, lsn);
PageSetSUI(page, ThisStartUpID);
PageSetTLI(page, ThisTimeLineID);
LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
WriteBuffer(buffer);

8
src/backend/access/transam/recovery.conf.sample
View File

@@ -63,4 +63,12 @@
#
#recovery_target_inclusive = 'true' # 'true' or 'false'
#
#
# If you want to recover into a timeline other than the "main line" shown in
# pg_control, specify the timeline number here, or write 'latest' to get
# the latest branch for which there's a history file.
#
#recovery_target_timeline = '33' # number or 'latest'
#
#
#---------------------------------------------------------------------------

6
src/backend/access/transam/rmgr.c
View File

@@ -3,7 +3,7 @@
*
* Resource managers definition
*
* $PostgreSQL: pgsql/src/backend/access/transam/rmgr.c,v 1.13 2004/07/01 00:49:42 tgl Exp $
* $PostgreSQL: pgsql/src/backend/access/transam/rmgr.c,v 1.14 2004/07/21 22:31:20 tgl Exp $
*/
#include "postgres.h"
@@ -14,12 +14,12 @@
#include "access/rtree.h"
#include "access/slru.h"
#include "access/xact.h"
#include "access/xlog.h"
#include "access/xlog_internal.h"
#include "storage/smgr.h"
#include "commands/sequence.h"
RmgrData RmgrTable[RM_MAX_ID + 1] = {
const RmgrData RmgrTable[RM_MAX_ID + 1] = {
{"XLOG", xlog_redo, xlog_undo, xlog_desc, NULL, NULL},
{"Transaction", xact_redo, xact_undo, xact_desc, NULL, NULL},
{"Storage", smgr_redo, smgr_undo, smgr_desc, NULL, NULL},

3
src/backend/access/transam/slru.c
View File

@@ -6,7 +6,7 @@
* Portions Copyright (c) 1996-2003, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
* $PostgreSQL: pgsql/src/backend/access/transam/slru.c,v 1.17 2004/07/01 00:49:42 tgl Exp $
* $PostgreSQL: pgsql/src/backend/access/transam/slru.c,v 1.18 2004/07/21 22:31:20 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@@ -22,6 +22,7 @@
#include "postmaster/bgwriter.h"
#include "storage/fd.h"
#include "storage/lwlock.h"
#include "storage/shmem.h"
#include "miscadmin.h"

1436
src/backend/access/transam/xlog.c
View File

File diff suppressed because it is too large Load Diff

156
src/backend/access/transam/xlogutils.c
View File

@@ -2,169 +2,31 @@
*
* xlogutils.c
*
* PostgreSQL transaction log manager utility routines
*
* This file contains support routines that are used by XLOG replay functions.
* None of this code is used during normal system operation.
*
*
* Portions Copyright (c) 1996-2003, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
* $PostgreSQL: pgsql/src/backend/access/transam/xlogutils.c,v 1.31 2004/06/18 06:13:15 tgl Exp $
* $PostgreSQL: pgsql/src/backend/access/transam/xlogutils.c,v 1.32 2004/07/21 22:31:20 tgl Exp $
*
*-------------------------------------------------------------------------
*/
#include "postgres.h"
#include "access/htup.h"
#include "access/xlogutils.h"
#include "catalog/pg_database.h"
#include "storage/bufpage.h"
#include "storage/bufmgr.h"
#include "storage/smgr.h"
#include "utils/hsearch.h"
#include "utils/relcache.h"
/*
* ---------------------------------------------------------------
*
* Index support functions
*
*----------------------------------------------------------------
*/
/*
* Check if specified heap tuple was inserted by given
* xaction/command and return
*
* - -1 if not
* - 0 if there is no tuple at all
* - 1 if yes
*/
int
XLogIsOwnerOfTuple(RelFileNode hnode, ItemPointer iptr,
TransactionId xid, CommandId cid)
{
Relation reln;
Buffer buffer;
Page page;
ItemId lp;
HeapTupleHeader htup;
reln = XLogOpenRelation(false, RM_HEAP_ID, hnode);
if (!RelationIsValid(reln))
return (0);
buffer = ReadBuffer(reln, ItemPointerGetBlockNumber(iptr));
if (!BufferIsValid(buffer))
return (0);
LockBuffer(buffer, BUFFER_LOCK_SHARE);
page = (Page) BufferGetPage(buffer);
if (PageIsNew((PageHeader) page) ||
ItemPointerGetOffsetNumber(iptr) > PageGetMaxOffsetNumber(page))
{
LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
ReleaseBuffer(buffer);
return (0);
}
lp = PageGetItemId(page, ItemPointerGetOffsetNumber(iptr));
if (!ItemIdIsUsed(lp) || ItemIdDeleted(lp))
{
LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
ReleaseBuffer(buffer);
return (0);
}
htup = (HeapTupleHeader) PageGetItem(page, lp);
Assert(PageGetSUI(page) == ThisStartUpID);
if (!TransactionIdEquals(HeapTupleHeaderGetXmin(htup), xid) ||
HeapTupleHeaderGetCmin(htup) != cid)
{
LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
ReleaseBuffer(buffer);
return (-1);
}
LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
ReleaseBuffer(buffer);
return (1);
}
/*
* MUST BE CALLED ONLY ON RECOVERY.
*
* Check if exists valid (inserted by not aborted xaction) heap tuple
* for given item pointer
*/
bool
XLogIsValidTuple(RelFileNode hnode, ItemPointer iptr)
{
Relation reln;
Buffer buffer;
Page page;
ItemId lp;
HeapTupleHeader htup;
reln = XLogOpenRelation(false, RM_HEAP_ID, hnode);
if (!RelationIsValid(reln))
return (false);
buffer = ReadBuffer(reln, ItemPointerGetBlockNumber(iptr));
if (!BufferIsValid(buffer))
return (false);
LockBuffer(buffer, BUFFER_LOCK_SHARE);
page = (Page) BufferGetPage(buffer);
if (PageIsNew((PageHeader) page) ||
ItemPointerGetOffsetNumber(iptr) > PageGetMaxOffsetNumber(page))
{
LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
ReleaseBuffer(buffer);
return (false);
}
if (PageGetSUI(page) != ThisStartUpID)
{
Assert(PageGetSUI(page) < ThisStartUpID);
LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
ReleaseBuffer(buffer);
return (true);
}
lp = PageGetItemId(page, ItemPointerGetOffsetNumber(iptr));
if (!ItemIdIsUsed(lp) || ItemIdDeleted(lp))
{
LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
ReleaseBuffer(buffer);
return (false);
}
htup = (HeapTupleHeader) PageGetItem(page, lp);
/* MUST CHECK WASN'T TUPLE INSERTED IN PREV STARTUP */
if (!(htup->t_infomask & HEAP_XMIN_COMMITTED))
{
if (htup->t_infomask & HEAP_XMIN_INVALID ||
(htup->t_infomask & HEAP_MOVED_IN &&
TransactionIdDidAbort(HeapTupleHeaderGetXvac(htup))) ||
TransactionIdDidAbort(HeapTupleHeaderGetXmin(htup)))
{
LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
ReleaseBuffer(buffer);
return (false);
}
}
LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
ReleaseBuffer(buffer);
return (true);
}
/*
* ---------------------------------------------------------------
*
* Storage related support functions
*
*----------------------------------------------------------------
*/
Buffer
@@ -198,8 +60,10 @@ XLogReadBuffer(bool extend, Relation reln, BlockNumber blkno)
return (buffer);
}
/*
* "Relation" cache
* Lightweight "Relation" cache --- this substitutes for the normal relcache
* during XLOG replay.
*/
typedef struct XLogRelDesc