8000 The GiST scan algorithm uses LSNs to detect concurrent pages splits, but · justtesting112233/postgres@7ff02ad · GitHub
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hlinnakahlinnaka
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The GiST scan algorithm uses LSNs to detect concurrent pages splits, but
temporary indexes are not WAL-logged. We used a constant LSN for temporary indexes, on the assumption that we don't need to worry about concurrent page splits in temporary indexes because they're only visible to the current session. But that assumption is wrong, it's possible to insert rows and split pages in the same session, while a scan is in progress. For example, by opening a cursor and fetching some rows, and INSERTing new rows before fetching some more. Fix by generating fake increasing LSNs, used in place of real LSNs in temporary GiST indexes.
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4 files changed

+31
-11
lines changed

4 files changed

+31
-11
lines changed

‎src/backend/access/gist/gist.c

Lines changed: 4 additions & 6 deletions
Original file line numberDiff line numberDiff line change
@@ -22,8 +22,6 @@
2222
#include "storage/indexfsm.h"
2323
#include "utils/memutils.h"
2424

25-
const XLogRecPtr XLogRecPtrForTemp = {1, 1};
26-
2725
/* Working state for gistbuild and its callback */
2826
typedef struct
2927
{
@@ -132,7 +130,7 @@ gistbuild(PG_FUNCTION_ARGS)
132130
PageSetTLI(page, ThisTimeLineID);
133131
}
134132
else
135-
PageSetLSN(page, XLogRecPtrForTemp);
133+
PageSetLSN(page, GetXLogRecPtrForTemp());
136134

137135
UnlockReleaseBuffer(buffer);
138136

@@ -423,7 +421,7 @@ gistplacetopage(GISTInsertState *state, GISTSTATE *giststate)
423421
{
424422
for (ptr = dist; ptr; ptr = ptr->next)
425423
{
426-
PageSetLSN(ptr->page, XLogRecPtrForTemp);
424+
PageSetLSN(ptr->page, GetXLogRecPtrForTemp());
427425
}
428426
}
429427

@@ -491,7 +489,7 @@ gistplacetopage(GISTInsertState *state, GISTSTATE *giststate)
491489
PageSetTLI(state->stack->page, ThisTimeLineID);
492490
}
493491
else
494-
PageSetLSN(state->stack->page, XLogRecPtrForTemp);
492+
PageSetLSN(state->stack->page, GetXLogRecPtrForTemp());
495493

496494
if (state->stack->blkno == GIST_ROOT_BLKNO)
497495
state->needInsertComplete = false;
@@ -1027,7 +1025,7 @@ gistnewroot(Relation r, Buffer buffer, IndexTuple *itup, int len, ItemPointer ke
10271025
PageSetTLI(page, ThisTimeLineID);
10281026
}
10291027
else
1030-
PageSetLSN(page, XLogRecPtrForTemp);
1028+
PageSetLSN(page, GetXLogRecPtrForTemp());
10311029

10321030
END_CRIT_SECTION();
10331031
}

‎src/backend/access/gist/gistutil.c

Lines changed: 21 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -677,3 +677,24 @@ gistoptions(PG_FUNCTION_ARGS)
677677
PG_RETURN_BYTEA_P(result);
678678
PG_RETURN_NULL();
679679
}
680+
681+
/*
682+
* Temporary GiST indexes are not WAL-logged, but we need LSNs to detect
683+
* concurrent page splits anyway. GetXLogRecPtrForTemp() provides a fake
684+
* sequence of LSNs for that purpose. Each call generates an LSN that is
685+
* greater than any previous value returned by this function in the same
686+
* session.
687+
*/
688+
XLogRecPtr
689+
GetXLogRecPtrForTemp(void)
690+
{
691+
static XLogRecPtr counter = {0, 1};
692+
693+
counter.xrecoff++;
694+
if (counter.xrecoff == 0)
695+
{
696+
counter.xlogid++;
697+
counter.xrecoff++;
698+
}
699+
return counter;
700+
}

‎src/backend/access/gist/gistvacuum.c

Lines changed: 4 additions & 4 deletions
Original file line numberDiff line numberDiff line change
@@ -138,7 +138,7 @@ gistDeleteSubtree(GistVacuum *gv, BlockNumber blkno)
138138
PageSetTLI(page, ThisTimeLineID);
139139
}
140140
else
141-
PageSetLSN(page, XLogRecPtrForTemp);
141+
PageSetLSN(page, GetXLogRecPtrForTemp());
142142

143143
END_CRIT_SECTION();
144144

@@ -245,7 +245,7 @@ vacuumSplitPage(GistVacuum *gv, Page tempPage, Buffer buffer, IndexTuple *addon,
245245
else
246246
{
247247
for (ptr = dist; ptr; ptr = ptr->next)
248-
PageSetLSN(BufferGetPage(ptr->buffer), XLogRecPtrForTemp);
248+
PageSetLSN(BufferGetPage(ptr->buffer), GetXLogRecPtrForTemp());
249249
}
250250

251251
for (ptr = dist; ptr; ptr = ptr->next)
@@ -460,7 +460,7 @@ gistVacuumUpdate(GistVacuum *gv, BlockNumber blkno, bool needunion)
460460
pfree(rdata);
461461
}
462462
else
463-
PageSetLSN(page, XLogRecPtrForTemp);
463+
PageSetLSN(page, GetXLogRecPtrForTemp());
464464
}
465465

466466
END_CRIT_SECTION();
@@ -774,7 +774,7 @@ gistbulkdelete(PG_FUNCTION_ARGS)
774774
pfree(rdata);
775775
}
776776
else
777-
PageSetLSN(page, XLogRecPtrForTemp);
777+
PageSetLSN(page, GetXLogRecPtrForTemp());
778778

779779
END_CRIT_SECTION();
780780
}

‎src/include/access/gist_private.h

Lines changed: 2 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -87,7 +87,6 @@ typedef struct GISTScanOpaqueData
8787
typedef GISTScanOpaqueData *GISTScanOpaque;
8888

8989
/* XLog stuff */
90-
extern const XLogRecPtr XLogRecPtrForTemp;
9190

9291
#define XLOG_GIST_PAGE_UPDATE 0x00
9392
#define XLOG_GIST_NEW_ROOT 0x20
@@ -326,6 +325,8 @@ extern void gistMakeUnionKey(GISTSTATE *giststate, int attno,
326325
GISTENTRY *entry2, bool isnull2,
327326
Datum *dst, bool *dstisnull);
328327

328+
extern XLogRecPtr GetXLogRecPtrForTemp(void);
329+
329330
/* gistvacuum.c */
330331
extern Datum gistbulkdelete(PG_FUNCTION_ARGS);
331332
extern Datum gistvacuumcleanup(PG_FUNCTION_ARGS);

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