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Commit786afc1

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Revert patch for taking fewer snapshots.
This reverts commitd573e23, "Take fewersnapshots". While that seemed like a good idea at the time, it causedexecution to use a snapshot that had been acquired before locking any ofthe tables mentioned in the query. This created user-visible anomaliesthat were not present in any prior release of Postgres, as reported byTomas Vondra. While this whole area could do with a redesign (since thereare related cases that have anomalies anyway), it doesn't seem likely thatany future patch would be reasonably back-patchable; and we don't want 9.2to exhibit a behavior that's subtly unlike either past or future releases.Hence, revert to prior code while we rethink the problem.
1 parenteea6ada commit786afc1

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7 files changed

+32
-46
lines changed

7 files changed

+32
-46
lines changed

‎doc/src/sgml/release-9.2.sgml‎

Lines changed: 0 additions & 7 deletions
Original file line numberDiff line numberDiff line change
@@ -809,13 +809,6 @@
809809
</para>
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</listitem>
811811

812-
<listitem>
813-
<para>
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Take fewer <acronym>MVCC</acronym> snapshots
815-
(Robert Haas)
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</para>
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</listitem>
818-
819812
<listitem>
820813
<para>
821814
Make the number of CLOG buffers scale based on <link

‎src/backend/commands/portalcmds.c‎

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -121,7 +121,7 @@ PerformCursorOpen(PlannedStmt *stmt, ParamListInfo params,
121121
/*
122122
* Start execution, inserting parameters if any.
123123
*/
124-
PortalStart(portal,params,0,true);
124+
PortalStart(portal,params,0,GetActiveSnapshot());
125125

126126
Assert(portal->strategy==PORTAL_ONE_SELECT);
127127

‎src/backend/commands/prepare.c‎

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -289,7 +289,7 @@ ExecuteQuery(ExecuteStmt *stmt, IntoClause *intoClause,
289289
/*
290290
* Run the portal as appropriate.
291291
*/
292-
PortalStart(portal,paramLI,eflags,true);
292+
PortalStart(portal,paramLI,eflags,GetActiveSnapshot());
293293

294294
(void)PortalRun(portal,count, false,dest,dest,completionTag);
295295

‎src/backend/executor/spi.c‎

Lines changed: 11 additions & 7 deletions
Original file line numberDiff line numberDiff line change
@@ -1126,6 +1126,7 @@ SPI_cursor_open_internal(const char *name, SPIPlanPtr plan,
11261126
CachedPlan*cplan;
11271127
List*stmt_list;
11281128
char*query_string;
1129+
Snapshotsnapshot;
11291130
MemoryContextoldcontext;
11301131
Portalportal;
11311132

@@ -1268,6 +1269,15 @@ SPI_cursor_open_internal(const char *name, SPIPlanPtr plan,
12681269
}
12691270
}
12701271

1272+
/* Set up the snapshot to use. */
1273+
if (read_only)
1274+
snapshot=GetActiveSnapshot();
1275+
else
1276+
{
1277+
CommandCounterIncrement();
1278+
snapshot=GetTransactionSnapshot();
1279+
}
1280+
12711281
/*
12721282
* If the plan has parameters, copy them into the portal. Note that this
12731283
* must be done after revalidating the plan, because in dynamic parameter
@@ -1283,13 +1293,7 @@ SPI_cursor_open_internal(const char *name, SPIPlanPtr plan,
12831293
/*
12841294
* Start portal execution.
12851295
*/
1286-
if (read_only)
1287-
PortalStart(portal,paramLI,0, true);
1288-
else
1289-
{
1290-
CommandCounterIncrement();
1291-
PortalStart(portal,paramLI,0, false);
1292-
}
1296+
PortalStart(portal,paramLI,0,snapshot);
12931297

12941298
Assert(portal->strategy!=PORTAL_MULTI_QUERY);
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‎src/backend/tcop/postgres.c‎

Lines changed: 11 additions & 21 deletions
Original file line numberDiff line numberDiff line change
@@ -974,6 +974,10 @@ exec_simple_query(const char *query_string)
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975975
plantree_list=pg_plan_queries(querytree_list,0,NULL);
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977+
/* Done with the snapshot used for parsing/planning */
978+
if (snapshot_set)
979+
PopActiveSnapshot();
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977981
/* If we got a cancel signal in analysis or planning, quit */
978982
CHECK_FOR_INTERRUPTS();
979983

@@ -998,19 +1002,9 @@ exec_simple_query(const char *query_string)
9981002
NULL);
9991003

10001004
/*
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* Start the portal.
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*
1003-
* If we took a snapshot for parsing/planning, the portal may be able
1004-
* to reuse it for the execution phase. Currently, this will only
1005-
* happen in PORTAL_ONE_SELECT mode. But even if PortalStart doesn't
1006-
* end up being able to do this, keeping the parse/plan snapshot
1007-
* around until after we start the portal doesn't cost much.
1005+
* Start the portal. No parameters here.
10081006
*/
1009-
PortalStart(portal,NULL,0,snapshot_set);
1010-
1011-
/* Done with the snapshot used for parsing/planning */
1012-
if (snapshot_set)
1013-
PopActiveSnapshot();
1007+
PortalStart(portal,NULL,0,InvalidSnapshot);
10141008

10151009
/*
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* Select the appropriate output format: text unless we are doing a
@@ -1733,19 +1727,15 @@ exec_bind_message(StringInfo input_message)
17331727
cplan->stmt_list,
17341728
cplan);
17351729

1736-
/*
1737-
* And we're ready to start portal execution.
1738-
*
1739-
* If we took a snapshot for parsing/planning, we'll try to reuse it for
1740-
* query execution (currently, reuse will only occur if PORTAL_ONE_SELECT
1741-
* mode is chosen).
1742-
*/
1743-
PortalStart(portal,params,0,snapshot_set);
1744-
17451730
/* Done with the snapshot used for parameter I/O and parsing/planning */
17461731
if (snapshot_set)
17471732
PopActiveSnapshot();
17481733

1734+
/*
1735+
* And we're ready to start portal execution.
1736+
*/
1737+
PortalStart(portal,params,0,InvalidSnapshot);
1738+
17491739
/*
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* Apply the result format requests to the portal.
17511741
*/

‎src/backend/tcop/pquery.c‎

Lines changed: 7 additions & 8 deletions
Original file line numberDiff line numberDiff line change
@@ -447,18 +447,17 @@ FetchStatementTargetList(Node *stmt)
447447
* currently only honored for PORTAL_ONE_SELECT portals). Most callers
448448
* should simply pass zero.
449449
*
450-
* The use_active_snapshot parameter is currently used only for
451-
* PORTAL_ONE_SELECT portals. If it is true, the active snapshot will
452-
* be used when starting up the executor; if false, a new snapshot will
453-
* be taken. This is used both for cursors and to avoid taking an entirely
454-
* new snapshot when it isn't necessary.
450+
* The caller can optionally pass a snapshot to be used; pass InvalidSnapshot
451+
* for the normal behavior of setting a new snapshot. This parameter is
452+
* presently ignored for non-PORTAL_ONE_SELECT portals (it's only intended
453+
* to be used for cursors).
455454
*
456455
* On return, portal is ready to accept PortalRun() calls, and the result
457456
* tupdesc (if any) is known.
458457
*/
459458
void
460459
PortalStart(Portalportal,ParamListInfoparams,
461-
inteflags,booluse_active_snapshot)
460+
inteflags,Snapshotsnapshot)
462461
{
463462
PortalsaveActivePortal;
464463
ResourceOwnersaveResourceOwner;
@@ -500,8 +499,8 @@ PortalStart(Portal portal, ParamListInfo params,
500499
casePORTAL_ONE_SELECT:
501500

502501
/* Must set snapshot before starting executor. */
503-
if (use_active_snapshot)
504-
PushActiveSnapshot(GetActiveSnapshot());
502+
if (snapshot)
503+
PushActiveSnapshot(snapshot);
505504
else
506505
PushActiveSnapshot(GetTransactionSnapshot());
507506

‎src/include/tcop/pquery.h‎

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -28,7 +28,7 @@ extern List *FetchPortalTargetList(Portal portal);
2828
externList*FetchStatementTargetList(Node*stmt);
2929

3030
externvoidPortalStart(Portalportal,ParamListInfoparams,
31-
inteflags,booluse_active_snapshot);
31+
inteflags,Snapshotsnapshot);
3232

3333
externvoidPortalSetResultFormat(Portalportal,intnFormats,
3434
int16*formats);

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