From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Mon, 26 Nov 2012 20:55:51 +0000 (-0500)
Subject: Revert patch for taking fewer snapshots.
X-Git-Tag: REL9_2_2~25
X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=786afc1ce53126feecf4d02e96e7455669ccbf5a;p=postgresql
Revert patch for taking fewer snapshots.
This reverts commit d573e239f03506920938bf0be56c868d9c3416da, "Take fewer
snapshots". While that seemed like a good idea at the time, it caused
execution to use a snapshot that had been acquired before locking any of
the tables mentioned in the query. This created user-visible anomalies
that were not present in any prior release of Postgres, as reported by
Tomas Vondra. While this whole area could do with a redesign (since there
are related cases that have anomalies anyway), it doesn't seem likely that
any future patch would be reasonably back-patchable; and we don't want 9.2
to exhibit a behavior that's subtly unlike either past or future releases.
Hence, revert to prior code while we rethink the problem.
---
diff --git a/doc/src/sgml/release-9.2.sgml b/doc/src/sgml/release-9.2.sgml
index 840e7a7a0d..c641606fd5 100644
--- a/doc/src/sgml/release-9.2.sgml
+++ b/doc/src/sgml/release-9.2.sgml
@@ -809,13 +809,6 @@
</para>
</listitem>
- <listitem>
- <para>
- Take fewer <acronym>MVCC</acronym> snapshots
- (Robert Haas)
- </para>
- </listitem>
-
<listitem>
<para>
Make the number of CLOG buffers scale based on <link
diff --git a/src/backend/commands/portalcmds.c b/src/backend/commands/portalcmds.c
index e402042332..b5ce87a29d 100644
--- a/src/backend/commands/portalcmds.c
+++ b/src/backend/commands/portalcmds.c
@@ -121,7 +121,7 @@ PerformCursorOpen(PlannedStmt *stmt, ParamListInfo params,
/*
* Start execution, inserting parameters if any.
*/
- PortalStart(portal, params, 0, true);
+ PortalStart(portal, params, 0, GetActiveSnapshot());
Assert(portal->strategy == PORTAL_ONE_SELECT);
diff --git a/src/backend/commands/prepare.c b/src/backend/commands/prepare.c
index 2d87b1c690..e3de7f2f1d 100644
--- a/src/backend/commands/prepare.c
+++ b/src/backend/commands/prepare.c
@@ -289,7 +289,7 @@ ExecuteQuery(ExecuteStmt *stmt, IntoClause *intoClause,
/*
* Run the portal as appropriate.
*/
- PortalStart(portal, paramLI, eflags, true);
+ PortalStart(portal, paramLI, eflags, GetActiveSnapshot());
(void) PortalRun(portal, count, false, dest, dest, completionTag);
diff --git a/src/backend/executor/spi.c b/src/backend/executor/spi.c
index e222365d11..5a11c6f739 100644
--- a/src/backend/executor/spi.c
+++ b/src/backend/executor/spi.c
@@ -1126,6 +1126,7 @@ SPI_cursor_open_internal(const char *name, SPIPlanPtr plan,
CachedPlan *cplan;
List *stmt_list;
char *query_string;
+ Snapshot snapshot;
MemoryContext oldcontext;
Portal portal;
@@ -1268,6 +1269,15 @@ SPI_cursor_open_internal(const char *name, SPIPlanPtr plan,
}
}
+ /* Set up the snapshot to use. */
+ if (read_only)
+ snapshot = GetActiveSnapshot();
+ else
+ {
+ CommandCounterIncrement();
+ snapshot = GetTransactionSnapshot();
+ }
+
/*
* If the plan has parameters, copy them into the portal. Note that this
* must be done after revalidating the plan, because in dynamic parameter
@@ -1283,13 +1293,7 @@ SPI_cursor_open_internal(const char *name, SPIPlanPtr plan,
/*
* Start portal execution.
*/
- if (read_only)
- PortalStart(portal, paramLI, 0, true);
- else
- {
- CommandCounterIncrement();
- PortalStart(portal, paramLI, 0, false);
- }
+ PortalStart(portal, paramLI, 0, snapshot);
Assert(portal->strategy != PORTAL_MULTI_QUERY);
diff --git a/src/backend/tcop/postgres.c b/src/backend/tcop/postgres.c
index 51b6df54f4..f1633f9de3 100644
--- a/src/backend/tcop/postgres.c
+++ b/src/backend/tcop/postgres.c
@@ -974,6 +974,10 @@ exec_simple_query(const char *query_string)
plantree_list = pg_plan_queries(querytree_list, 0, NULL);
+ /* Done with the snapshot used for parsing/planning */
+ if (snapshot_set)
+ PopActiveSnapshot();
+
/* If we got a cancel signal in analysis or planning, quit */
CHECK_FOR_INTERRUPTS();
@@ -998,19 +1002,9 @@ exec_simple_query(const char *query_string)
NULL);
/*
- * Start the portal.
- *
- * If we took a snapshot for parsing/planning, the portal may be able
- * to reuse it for the execution phase. Currently, this will only
- * happen in PORTAL_ONE_SELECT mode. But even if PortalStart doesn't
- * end up being able to do this, keeping the parse/plan snapshot
- * around until after we start the portal doesn't cost much.
+ * Start the portal. No parameters here.
*/
- PortalStart(portal, NULL, 0, snapshot_set);
-
- /* Done with the snapshot used for parsing/planning */
- if (snapshot_set)
- PopActiveSnapshot();
+ PortalStart(portal, NULL, 0, InvalidSnapshot);
/*
* Select the appropriate output format: text unless we are doing a
@@ -1733,19 +1727,15 @@ exec_bind_message(StringInfo input_message)
cplan->stmt_list,
cplan);
- /*
- * And we're ready to start portal execution.
- *
- * If we took a snapshot for parsing/planning, we'll try to reuse it for
- * query execution (currently, reuse will only occur if PORTAL_ONE_SELECT
- * mode is chosen).
- */
- PortalStart(portal, params, 0, snapshot_set);
-
/* Done with the snapshot used for parameter I/O and parsing/planning */
if (snapshot_set)
PopActiveSnapshot();
+ /*
+ * And we're ready to start portal execution.
+ */
+ PortalStart(portal, params, 0, InvalidSnapshot);
+
/*
* Apply the result format requests to the portal.
*/
diff --git a/src/backend/tcop/pquery.c b/src/backend/tcop/pquery.c
index d0db7ad62c..ce5e5e29dd 100644
--- a/src/backend/tcop/pquery.c
+++ b/src/backend/tcop/pquery.c
@@ -447,18 +447,17 @@ FetchStatementTargetList(Node *stmt)
* currently only honored for PORTAL_ONE_SELECT portals). Most callers
* should simply pass zero.
*
- * The use_active_snapshot parameter is currently used only for
- * PORTAL_ONE_SELECT portals. If it is true, the active snapshot will
- * be used when starting up the executor; if false, a new snapshot will
- * be taken. This is used both for cursors and to avoid taking an entirely
- * new snapshot when it isn't necessary.
+ * The caller can optionally pass a snapshot to be used; pass InvalidSnapshot
+ * for the normal behavior of setting a new snapshot. This parameter is
+ * presently ignored for non-PORTAL_ONE_SELECT portals (it's only intended
+ * to be used for cursors).
*
* On return, portal is ready to accept PortalRun() calls, and the result
* tupdesc (if any) is known.
*/
void
PortalStart(Portal portal, ParamListInfo params,
- int eflags, bool use_active_snapshot)
+ int eflags, Snapshot snapshot)
{
Portal saveActivePortal;
ResourceOwner saveResourceOwner;
@@ -500,8 +499,8 @@ PortalStart(Portal portal, ParamListInfo params,
case PORTAL_ONE_SELECT:
/* Must set snapshot before starting executor. */
- if (use_active_snapshot)
- PushActiveSnapshot(GetActiveSnapshot());
+ if (snapshot)
+ PushActiveSnapshot(snapshot);
else
PushActiveSnapshot(GetTransactionSnapshot());
diff --git a/src/include/tcop/pquery.h b/src/include/tcop/pquery.h
index 22aad2e96c..c77bb8be94 100644
--- a/src/include/tcop/pquery.h
+++ b/src/include/tcop/pquery.h
@@ -28,7 +28,7 @@ extern List *FetchPortalTargetList(Portal portal);
extern List *FetchStatementTargetList(Node *stmt);
extern void PortalStart(Portal portal, ParamListInfo params,
- int eflags, bool use_active_snapshot);
+ int eflags, Snapshot snapshot);
extern void PortalSetResultFormat(Portal portal, int nFormats,
int16 *formats);