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.

1 files changed, 7 insertions, 8 deletions

diff --git a/src/backend/tcop/pquery.c b/src/backend/tcop/pquery.c
index 2cb9a8ee2f..8ad0aaa55e 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());