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, 11 insertions, 7 deletions

diff --git a/src/backend/executor/spi.c b/src/backend/executor/spi.c
index e222365d111..5a11c6f7392 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);