Take fewer snapshots.

When a PORTAL_ONE_SELECT query is executed, we can opportunistically
reuse the parse/plan shot for the execution phase.  This cuts down the
number of snapshots per simple query from 2 to 1 for the simple
protocol, and 3 to 2 for the extended protocol.  Since we are only
reusing a snapshot taken early in the processing of the same protocol
message, the change shouldn't be user-visible, except that the remote
possibility of the planning and execution snapshots being different is
eliminated.

Note that this change does not make it safe to assume that the parse/plan
snapshot will certainly be reused; that will currently only happen if
PortalStart() decides to use the PORTAL_ONE_SELECT strategy.  It might
be worth trying to provide some stronger guarantees here in the future,
but for now we don't.

Patch by me; review by Dimitri Fontaine.

1 files changed, 21 insertions, 11 deletions

diff --git a/src/backend/tcop/postgres.c b/src/backend/tcop/postgres.c
index 5bb16e010cc..f789a1e6276 100644
--- a/src/backend/tcop/postgres.c
+++ b/src/backend/tcop/postgres.c
@@ -943,10 +943,6 @@ 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();
 
@@ -971,9 +967,19 @@ exec_simple_query(const char *query_string)
 						  NULL);
 
 		/*
-		 * Start the portal.  No parameters here.
+		 * 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.
 		 */
-		PortalStart(portal, NULL, InvalidSnapshot);
+		PortalStart(portal, NULL, snapshot_set);
+
+		/* Done with the snapshot used for parsing/planning */
+		if (snapshot_set)
+			PopActiveSnapshot();
 
 		/*
 		 * Select the appropriate output format: text unless we are doing a
@@ -1696,14 +1702,18 @@ exec_bind_message(StringInfo input_message)
 					  cplan->stmt_list,
 					  cplan);
 
-	/* Done with the snapshot used for parameter I/O and parsing/planning */
-	if (snapshot_set)
-		PopActiveSnapshot();
-
 	/*
 	 * 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, InvalidSnapshot);
+	PortalStart(portal, params, snapshot_set);
+
+	/* Done with the snapshot used for parameter I/O and parsing/planning */
+	if (snapshot_set)
+		PopActiveSnapshot();
 
 	/*
 	 * Apply the result format requests to the portal.