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.

src/backend/commands/portalcmds.c

@@ -121,7 +121,7 @@ PerformCursorOpen(PlannedStmt *stmt, ParamListInfo params,
 	/*
 	 * Start execution, inserting parameters if any.
 	 */
-	PortalStart(portal, params, GetActiveSnapshot());
+	PortalStart(portal, params, true);
 
 	Assert(portal->strategy == PORTAL_ONE_SELECT);
 

src/backend/commands/prepare.c

@@ -278,7 +278,7 @@ ExecuteQuery(ExecuteStmt *stmt, const char *queryString,
 	/*
 	 * Run the portal to completion.
 	 */
-	PortalStart(portal, paramLI, GetActiveSnapshot());
+	PortalStart(portal, paramLI, true);
 
 	(void) PortalRun(portal, FETCH_ALL, false, dest, dest, completionTag);
 

src/backend/executor/spi.c

@@ -1126,7 +1126,6 @@ SPI_cursor_open_internal(const char *name, SPIPlanPtr plan,
 	CachedPlan *cplan;
 	List	   *stmt_list;
 	char	   *query_string;
-	Snapshot	snapshot;
 	MemoryContext oldcontext;
 	Portal		portal;
 
@@ -1269,15 +1268,6 @@ 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
@@ -1293,7 +1283,13 @@ SPI_cursor_open_internal(const char *name, SPIPlanPtr plan,
 	/*
 	 * Start portal execution.
 	 */
-	PortalStart(portal, paramLI, snapshot);
+	if (read_only)
+		PortalStart(portal, paramLI, true);
+	else
+	{
+		CommandCounterIncrement();
+		PortalStart(portal, paramLI, false);
+	}
 
 	Assert(portal->strategy != PORTAL_MULTI_QUERY);
 

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.

src/backend/tcop/pquery.c

@@ -446,16 +446,17 @@ FetchStatementTargetList(Node *stmt)
  * the query, they must be passed in here (caller is responsible for
  * giving them appropriate lifetime).
  *
- * 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).
+ * 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.
  *
  * On return, portal is ready to accept PortalRun() calls, and the result
  * tupdesc (if any) is known.
  */
 void
-PortalStart(Portal portal, ParamListInfo params, Snapshot snapshot)
+PortalStart(Portal portal, ParamListInfo params, bool use_active_snapshot)
 {
 	Portal		saveActivePortal;
 	ResourceOwner saveResourceOwner;
@@ -497,8 +498,8 @@ PortalStart(Portal portal, ParamListInfo params, Snapshot snapshot)
 			case PORTAL_ONE_SELECT:
 
 				/* Must set snapshot before starting executor. */
-				if (snapshot)
-					PushActiveSnapshot(snapshot);
+				if (use_active_snapshot)
+					PushActiveSnapshot(GetActiveSnapshot());
 				else
 					PushActiveSnapshot(GetTransactionSnapshot());
 

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,
-			Snapshot snapshot);
+			bool use_active_snapshot);
 
 extern void PortalSetResultFormat(Portal portal, int nFormats,
 					  int16 *formats);