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, 7 insertions, 11 deletions

diff --git a/src/backend/executor/spi.c b/src/backend/executor/spi.c
index 688279c716e..7af3abf3f7a 100644
--- a/src/backend/executor/spi.c
+++ b/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);