Avoid projecting tuples unnecessarily in Gather and Gather Merge.

It's most often the case that the target list for the Gather (Merge)
node matches the target list supplied by the underlying plan node;
when this is so, we can avoid the overhead of projecting.

This depends on commit f455e1125e2588d4cd4fc663c6a10da4e003a3b5 for
proper functioning.

Idea by Andres Freund.  Patch by me.  Review by Amit Kapila.

Discussion: http://postgr.es/m/CA+TgmoZ0ZL=cesZFq8c9NnfK6bqy-wwUd3_74iYGodYrSoQ7Fw@mail.gmail.com

1 files changed, 14 insertions, 10 deletions

diff --git a/src/backend/executor/nodeGatherMerge.c b/src/backend/executor/nodeGatherMerge.c
index d81462e72b4..166f2064ff7 100644
--- a/src/backend/executor/nodeGatherMerge.c
+++ b/src/backend/executor/nodeGatherMerge.c
@@ -116,10 +116,19 @@ ExecInitGatherMerge(GatherMerge *node, EState *estate, int eflags)
 	outerPlanState(gm_state) = ExecInitNode(outerNode, estate, eflags);
 
 	/*
+	 * Store the tuple descriptor into gather merge state, so we can use it
+	 * while initializing the gather merge slots.
+	 */
+	if (!ExecContextForcesOids(outerPlanState(gm_state), &hasoid))
+		hasoid = false;
+	tupDesc = ExecTypeFromTL(outerNode->targetlist, hasoid);
+	gm_state->tupDesc = tupDesc;
+
+	/*
 	 * Initialize result tuple type and projection info.
 	 */
 	ExecAssignResultTypeFromTL(&gm_state->ps);
-	ExecAssignProjectionInfo(&gm_state->ps, NULL);
+	ExecConditionalAssignProjectionInfo(&gm_state->ps, tupDesc, OUTER_VAR);
 
 	/*
 	 * initialize sort-key information
@@ -151,15 +160,6 @@ ExecInitGatherMerge(GatherMerge *node, EState *estate, int eflags)
 		}
 	}
 
-	/*
-	 * Store the tuple descriptor into gather merge state, so we can use it
-	 * while initializing the gather merge slots.
-	 */
-	if (!ExecContextForcesOids(outerPlanState(gm_state), &hasoid))
-		hasoid = false;
-	tupDesc = ExecTypeFromTL(outerNode->targetlist, hasoid);
-	gm_state->tupDesc = tupDesc;
-
 	/* Now allocate the workspace for gather merge */
 	gather_merge_setup(gm_state);
 
@@ -257,6 +257,10 @@ ExecGatherMerge(PlanState *pstate)
 	if (TupIsNull(slot))
 		return NULL;
 
+	/* If no projection is required, we're done. */
+	if (node->ps.ps_ProjInfo == NULL)
+		return slot;
+
 	/*
 	 * Form the result tuple using ExecProject(), and return it.
 	 */