Optimize tuplestore usage for WITH RECURSIVE CTEs

david-rowley · david-rowley · commit 5d56d07ca343 · 2024-09-19T15:20:35.000+12:00
nodeRecursiveunion.c makes use of two tuplestores and, until now, would delete and recreate one of these tuplestores after every recursive iteration. Here we adjust that behavior and instead reuse one of the existing tuplestores and just empty it of all tuples using tuplestore_clear(). This saves some free/malloc roundtrips and has shown a 25-30% performance improvement for queries that perform very little work between recursive iterations. This also paves the way to add some EXPLAIN ANALYZE telemetry output for recursive common table expressions, similar to what was done in 1eff827 and 95d6e9a. Previously calling tuplestore_end() would have caused the maximum storage space used to be lost. Reviewed-by: Tatsuo Ishii Discussion: https://postgr.es/m/CAApHDvr9yW0YRiK8A2J7nvyT8g17YzbSfOviEWrghazKZbHbig@mail.gmail.com
diff --git a/src/backend/executor/nodeRecursiveunion.c b/src/backend/executor/nodeRecursiveunion.c
@@ -115,19 +115,26 @@ ExecRecursiveUnion(PlanState *pstate)
 		slot = ExecProcNode(innerPlan);
 		if (TupIsNull(slot))
 		{
+			Tuplestorestate *swaptemp;
+
 			/* Done if there's nothing in the intermediate table */
 			if (node->intermediate_empty)
 				break;
 
-			/* done with old working table ... */
-			tuplestore_end(node->working_table);
+			/*
+			 * Now we let the intermediate table become the work table.  We
+			 * need a fresh intermediate table, so delete the tuples from the
+			 * current working table and use that as the new intermediate
+			 * table.  This saves a round of free/malloc from creating a new
+			 * tuple store.
+			 */
+			tuplestore_clear(node->working_table);
 
-			/* intermediate table becomes working table */
+			swaptemp = node->working_table;
 			node->working_table = node->intermediate_table;
+			node->intermediate_table = swaptemp;
 
-			/* create new empty intermediate table */
-			node->intermediate_table = tuplestore_begin_heap(false, false,
-															 work_mem);
+			/* mark the intermediate table as empty */
 			node->intermediate_empty = true;
 
 			/* reset the recursive term */
