plpgsql: Don't generate parallel plans for RETURN QUERY.

robertmhaas · robertmhaas · commit f120b614e070 · 2017-03-24T12:30:39.000-04:00
Commit 7aea8e4 allowed a parallel plan to be generated when for a RETURN QUERY or RETURN QUERY EXECUTE statement in a PL/pgsql block, but that's a bad idea because plplgsql asks the executor for 50 rows at a time. That means that we'll always be running serially a plan that was intended for parallel execution, which is not a good idea. Fix by not requesting a parallel plan from the outset. Per discussion, back-patch to 9.6. There is a slight risk that, due to optimizer error, somebody could have a case where the parallel plan executed serially is actually faster than the supposedly-best serial plan, but the consensus seems to be that that's not sufficient justification for leaving 9.6 unpatched. Discussion: http://postgr.es/m/CA+TgmoZ_ZuH+auEeeWnmtorPsgc_SmP+XWbDsJ+cWvWBSjNwDQ@mail.gmail.com Discussion: http://postgr.es/m/CA+TgmobXEhvHbJtWDuPZM9bVSLiTj-kShxQJ2uM5GPDze9fRYA@mail.gmail.com
diff --git a/src/pl/plpgsql/src/pl_exec.c b/src/pl/plpgsql/src/pl_exec.c
@@ -3023,15 +3023,15 @@ exec_stmt_return_query(PLpgSQL_execstate *estate,
 	if (stmt->query != NULL)
 	{
 		/* static query */
-		exec_run_select(estate, stmt->query, 0, &portal, true);
+		exec_run_select(estate, stmt->query, 0, &portal, false);
 	}
 	else
 	{
 		/* RETURN QUERY EXECUTE */
 		Assert(stmt->dynquery != NULL);
 		portal = exec_dynquery_with_params(estate, stmt->dynquery,
 										   stmt->params, NULL,
-										   CURSOR_OPT_PARALLEL_OK);
+										   0);
 	}
 
 	/* Use eval_mcontext for tuple conversion work */

Original file line number	Diff line number	Diff line change
`@@ -3023,15 +3023,15 @@ exec_stmt_return_query(PLpgSQL_execstate *estate,`
`3023`	`3023`	`if (stmt->query != NULL)`
`3024`	`3024`	`{`
`3025`	`3025`	`/* static query */`
`3026`		`- exec_run_select(estate, stmt->query, 0, &portal, true);`
	`3026`	`+ exec_run_select(estate, stmt->query, 0, &portal, false);`
`3027`	`3027`	`}`
`3028`	`3028`	`else`
`3029`	`3029`	`{`
`3030`	`3030`	`/* RETURN QUERY EXECUTE */`
`3031`	`3031`	`Assert(stmt->dynquery != NULL);`
`3032`	`3032`	`portal = exec_dynquery_with_params(estate, stmt->dynquery,`
`3033`	`3033`	`stmt->params, NULL,`
`3034`		`- CURSOR_OPT_PARALLEL_OK);`
	`3034`	`+ 0);`
`3035`	`3035`	`}`
`3036`	`3036`
`3037`	`3037`	`/* Use eval_mcontext for tuple conversion work */`