Request small targetlist for input to WindowAgg.

RhodiumToad · RhodiumToad · commit f57c63107007 · 2019-11-06T04:33:35.000Z
WindowAgg will potentially store large numbers of input rows into tuplestores to allow access to other rows in the frame. If the input is coming via an explicit Sort node, then unneeded columns will already have been discarded (since Sort requests a small tlist); but there are idioms like COUNT(*) OVER () that result in the input not being sorted at all, and cases where the input is being sorted by some means other than a Sort; if we don't request a small tlist, then WindowAgg's storage requirement is inflated by the unneeded columns. Backpatch back to 9.6, where the current tlist handling was added. (Prior to that, WindowAgg would always use a small tlist.) Discussion: https://postgr.es/m/87a7ator8n.fsf@news-spur.riddles.org.uk
diff --git a/src/backend/optimizer/plan/createplan.c b/src/backend/optimizer/plan/createplan.c
@@ -2398,10 +2398,13 @@ create_windowagg_plan(PlannerInfo *root, WindowAggPath *best_path)
 	ListCell   *lc;
 
 	/*
-	 * WindowAgg can project, so no need to be terribly picky about child
-	 * tlist, but we do need grouping columns to be available
+	 * Choice of tlist here is motivated by the fact that WindowAgg will be
+	 * storing the input rows of window frames in a tuplestore; it therefore
+	 * behooves us to request a small tlist to avoid wasting space. We do of
+	 * course need grouping columns to be available.
 	 */
-	subplan = create_plan_recurse(root, best_path->subpath, CP_LABEL_TLIST);
+	subplan = create_plan_recurse(root, best_path->subpath,
+								  CP_LABEL_TLIST | CP_SMALL_TLIST);
 
 	tlist = build_path_tlist(root, &best_path->path);
 
