Request small targetlist for input to WindowAgg.

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
index 5f6d2bad7bed46e630fa29ccd760b1cc397dd381..5f629adeb25411bb4fcc87e5220c12c6a9d4e992 100644 (file)
--- a/src/backend/optimizer/plan/createplan.c
+++ b/src/backend/optimizer/plan/createplan.c
@@ -2216,10 +2216,13 @@ 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);