Remove redundant null-treatment check in window function dedup.

Commit 25a30bbd423 (IGNORE NULLS / RESPECT NULLS for window functions)
made ExecInitWindowAgg() treat two otherwise-equal window functions as
duplicates only when their ignore_nulls settings also matched:

    if (i <= wfuncno && wfunc->ignore_nulls == perfunc[i].ignore_nulls)

That extra term reads WindowStatePerFuncData.ignore_nulls, but the field
was never populated when a per-function entry was filled in, so it stayed
zero from palloc0_array().  Consequently a duplicate call carrying
IGNORE NULLS or an explicit RESPECT NULLS never matched an identical
earlier entry and was needlessly given its own per-function slot and
evaluated twice.  (Results stayed correct; this was a missed sharing, not
a wrong answer.)

The extra term is in fact redundant.  WindowFunc.ignore_nulls is a plain
scalar field with no pg_node_attr, so _equalWindowFunc() already compares
it; the preceding equal() call therefore never matches two WindowFuncs
that differ only in null treatment.  If equal() matches, ignore_nulls
necessarily matched too, so the term can never change the outcome, and
WindowStatePerFuncData.ignore_nulls existed only to feed it.

Rather than populate the shadow field, drop the redundant term and the
field (and adjust the now-stale comment) and let equal() do the work.
That fixes the same bug while removing the hand-maintained duplicate
state that caused it, so it cannot silently drift again.

Author: Chao Li <li.evan.chao@gmail.com>
Co-authored-by: Ewan Young <kdbase.hack@gmail.com>
Reviewed-by: Tatsuo Ishii <ishii@postgresql.org>
Discussion: https://postgr.es/m/5D2C9081-5DFE-4E27-AB14-7358238EA1BC%40gmail.com
Backpatch-through: 19

diff --git a/src/backend/executor/nodeWindowAgg.c b/src/backend/executor/nodeWindowAgg.c
index f1c524d00df9292161f2e0ab481d4df56bad0c7e..7d6ec2dfc4bf08195a0c983affcb29ad8681d191 100644 (file)
--- a/src/backend/executor/nodeWindowAgg.c
+++ b/src/backend/executor/nodeWindowAgg.c
@@ -110,7 +110,6 @@ typedef struct WindowStatePerFuncData
 
        bool            plain_agg;              /* is it just a plain aggregate function? */
        int                     aggno;                  /* if so, index of its WindowStatePerAggData */
-       uint8           ignore_nulls;   /* ignore nulls */
 
        WindowObject winobj;            /* object used in window function API */
 } WindowStatePerFuncData;
@@ -2737,17 +2736,14 @@ ExecInitWindowAgg(WindowAgg *node, EState *estate, int eflags)
                        elog(ERROR, "WindowFunc with winref %u assigned to WindowAgg with winref %u",
                                 wfunc->winref, node->winref);
 
-               /*
-                * Look for a previous duplicate window function, which needs the same
-                * ignore_nulls value
-                */
+               /* Look for a previous duplicate window function */
                for (i = 0; i <= wfuncno; i++)
                {
                        if (equal(wfunc, perfunc[i].wfunc) &&
                                !contain_volatile_functions((Node *) wfunc))
                                break;
                }
-               if (i <= wfuncno && wfunc->ignore_nulls == perfunc[i].ignore_nulls)
+               if (i <= wfuncno)
                {
                        /* Found a match to an existing entry, so just mark it */
                        wfuncstate->wfuncno = i;