From 25a9ff6cadbb2099a5f53876d9e29f35a080df43 Mon Sep 17 00:00:00 2001
From: Tomas Vondra <tomas.vondra@postgresql.org>
Date: Sat, 16 Nov 2019 01:17:15 +0100
Subject: [PATCH] Skip system attributes when applying mvdistinct stats

When estimating number of distinct groups, we failed to ignore system
attributes when matching the group expressions to mvdistinct stats,
causing failures like

  ERROR: negative bitmapset member not allowed

Fix that by simply skipping anything that is not a regular attribute.
Backpatch to PostgreSQL 10, where the extended stats were introduced.

Bug: #16111
Reported-by: Tuomas Leikola
Author: Tomas Vondra
Backpatch-through: 10
Discussion: https://postgr.es/m/16111-687799584c3a7e73@postgresql.org
---
 src/backend/utils/adt/selfuncs.c        | 19 ++++++++++++++-----
 src/test/regress/expected/stats_ext.out | 12 ++++++++++++
 src/test/regress/sql/stats_ext.sql      |  4 ++++
 3 files changed, 30 insertions(+), 5 deletions(-)

diff --git a/src/backend/utils/adt/selfuncs.c b/src/backend/utils/adt/selfuncs.c
index 7622edb9dd..126ffad67d 100644
--- a/src/backend/utils/adt/selfuncs.c
+++ b/src/backend/utils/adt/selfuncs.c
@@ -3931,14 +3931,19 @@ estimate_multivariate_ndistinct(PlannerInfo *root, RelOptInfo *rel,
 	foreach(lc, *varinfos)
 	{
 		GroupVarInfo *varinfo = (GroupVarInfo *) lfirst(lc);
+		AttrNumber	attnum;
 
 		Assert(varinfo->rel == rel);
 
-		if (IsA(varinfo->var, Var))
-		{
-			attnums = bms_add_member(attnums,
-									 ((Var *) varinfo->var)->varattno);
-		}
+		if (!IsA(varinfo->var, Var))
+			continue;
+
+		attnum = ((Var *) varinfo->var)->varattno;
+
+		if (!AttrNumberIsForUserDefinedAttr(attnum))
+			continue;
+
+		attnums = bms_add_member(attnums, attnum);
 	}
 
 	/* look for the ndistinct statistics matching the most vars */
@@ -4018,6 +4023,10 @@ estimate_multivariate_ndistinct(PlannerInfo *root, RelOptInfo *rel,
 			}
 
 			attnum = ((Var *) varinfo->var)->varattno;
+
+			if (!AttrNumberIsForUserDefinedAttr(attnum))
+				continue;
+
 			if (!bms_is_member(attnum, matched))
 				newlist = lappend(newlist, varinfo);
 		}
diff --git a/src/test/regress/expected/stats_ext.out b/src/test/regress/expected/stats_ext.out
index eebf250998..5b7e1d4660 100644
--- a/src/test/regress/expected/stats_ext.out
+++ b/src/test/regress/expected/stats_ext.out
@@ -217,6 +217,18 @@ SELECT stxkind, stxndistinct
  {d,f}   | {"3, 4": 301, "3, 6": 301, "4, 6": 301, "3, 4, 6": 301}
 (1 row)
 
+-- minor improvement, make sure the ctid does not break the matching
+EXPLAIN (COSTS off)
+SELECT COUNT(*) FROM ndistinct GROUP BY ctid, a, b;
+            QUERY PLAN             
+-----------------------------------
+ GroupAggregate
+   Group Key: ctid, a, b
+   ->  Sort
+         Sort Key: ctid, a, b
+         ->  Seq Scan on ndistinct
+(5 rows)
+
 -- Hash Aggregate, thanks to estimates improved by the statistic
 EXPLAIN (COSTS off)
  SELECT COUNT(*) FROM ndistinct GROUP BY a, b;
diff --git a/src/test/regress/sql/stats_ext.sql b/src/test/regress/sql/stats_ext.sql
index 43ff77c534..2543dbfffa 100644
--- a/src/test/regress/sql/stats_ext.sql
+++ b/src/test/regress/sql/stats_ext.sql
@@ -144,6 +144,10 @@ ANALYZE ndistinct;
 SELECT stxkind, stxndistinct
   FROM pg_statistic_ext WHERE stxrelid = 'ndistinct'::regclass;
 
+-- minor improvement, make sure the ctid does not break the matching
+EXPLAIN (COSTS off)
+SELECT COUNT(*) FROM ndistinct GROUP BY ctid, a, b;
+
 -- Hash Aggregate, thanks to estimates improved by the statistic
 EXPLAIN (COSTS off)
  SELECT COUNT(*) FROM ndistinct GROUP BY a, b;