From 33e52ad9a32929a6d14dfd98a8440d57028f2e3e Mon Sep 17 00:00:00 2001
From: Tomas Vondra
Date: Fri, 26 Mar 2021 22:34:53 +0100
Subject: Fix ndistinct estimates with system attributes

When estimating the number of groups using extended statistics, the code
was discarding information about system attributes. This led to strange
situation that

    SELECT 1 FROM t GROUP BY ctid;

could have produced higher estimate (equal to pg_class.reltuples) than

    SELECT 1 FROM t GROUP BY a, b, ctid;

with extended statistics on (a,b). Fixed by retaining information about
the system attribute.

Backpatch all the way to 10, where extended statistics were introduced.

Author: Tomas Vondra
Backpatch-through: 10
---
 src/test/regress/expected/stats_ext.out | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

(limited to 'src/test')

diff --git a/src/test/regress/expected/stats_ext.out b/src/test/regress/expected/stats_ext.out
index 431b3fa3de..d80e6a3907 100644
--- a/src/test/regress/expected/stats_ext.out
+++ b/src/test/regress/expected/stats_ext.out
@@ -260,7 +260,7 @@ SELECT s.stxkind, d.stxdndistinct
 SELECT * FROM check_estimated_rows('SELECT COUNT(*) FROM ndistinct GROUP BY ctid, a, b');
  estimated | actual 
 -----------+--------
-        11 |   1000
+      1000 |   1000
 (1 row)
 
 -- Hash Aggregate, thanks to estimates improved by the statistic
-- 
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