From: Tomas Vondra <tomas.vondra@postgresql.org>
Date: Fri, 26 Mar 2021 21:34:53 +0000 (+0100)
Subject: Fix ndistinct estimates with system attributes
X-Git-Tag: REL_11_12~43
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=2a307df1bba7178b3903e01bee5012707f950473;p=thirdparty%2Fpostgresql.git

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
---

diff --git a/src/backend/utils/adt/selfuncs.c b/src/backend/utils/adt/selfuncs.c
index 17640b2328f..2240636e6c4 100644
--- a/src/backend/utils/adt/selfuncs.c
+++ b/src/backend/utils/adt/selfuncs.c
@@ -4025,11 +4025,11 @@ estimate_multivariate_ndistinct(PlannerInfo *root, RelOptInfo *rel,
 
 			attnum = ((Var *) varinfo->var)->varattno;
 
-			if (!AttrNumberIsForUserDefinedAttr(attnum))
+			if (AttrNumberIsForUserDefinedAttr(attnum) &&
+				bms_is_member(attnum, matched))
 				continue;
 
-			if (!bms_is_member(attnum, matched))
-				newlist = lappend(newlist, varinfo);
+			newlist = lappend(newlist, varinfo);
 		}
 
 		*varinfos = newlist;