From 33e52ad9a32929a6d14dfd98a8440d57028f2e3e Mon Sep 17 00:00:00 2001 From: Tomas Vondra Date: Fri, 26 Mar 2021 22:34:53 +0100 Subject: [PATCH] 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/backend/utils/adt/selfuncs.c | 6 +++--- src/test/regress/expected/stats_ext.out | 2 +- 2 files changed, 4 insertions(+), 4 deletions(-) diff --git a/src/backend/utils/adt/selfuncs.c b/src/backend/utils/adt/selfuncs.c index 52314d3aa1..2348d4a772 100644 --- a/src/backend/utils/adt/selfuncs.c +++ b/src/backend/utils/adt/selfuncs.c @@ -3987,11 +3987,11 @@ 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; 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