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Fix ndistinct estimates with system attributes

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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
This commit is contained in:
Tomas Vondra
2021-03-26 22:34:53 +01:00
parent 366aa374b3
commit 88acb01357
2 changed files with 4 additions and 4 deletions
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4
src/backend/utils/adt/selfuncs.c
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@@ -3743,10 +3743,10 @@ estimate_multivariate_ndistinct(PlannerInfo *root, RelOptInfo *rel,
attnum = ((Var *) varinfo->var)->varattno; attnum = ((Var *) varinfo->var)->varattno;
if (!AttrNumberIsForUserDefinedAttr(attnum)) if (AttrNumberIsForUserDefinedAttr(attnum) &&
bms_is_member(attnum, matched))
continue; continue;
if (!bms_is_member(attnum, matched))
newlist = lappend(newlist, varinfo); newlist = lappend(newlist, varinfo);
} }

2
src/test/regress/expected/stats_ext.out
View File

@@ -222,7 +222,7 @@ SELECT s.stxkind, d.stxdndistinct
SELECT * FROM check_estimated_rows('SELECT COUNT(*) FROM ndistinct GROUP BY ctid, a, b'); SELECT * FROM check_estimated_rows('SELECT COUNT(*) FROM ndistinct GROUP BY ctid, a, b');
estimated | actual estimated | actual
-----------+-------- -----------+--------
11 | 1000 1000 | 1000
(1 row) (1 row)
-- Hash Aggregate, thanks to estimates improved by the statistic -- Hash Aggregate, thanks to estimates improved by the statistic