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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 d544671f15
commit e5eb40eed4
1 changed files with 3 additions and 3 deletions
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6
src/backend/utils/adt/selfuncs.c
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@ -3851,11 +3851,11 @@ 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;