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Fix ndistinct estimates with system attributes

author Tomas Vondra <tomas.vondra@postgresql.org>

2021年3月26日 21:34:53 +0000 (22:34 +0100)

committer Tomas Vondra <tomas.vondra@postgresql.org>

2021年3月26日 21:34:58 +0000 (22:34 +0100)

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 patch | blob | blame | history

src/test/regress/expected/stats_ext.out patch | blob | blame | history

diff --git a/src/backend/utils/adt/selfuncs.c b/src/backend/utils/adt/selfuncs.c

index 52314d3aa1c5c1829c161c98783b8bc2b425a839..2348d4a772a0a1a9901e59b36a34d70bb532fc0e 100644 (file)

--- 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;