Re: PoC/WIP: Extended statistics on expressions

On Sun, Jan 17, 2021 at 01:23:39AM +0100, Tomas Vondra wrote:
> > CREATE TABLE t AS SELECT generate_series(1,9) AS i;
> > CREATE STATISTICS s ON (i+1) ,(i+1+0) FROM t;
> > ANALYZE t;
> > SELECT i+1 FROM t GROUP BY 1;
> > ERROR: corrupt MVNDistinct entry
>
> Thanks. There was a thinko in estimate_multivariate_ndistinct, resulting in
> mismatching the ndistinct coefficient items. The attached patch fixes that,
> but I've realized the way we pick the "best" statistics may need some
> improvements (I added an XXX comment about that).

That maybe indicates a deficiency in testing and code coverage.

| postgres=# CREATE TABLE t(i int);
| postgres=# CREATE STATISTICS s2 ON (i+1) ,(i+1+0) FROM t;
| postgres=# \d t
| Table "public.t"
| Column | Type | Collation | Nullable | Default
| --------+---------+-----------+----------+---------
| i | integer | | |
| Indexes:
| "t_i_idx" btree (i)
| Statistics objects:
| "public"."s2" (ndistinct, dependencies, mcv) ON FROM t

on ... what ?

--
Justin