On 07/18/2018 12:41 AM, Konstantin Knizhnik wrote:
> ...
>
> Teodor Sigaev has proposed an alternative approach for calculating
> selectivity of multicolumn join or compound index search.
> Usually DBA creates compound indexes which can be used by optimizer to
> build efficient query execution plan based on index search.
> We can stores statistic for compound keys of such indexes and (as it is
> done now for functional indexes) and use it to estimate selectivity
> of clauses. I have implemented this idea and will publish patch in
> separate thread soon.
> Now I just want to share some results for the Tomas examples.
>
> So for Vanilla Postges without extra statistic estimated number of rows
> is about 4 times smaller than real.
>
Can you please post plans with parallelism disabled, and perhaps without
the aggregate? Both makes reading the plans unnecessarily difficult ...
> postgres=# explain analyze SELECT count(*) FROM foo WHERE a=1 and (b=1
> or b=2);
> QUERY PLAN
>
>
--------------------------------------------------------------------------------------------------------------------------------------------
>
> Aggregate (cost=10964.76..10964.77 rows=1 width=8) (actual
> time=49.251..49.251 rows=1 loops=1)
> -> Bitmap Heap Scan on foo (cost=513.60..10906.48 rows=23310
> width=0) (actual time=17.368..39.928 rows=90000 loops=1)
ok, 23k vs. 90k
>
> If we add statistic for a and b columns:
>
> create statistics ab on a,b from foo;
> analyze foo;
>
> then expected results is about 30% larger then real: 120k vs 90k:
>
Eh? The plan however says 9k vs. 30k ...
> -> Parallel Bitmap Heap Scan on foo
> (cost=2561.33..13424.24 rows=9063 width=0) (actual time=15.551..26.057
> rows=30000 loops=3)
...
> With compound index statistic estimation is almost equal to real value:
>
> -> Bitmap Heap Scan on foo (cost=1880.20..13411.66 rows=23310
> width=0) (actual time=38.776..61.050 rows=90000 loops=1)
Well, this says 23k vs. 90k.
regards
--
Tomas Vondra http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
