Re: patternsel() and histogram_selectivity() and the hard cutoff of 100 - Mailing list pgsql-hackers

"Gregory Stark" <stark@enterprisedb.com> writes:

> So I had a thought about how to soften the controversial hard cutoff of 100
> for the use of the histogram selectivity. Instead of switching 100% one way or
> the other between the two heuristics why not calculate both and combine them.
> The larger the sample size from the histogram the more we can weight the
> histogram calculation. The smaller the histogram size the more we weight the
> heuristic.
>
> My first thought was to scale it linearly so we use 10% of the histogram
> sample + 90% of the heuristic for default statistic sizes of 10 samples. That
> degenerates to the status quo for 100 samples and up.

Incidentally I hacked up a patch to do this:


postgres=# create table xx as (select i||'x'||i from generate_series(1,10000) as i(i));
SELECT

postgres=# alter table xx alter x set statistics 1;
ANALYZE

postgres=# analyze xx;
ALTER TABLE

postgres=# explain analyze select * from xx where x like '%x1%';
                                             QUERY PLAN
-----------------------------------------------------------------------------------------------------
 Seq Scan on xx  (cost=0.00..174.00 rows=2000 width=9) (actual time=0.095..11.814 rows=1112 loops=1)
   Filter: (x ~~ '%x1%'::text)
 Total runtime: 13.957 ms
(3 rows)

postgres=# alter table xx alter x set statistics 10; analyze xx; explain analyze select * from xx where x like '%x1%';
...
 Seq Scan on xx  (cost=0.00..174.00 rows=1920 width=9) (actual time=0.036..11.454 rows=1112 loops=1)
...
postgres=# alter table xx alter x set statistics 20; analyze xx; explain analyze select * from xx where x like '%x1%';
 Seq Scan on xx  (cost=0.00..174.00 rows=1820 width=9) (actual time=0.036..11.446 rows=1112 loops=1)
...
postgres=# alter table xx alter x set statistics 50; analyze xx; explain analyze select * from xx where x like '%x1%';
 Seq Scan on xx  (cost=0.00..174.00 rows=1520 width=9) (actual time=0.036..11.406 rows=1112 loops=1)
...
postgres=# alter table xx alter x set statistics 70; analyze xx; explain analyze select * from xx where x like '%x1%';
 Seq Scan on xx  (cost=0.00..174.00 rows=1320 width=9) (actual time=0.036..10.725 rows=1112 loops=1)
...
postgres=# alter table xx alter x set statistics 90; analyze xx; explain analyze select * from xx where x like '%x1%';
 Seq Scan on xx  (cost=0.00..174.00 rows=1220 width=9) (actual time=0.036..10.326 rows=1112 loops=1)
...
postgres=# alter table xx alter x set statistics 100; analyze xx; explain analyze select * from xx where x like '%x1%';
 Seq Scan on xx  (cost=0.00..174.00 rows=1120 width=9) (actual time=0.037..11.411 rows=1112 loops=1)
...
postgres=# alter table xx alter x set statistics 200; analyze xx; explain analyze select * from xx where x like '%x1%';
 Seq Scan on xx  (cost=0.00..174.00 rows=1106 width=9) (actual time=0.037..11.328 rows=1112 loops=1)
...
postgres=# alter table xx alter x set statistics 1; analyze xx; explain analyze select * from xx where x like '%x1%';
 Seq Scan on xx  (cost=0.00..174.00 rows=2000 width=9) (actual time=0.037..11.810 rows=1112 loops=1)



--
  Gregory Stark
  EnterpriseDB          http://www.enterprisedb.com
  Get trained by Bruce Momjian - ask me about EnterpriseDB's PostgreSQL training!