Re: LIMIT confuses the planner

Richard Huxton wrote:
> Since it's expecting 7914 rows for "kouber" it thinks it will find the
> 20 rows you want fairly quickly by just looking backward through the
> login_attempt_pkey index.
>
> Try increasing the stats on the username column.
>
> ALTER TABLE login_attempt ALTER COLUMN username SET STATISTICS 100;
> ANALYZE login_attempt;
>
> You can try different values of statistics up to 1000, but there's no
> point in setting it too high.
>

Hmmm, that did the trick, thank you. I updated the statistics of the
column to 300, so now the query plan changed to:


Limit  (cost=127.65..127.70 rows=20 width=38) (actual time=0.085..0.086
rows=3 loops=1)
->  Sort  (cost=127.65..129.93 rows=910 width=38) (actual
time=0.084..0.085 rows=3 loops=1)
  Sort Key: login_attempt_sid
  Sort Method:  quicksort  Memory: 25kB
  ->  Bitmap Heap Scan on login_attempt  (cost=7.74..103.44 rows=910
width=38) (actual time=0.075..0.078 rows=3 loops=1)
        Recheck Cond: ((username)::text = 'kouber'::text)
        ->  Bitmap Index Scan on login_attempt_username_idx
(cost=0.00..7.51 rows=910 width=0) (actual time=0.069..0.069 rows=3 loops=1)
         Index Cond: ((username)::text = 'kouber'::text)
Total runtime: 0.114 ms


Now the planner believes there're 910 rows, which is a bit closer to the
real data:

swing=# select avg(length) from (select username, count(*) as length
from login_attempt group by username) as freq;
          avg
----------------------
  491.6087310427555479
(1 row)


--
Kouber Saparev
http://kouber.saparev.com