Re: Select performance variation based on the different combinations of using where lower(), order by, and limit

Tyler Reese <jukey91@gmail.com> wrote:

> I don't understand why the performance of case 4 is so much slower

>case 4:
>mydb=> explain analyze SELECT * FROM "cdr" WHERE lower("CallingPartyNumber") = '9725551212' order by "key" limit 100;

> Limit  (cost=0.00..72882.05 rows=100 width=757) (actual time=20481.083..30464.960 rows=11 loops=1)
>   ->  Index Scan using cdr_pkey on cdr  (cost=0.00..30036152.32 rows=41212 width=757) (actual
time=20481.049..30464.686rows=11 loops=1) 
>         Filter: (lower("CallingPartyNumber") = '9725551212'::text)
> Total runtime: 30465.246 ms

It thinks that it will only need to read 1/412th of the table to
find 100 matching rows, and using that index it will be able to
skip the sort.  Since there aren't 100 matching rows, it has to
read the whole table through the index.  Raising the statistics
target and running ANALYZE might allow it to use a more accurate
estimate, and thereby make a better choice.

--
Kevin Grittner
EDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company