Re: [HACKERS] optimization join on random value

On Mon, May 04, 2015 at 12:15:54AM +0300, Anton wrote:
> Hello guru of postgres, it's possoble to tune query with join on
> random string ?
> i know that it is not real life example, but i need it for tests.
>
> soe=# explain
> soe-# SELECT ADDRESS_ID,
> soe-# CUSTOMER_ID,
> soe-# DATE_CREATED,
> soe-# HOUSE_NO_OR_NAME,
> soe-# STREET_NAME,
> soe-# TOWN,
> soe-# COUNTY,
> soe-# COUNTRY,
> soe-# POST_CODE,
> soe-# ZIP_CODE
> soe-# FROM ADDRESSES
> soe-# WHERE customer_id = trunc( random()*45000) ;
> QUERY PLAN
> -------------------------------------------------------------------------------------------
> Seq Scan on addresses (cost=0.00..165714.00 rows=22500 width=84)
> Filter: ((customer_id)::double precision = trunc((random() *
> 45000::double precision)))
> (2 rows)

If you look carefully you'll see that the comparison here is done as a
"double precision" and so can't use the index. If you say something
like:

WHERE customer_id = trunc( random()*45000)::bigint

it will probably work fine.

Have a nice day,
--
Martijn van Oosterhout <kleptog(at)svana(dot)org> http://svana.org/kleptog/
> He who writes carelessly confesses thereby at the very outset that he does
> not attach much importance to his own thoughts.
-- Arthur Schopenhauer