On 05/04/2015 12:23 AM, Anton Bushmelev 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)
There are two problems here that prohibit the index from being used:
1. random() is volatile, so it's recalculated for each row.
2. For the comparison, customer_id is cast to a float, and the index is
on the bigint value.
To work around the first problem, put the random() call inside a
subquery. And for the second problem, cast to bigint.
SELECT ... FROM addresses
WHERE customer_id = (SELECT random()*45000)::bigint
- Heikki
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