Re: Performance Problem with sub-select using array - Mailing list pgsql-sql

From Aaron Bono
Subject Re: Performance Problem with sub-select using array
Date
Msg-id bf05e51c0608272110t6e994e09n7922e494fe62cd2f@mail.gmail.com
Whole thread Raw
In response to Performance Problem with sub-select using array  ("Travis Whitton" <tinymountain@gmail.com>)
List pgsql-sql
On 8/24/06, Travis Whitton <tinymountain@gmail.com> wrote:
Hello all, I'm running the following query on about 6,000 records worth of data, and it takes about 8 seconds to complete. Can anyone provide any suggestions to improve performance? I have an index on two columns in the transacts table (program_id, customer_id). If I specify a number for customer.id in the sub-select, query time is reduced to about 2 seconds, which still seems like a million years for only 6,000 records, but I'm guessing that the sub-select can't resolve the id since it's done before the outer query, so it scans the entre recordset for every row? Transacts is a many to many table for customers and programs. I know this query doesn't even reference any columns from programs; however, I dynamically insert where clauses to constrain the result set.

SELECT distinct customers.id, first_name, last_name, address1, contact_city, contact_state, primary_phone, email, array(select programs.program_name from transacts, programs where customer_id = customers.id and programs.id = transacts.program_id and submit_status = 'success') AS partners from customers, transacts, programs where transacts.customer_id = customers.id and transacts.program_id = programs.id

 
My guess is that your problem is that you may be getting 6000 rows, but the array(select ....) is having to run once for each of record returned (so it is running 6000 times).

Try an explain analyze: http://www.postgresql.org/docs/7.4/interactive/sql-explain.html - that will reveal more of where the performance problem is.

==================================================================
   Aaron Bono
   Aranya Software Technologies, Inc.
   http://www.aranya.com
   http://codeelixir.com
==================================================================

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