Thread: slowness when subselect uses DISTINCT
I have several times now run into what seems like similar performance problems with some of my postgresql queries. I have a view that runs reasonably quicky. I use this view in a subselect in another query and that query too runs reasonably quicky. The view returns some unwanted duplicate rows so I modify it using either DISTINCT or GROUP BY to eliminate them. View still runs reasonably quickly. I use the modified view as a subselect as above, but now the query runs 2-3 orders of magnitude more slowly than before. Before I go through the effort of putting together a specific and concise test case, has anyone seen this general pattern and have an explanation or advice? (PG-8.2.3)
May I suggest you post an EXPLAIN ANALYZE to the group for the query you're
having problems with...?
-----Original Message-----
From: pgsql-sql-owner@postgresql.org [mailto:pgsql-sql-owner@postgresql.org]
On Behalf Of Stuart McGraw
Sent: Thursday, 19 April 2007 04:17
To: pgsql-sql@postgresql.org
Subject: [SQL] slowness when subselect uses DISTINCT
I have several times now run into what seems
like similar performance problems with some
of my postgresql queries.
I have a view that runs reasonably quicky.
I use this view in a subselect in another
query and that query too runs reasonably
quicky.
The view returns some unwanted duplicate
rows so I modify it using either DISTINCT
or GROUP BY to eliminate them.
View still runs reasonably quickly.
I use the modified view as a subselect as
above, but now the query runs 2-3 orders
of magnitude more slowly than before.
Before I go through the effort of putting
together a specific and concise test case,
has anyone seen this general pattern and
have an explanation or advice? (PG-8.2.3)
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Phillip Smith wrote: > May I suggest you post an EXPLAIN ANALYZE to the group for the query you're > having problems with...? I will do that but it has happened to me enough that it seems to be a general pattern, not something specific to one of my queries, so I thought some communal knowledge may exist. But apparently not.
Let's start with one, and whatever is causing it in one, is perhaps in theory, causing the rest :) Cheers, ~p -----Original Message----- From: pgsql-sql-owner@postgresql.org [mailto:pgsql-sql-owner@postgresql.org] On Behalf Of Stuart McGraw Sent: Friday, 20 April 2007 00:20 To: pgsql-sql@postgresql.org Subject: Re: [SQL] slowness when subselect uses DISTINCT Phillip Smith wrote: > May I suggest you post an EXPLAIN ANALYZE to the group for the query you're > having problems with...? I will do that but it has happened to me enough that it seems to be a general pattern, not something specific to one of my queries, so I thought some communal knowledge may exist. But apparently not. ---------------------------(end of broadcast)--------------------------- TIP 9: In versions below 8.0, the planner will ignore your desire to choose an index scan if your joining column's datatypesdo not match *******************Confidentiality and Privilege Notice******************* The material contained in this message is privileged and confidential to the addressee. If you are not the addressee indicated in this message or responsible for delivery of the message to such person, you may not copy or deliver this message to anyone, and you should destroy it and kindly notify the sender by reply email. Information in this message that does not relate to the official business of Weatherbeeta must be treated as neither given nor endorsed by Weatherbeeta. Weatherbeeta, its employees, contractors or associates shall not be liable for direct, indirect or consequential loss arising from transmission of this message or any attachments