From: | Greg Smith <greg(at)2ndquadrant(dot)com> |
---|---|
To: | shulkae <shulkae(at)gmail(dot)com> |
Cc: | pgsql-general(at)postgresql(dot)org |
Subject: | Re: Deleting 100 rows which meets certain criteria |
Date: | 2009-12-30 20:15:15 |
Message-ID: | 4B3BB4D3.8070609@2ndquadrant.com |
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Thread: | |
Lists: | pgsql-general |
shulkae wrote:
> DELETE from mytable WHERE (now() - timestamp_field > INTERVAL '400
> hour' ) LIMIT 100;
>
Force of habit (not sure if the optimizer does this trick for you) is
first to rewrite this as follows:
DELETE from mytable WHERE timestamp_field < (now() - INTERVAL '400
hour' ) LIMIT 100;
Just to turn the comparison into a constant being compared with the field.
If there's a useful primary key on this table, you can do this to delete:
DELETE FROM mytable where pkey IN (SELECT pkey from mytable WHERE
timestamp_field < (now() - INTERVAL '400 hour' ) LIMIT 100);
If there's not a primary key, you can use a hidden field named ctid to
get your record list:
http://www.postgresql.org/docs/current/static/ddl-system-columns.html
And then use that as the way to communicate the candidate deletion list
out of the subselect:
DELETE FROM mytable where ctid IN (SELECT ctid from mytable WHERE
timestamp_field < (now() - INTERVAL '400 hour' ) LIMIT 100);
The main advantage of using the primary key is that the result will be
more portable to other databases--the ctid field is very much a
PostgreSQL specific hack.
--
Greg Smith 2ndQuadrant Baltimore, MD
PostgreSQL Training, Services and Support
greg(at)2ndQuadrant(dot)com www.2ndQuadrant.com
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