Re: Very long deletion time on a 200 GB database

On 02/27/2012 02:08 AM, Reuven M. Lerner wrote:

> In the end, it was agreed that we could execute the deletes over
> time, deleting items in the background, or in parallel with the
> application's work. After all, if the disk is filling up at the rate
> of 2 GB/day, then so long as we delete 4 GB/day (which is pretty easy
> to do), we should be fine.

Please tell me you understand deleting rows from a PostgreSQL database
doesn't work like this. :) The MVCC storage system means you'll
basically just be marking all those deleted rows as reusable, so your
database will stop growing, but you'll eventually want to purge all the
accumulated dead rows.

One way to see how many there are is to use the pgstattuple contrib
module. You can just call it on the table name in question:

SELECT * FROM pgstattuple('my_table');

You may find that after your deletes are done, you'll have a free_pct of
80+%. In order to get rid of all that, you'll need to either run CLUSTER
on your table(s) or use the select->truncate->insert method anyway.

--
Shaun Thomas
OptionsHouse | 141 W. Jackson Blvd. | Suite 500 | Chicago IL, 60604
312-444-8534
sthomas(at)peak6(dot)com

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