Re: Slow duplicate deletes

On Mon, Mar 5, 2012 at 9:17 AM, DrYSG <ygutfreund(at)draper(dot)com> wrote:
> I have a large table (20M records) but mostly short text fields. There are
> duplicates that I am trying to remove.  I have a bigseriel index that I
> unique, but duplicates in another field.
>
> I have an 8 core, 12GB memory computer with RAID disks.
>
> This request has been running for 70 hours (is it safe to kill it?).

yes...generally speaking, it's safe to kill just about any query in
postgres any time.

> How can I make this run faster? This is a one time processing task, but it
> is taking a long time.
>
> DELETE FROM portal.metadata
> WHERE idx NOT IN
> (
>    SELECT MIN(idx)
>    FROM portal.metadata
>    GROUP BY "data_object.unique_id"
> );

compare the plan for that query (EXPLAIN) vs this one:

/* delete the records from m1 if there is another record with a lower
idx for the same unique_id */
DELETE FROM portal.metadata m1
WHERE EXISTS
(
SELECT 1 FROM portal.metadata m2
WHERE m1.unique_id = m2.unique_id
AND m2.idx < m1.idx
)

also, if you don't already have one, consider making an index on at
least unqiue_id, or possibly unique_id, idx.

back up your database before running this query :-).

merlin