On 2011-02-10, Josh wrote:
> Hi
>
> I'm trying to do a DELETE FROM on my large table (about 800 million
> rows) based on the contents of another, moderately large table (about
> 110 million rows). The command I'm using is:
>
> DELETE FROM records WHERE id NOT IN (SELECT id FROM unique_records);
>
> This process ran for about two weeks before I decided to stop it -- it
> was dragging down the DB server. I can understand long-running
> processes, but two weeks seems a bit much even for a big table.
I find that scripting deletes of smaller numbers of records can help
here, long-running queries do bad things to the efficiency of postgres.
on strategy that could work for your task would be to create a temp
table first:
create temp table planned_deletions as select id from records except
SELECT id FROM unique_records;
create index badids on planned_deletions(id);
the repeatedly
delete from records where id in ( select id from planned_deletions limit 1
order by id);
delete from planned_deletions where id in ( select id from planned_deletions
limit 1 order by id);
until there are none left.
possibly pausing a few seconds between each slug if there is a heavy
load on the server (that you were able to run the query for 2 weeks
suggests that there may not be).
> Is this the best way to approach the problem? Is there a better way?
>
> Some background: The server is version 8.3, running nothing but Pg.
> The 'records' table has 'id' as its primary key, and one other index
> on another column. The table is referenced by just about every other
> table in my DB (about 15 other tables) via foreign key constraints,
> which I don't want to break (which is why I'm not just recreating the
> table rather than deleting rows). Most of the dependent tables have ON
> DELETE CASCADE. The 'unique_records' table is a temp table I got via
> something like: SELECT DISTINCT (other_column) id INTO unique_records
> FROM records
if you can accept the down-time I would drop the constraints
(if you don't have them on file do a pg_dump --schema-only , and grep it
for the ADD CONSTRIANT commands, use sed or similar to create matching
DROP CONSTRAINT commands, run them)
then rebuild the table
then reintroduce the constraints, keep a copy of the grep output above
--
⚂⚃ 100% natural
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