Re: PSQL statement to delete 100 rows meeting certain criteria

shulkae <shulkae(at)gmail(dot)com> writes:
> I am newbie to postgres/SQL.
>
> I want to delete all rows exceeding 400 hours (10 days) and I want to
> limit deletion of only 100 records at a time.
>
> I was trying the following in PostgreSQL:
>
> DELETE from mytable WHERE (now() - timestamp_field > INTERVAL '400
> hour' ) LIMIT 100;
>
> Looks like DELETE syntax doesn't support LIMIT.
>
> Is there any other way to achieve this?

Sure...

Supposing mytable has, as a unique key, column "id"...

delete from mytable where
id in (select id from mytable
where timestamp_field < now() - '400 hours'::interval
limit 100);

I once set up a process where I did this exact sort of thing, complete
with a "back-off" scheme where each iteration would check a sequence to
see if a lot of new tuples had come in since last time, and:

a) If the system was looking busy, it would only delete a small
bit of data;
b) If the system was not busy at all, it would delete quite a bit
more obsolete data.
--
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