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List:       postgresql-sql
Subject:    Re: [SQL] efficient deletes on subqueries
From:       Tom Lane <tgl () sss ! pgh ! pa ! us>
Date:       2001-10-30 16:48:33
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"postgresql" <postgres@lg.ndirect.co.uk> writes:
> I want to remove all entries from AA where keyA matches that from
> obsolete_AA_entries,
> i.e. SELECT * FROM AA NATURAL JOIN obsolete_AA_entries;

> DELETE FROM AA WHERE EXISTS(
>           SELECT * from obsolete_AA_entries o where AA.keyA = o.keyA);
> seems to be faster than
> DELETE FROM AA where (keyA) in (SELECT * from obsolete_AA_entries);

> However, both are sequentially going through AA which is huge rather than
> looking up values one by one from obsolete_AA_entries which is small.
> How do I persuade the database to change its query strategy?

Try
	DELETE FROM AA WHERE AA.keyA = obsolete_AA_entries.keyA;

This is not valid SQL according to the SQL standard, but Postgres takes
it anyway.

Turning sub-SELECT queries into plannable joins is on the todo list,
but it's not done yet :-(

			regards, tom lane

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