Re: optimizing queries using IN and EXISTS

Interesting.
Thanks for the work-around.

Regards,

Nick

-----Original Message-----
From: pgsql-performance-owner(at)postgresql(dot)org [mailto:pgsql-performance-owner(at)postgresql(dot)org] On Behalf Of Tom Lane
Sent: donderdag 19 juli 2012 0:36
To: Nick Hofstede
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: [PERFORM] optimizing queries using IN and EXISTS

Nick Hofstede <Nick(dot)Hofstede(at)inventivegroup(dot)com> writes:
> I'm surprised at the difference in speed/execution plan between two
> logically equivalent queries, one using IN, the other using EXISTS.
> (At least I think they are logically equivalent)

> SELECT *
> FROM foo
> WHERE 'text6' IN (SELECT value
> FROM bar
> JOIN foo AS foo2
> ON bar.foo_ref = foo2.id
> WHERE foo2.id = foo.id)

Hm. convert_ANY_sublink_to_join() rejects subqueries that contain any Vars of the parent query level, so the reference to foo.id prevents this from being converted to a semijoin. However, it seems like that's overly restrictive. I'm not sure that we could remove the test altogether, but at least outer vars used in WHERE seem safe.

In the meantime, you can recast like this:

SELECT *
FROM foo
WHERE ('text6', id) IN (SELECT value, foo2.id
FROM bar
JOIN foo AS foo2
ON bar.foo_ref = foo2.id)

and still get a semijoin plan from an IN-style query.

regards, tom lane

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