Re: Query plan for "heavy" SELECT with "lite" sub-SELECTs

Arjen van der Meijden <acmmailing(at)tweakers(dot)net> writes:
> ... Rewriting it to something like this made the last iteration about as
> fast as the first:

> SELECT docid, (SELECT work to be done for each document)
> FROM documents
> WHERE docid IN (SELECT docid FROM documents
> ORDER BY docid
> LIMIT 1000
> OFFSET ?
> )

The reason for this, of course, is that the LIMIT/OFFSET filter is the
last step in a query plan --- it comes *after* computation of the SELECT
output list. (So does ORDER BY, if an explicit sort step is needed.)
So if you have an expensive-to-compute output list, a trick like Arjen's
will help. I don't think you can use an "IN" though, at least not if
you want to preserve the sort ordering in the final result.

regards, tom lane