| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Shrirang Chitnis <Shrirang(dot)Chitnis(at)hovservices(dot)com> |
| Cc: | Bryce Nesbitt <bryce2(at)obviously(dot)com>, "pgsql-performance(at)postgresql(dot)org" <pgsql-performance(at)postgresql(dot)org> |
| Subject: | Re: hashed subplan 5000x slower than two sequential operations |
| Date: | 2010-12-08 20:12:26 |
| Message-ID: | 20170.1291839146@sss.pgh.pa.us |
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| Lists: | pgsql-performance |
Shrirang Chitnis <Shrirang(dot)Chitnis(at)hovservices(dot)com> writes:
> Bryce,
> The two queries are different:
I suspect the second one is a typo and not what he really wanted.
> WHERE (contexts.parent_key = 392210
> OR contexts.context_key IN
> (SELECT collection_data.context_key
> FROM collection_data
> WHERE collection_data.collection_context_key = 392210)
The only really effective way the planner knows to optimize an
"IN (sub-SELECT)" is to turn it into a semi-join, which is not possible
here because of the unrelated OR clause. You might consider replacing
this with a UNION of two scans of "contexts". (And yes, I know it'd be
nicer if the planner did that for you.)
regards, tom lane
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