2010/12/8 Tom Lane <tgl@sss.pgh.pa.us>:
> Shrirang Chitnis <Shrirang.Chitnis@hovservices.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.)
I remeber a similar case - 9 years ago.
slow variant:
WHERE pk = C1 OR pk IN (SELECT .. FROM .. WHERE some = C2)
I had to rewrite to form
WHERE pk IN (SELECT .. FROM WHERE some = C2 UNION ALL SELECT C1)
Regards
Pavel Stehule
>
> regards, tom lane
>
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