Re: hashed subplan 5000x slower than two sequential operations

2010/12/8 Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>:
> 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.)

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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