Re: queries with subquery constraints on partitioned tables not optimized?

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От Nikolas Everett
Тема Re: queries with subquery constraints on partitioned tables not optimized?
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Msg-id d4e11e981002030754id1e3b2em7fa3e7a7ac0daef3@mail.gmail.com
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Ответ на Re: queries with subquery constraints on partitioned tables not optimized?  (Tom Lane <tgl@sss.pgh.pa.us>)
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On Tue, Feb 2, 2010 at 7:14 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
"Davor J." <DavorJ@live.com> writes:
> Now, if one takes a subquery for "1", the optimizer evaluates it first
> (let's say to "1"), but then searches for it (sequentially) in every
> partition, which, for large partitions, can be very time-consuming and goes
> beyond the point of partitioning.

No, the optimizer doesn't "evaluate it first".  Subqueries aren't ever
assumed to reduce to constants.  (If you actually do have a constant
expression, why don't you just leave out the word SELECT?)

                       regards, tom lane

If you don't have a constant expression then you can either explicitly loop in the calling code or a function or you could index the key in all the subtables.  The index isn't really optimal but it gets the job done.

Nik

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