Message-ID: <52C5F277.8070001@student.kit.edu>
Date: Fri, 3 Jan 2014 00:12:55 +0100
From: Thomas Mayer <thomas.mayer@student.kit.edu>
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To: Tom Lane <tgl@sss.pgh.pa.us>, Jeff Janes <jeff.janes@gmail.com>
CC: pgsql-performance <pgsql-performance@postgresql.org>
Subject: Re: window function induces full table scan
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Am 02.01.2014 23:43, schrieb Tom Lane:
> Jeff Janes <jeff.janes@gmail.com> writes:
>> On Thu, Jan 2, 2014 at 1:52 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>>> It's possible that in the specific case you exhibit here, pushing down
>>> the clause wouldn't result in changes in the window function's output for
>>> the selected rows, but the optimizer doesn't have enough knowledge about
>>> window functions to determine that.
>
>> A restriction in the WHERE clause which corresponds to the PARTITION BY
>> should be pushable, no?  I think it doesn't need to understand the internal
>> semantics of the window function itself, just of the PARTITION BY, which
>> should be doable, at least in principle.
>
> If the restriction clause must give the same answer for any two rows of
> the same partition, then yeah, we could in principle push it down without
> knowing anything about the specific window function.  It'd be a less than
> trivial test to make, I think.  In any case, it's not a "bug" that the
> optimizer doesn't do this currently.

I agree, this is not a "bug" in v9.3.2 in terms of correctness.

But it's a limitation, because the query plan is by far not optimal. You 
may consider this report as a feature request then.

The optimization I suggested is normally performed, when no window 
function occurs in the statement.

It seems like the optimizer is already capable of doing a check if the 
WHERE can be done first.

However, this check seems to be done too conservative: I guess, the 
check is ignoring the PARTITION-BY-sets of attributes completely.

>
> 			regards, tom lane
> .
>

Best regards
Thomas


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