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List:       postgresql-general
Subject:    Re: [HACKERS] Account for cost and selectivity of HAVING quals
From:       Tom Lane <tgl () sss ! pgh ! pa ! us>
Date:       2017-10-31 23:59:33
Message-ID: 1464.1509494373 () sss ! pgh ! pa ! us
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"David G. Johnston" <david.g.johnston@gmail.com> writes:
> On Tue, Oct 31, 2017 at 4:31 PM, Tels <nospam-pg-abuse@bloodgate.com> wrote:
>> That looks odd to me, it first uses output_tuples in a formula, then
>> overwrites the value with a new value. Should these lines be swapped?

> ​IIUC it is correct: the additional total_cost comes from processing every
> output group to check whether it is qualified - since every group is
> checked the incoming output_tuples from the prior grouping is used.

Right --- we'll expend the effort to compute the HAVING expression once
per group row, whether the row passes the qual or not.

			regards, tom lane


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