From: Tom Lane <tgl@sss.pgh.pa.us>
To: "David G. Johnston" <david.g.johnston@gmail.com>
cc: Tels <nospam-pg-abuse@bloodgate.com>,
        "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Subject: Re: Account for cost and selectivity of HAVING quals
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Comments: In-reply-to "David G. Johnston" <david.g.johnston@gmail.com>
	message dated "Tue, 31 Oct 2017 16:54:08 -0700"
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Date: Tue, 31 Oct 2017 19:59:33 -0400
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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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