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Date: Wed, 1 Nov 2017 03:51:38 -0400
Subject: Re: Account for cost and selectivity of HAVING quals
From: "Tels" <nospam-pg-abuse@bloodgate.com>
To: "David G. Johnston" <david.g.johnston@gmail.com>
Cc: "Tom Lane" <tgl@sss.pgh.pa.us>,
 "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
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Hello David,

On Tue, October 31, 2017 7:54 pm, David G. Johnston wrote:
> 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.  The
> side-effect of the effort is that the number of output_tuples has now been
> reduced to only those matching the qual - and so it now must take on a new
> value to represent this.

Ah, makes sense. Learned something new today.

Maybe it's worth to add a comment, or would everybody else beside me
understand it easily by looking at the code? :)

Thank you,

Tels


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