From: Tels <nospam-pg-abuse@bloodgate.com>
To: David G. Johnston <david.g.johnston@gmail.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>
Cc: pgsql-hackers@postgresql.org <pgsql-hackers@postgresql.org>
Subject: Re: Account for cost and selectivity of HAVING quals
Date: Wed, 1 Nov 2017 03:51:38 -0400
Message-ID: <61b0807a4afa2bc3f421c0fd5959e0a1.squirrel@sm.webmail.pair.com> (raw)
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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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To: pgsql-hackers@postgresql.org
Cc: nospam-pg-abuse@bloodgate.com, david.g.johnston@gmail.com, tgl@sss.pgh.pa.us
Subject: Re: Account for cost and selectivity of HAVING quals
In-Reply-To: <61b0807a4afa2bc3f421c0fd5959e0a1.squirrel@sm.webmail.pair.com>
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