| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | "David G(dot) Johnston" <david(dot)g(dot)johnston(at)gmail(dot)com> |
| Cc: | Tels <nospam-pg-abuse(at)bloodgate(dot)com>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Account for cost and selectivity of HAVING quals |
| Date: | 2017-10-31 23:59:33 |
| Message-ID: | 1464.1509494373@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
"David G. Johnston" <david(dot)g(dot)johnston(at)gmail(dot)com> writes:
> On Tue, Oct 31, 2017 at 4:31 PM, Tels <nospam-pg-abuse(at)bloodgate(dot)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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