| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
| Cc: | David Rowley <dgrowleyml(at)gmail(dot)com>, Heikki Linnakangas <hlinnaka(at)iki(dot)fi>, Peter Geoghegan <pg(at)bowt(dot)ie>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org> |
| Subject: | Re: On disable_cost |
| Date: | 2024-08-02 16:51:12 |
| Message-ID: | 3985903.1722617472@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> On Fri, Aug 2, 2024 at 9:13 AM David Rowley <dgrowleyml(at)gmail(dot)com> wrote:
>> ... That way you maintain the
>> existing behaviour of not optimising for disabled node types and don't
>> risk plan changes if the final cost comes out cheaper than the initial
>> cost.
> All three initial_cost_XXX functions have a comment that says "This
> must quickly produce lower-bound estimates of the path's startup and
> total costs," i.e. the final cost should never be cheaper. I'm pretty
> sure that it was the design intention here that no path ever gets
> rejected at the initial cost stage that would have been accepted at
> the final cost stage.
That absolutely is the expectation, and we'd better be careful not
to break it.
regards, tom lane
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