| From: | Robert Haas <robertmhaas(at)gmail(dot)com> |
|---|---|
| To: | David Rowley <dgrowleyml(at)gmail(dot)com> |
| Cc: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Jian Guo <gjian(at)vmware(dot)com>, Zhenghua Lyu <zlyu(at)vmware(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org> |
| Subject: | Re: On disable_cost |
| Date: | 2024-03-13 13:05:00 |
| Message-ID: | CA+TgmoYYNnJk-qvwQY5w16An1P1uPB74co9Szdj2RpJE8fgoKw@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Tue, Mar 12, 2024 at 4:55 PM David Rowley <dgrowleyml(at)gmail(dot)com> wrote:
> The primary place I see issues with disabled_cost is caused by
> STD_FUZZ_FACTOR. When you add 1.0e10 to a couple of modestly costly
> paths, it makes them appear fuzzily the same in cases where one could
> be significantly cheaper than the other. If we were to bump up the
> disable_cost it would make this problem worse.
Hmm, good point.
> So maybe the fix could be to set disable_cost to something like
> 1.0e110 and adjust compare_path_costs_fuzzily to not apply the
> fuzz_factor for paths >= disable_cost. However, I wonder if that
> risks the costs going infinite after a couple of cartesian joins.
Yeah, I think the disabled flag is a better answer if we can make it
work. No matter what value we pick for disable_cost, it's bound to
skew the planning sometimes.
--
Robert Haas
EDB: http://www.enterprisedb.com
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