| From: | Claudio Freire <klaussfreire(at)gmail(dot)com> |
|---|---|
| To: | Jim Nasby <jim(at)nasby(dot)net> |
| Cc: | Merlin Moncure <mmoncure(at)gmail(dot)com>, Stephen Frost <sfrost(at)snowman(dot)net>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, David Johnston <polobo(at)yahoo(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Planner hints in Postgresql |
| Date: | 2014-03-17 22:07:31 |
| Message-ID: | CAGTBQpaHMyyYCqX8U=X2CBCOipYX27qbbmg9Fyj8AhEN6Cq5Dg@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Mon, Mar 17, 2014 at 7:01 PM, Jim Nasby <jim(at)nasby(dot)net> wrote:
> Even better would be if the planner could estimate how bad a plan will
> become if we made assumptions that turn out to be wrong.
>
That's precisely what risk estimation was about.
Something like
SELECT * FROM wherever WHEN id > something LIMIT COST 10000;
Would forbid a sequential scan *if* the table is big enough to suspect the
plan might take that much, or a nested loop *if* the planner cannot *prove*
it will be faster than that.
I don't believe the limit unit is obscure at all (page fetches being a nice
measuring stick), but what is, is what do you do when no plan fits the
limits.
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