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Re: Shouldn't we have a way to avoid "risky" plans?

From: Віталій Тимчишин <tivv00(at)gmail(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Claudio Freire <klaussfreire(at)gmail(dot)com>, Josh Berkus <josh(at)agliodbs(dot)com>, postgres performance list <pgsql-performance(at)postgresql(dot)org>
Subject: Re: Shouldn't we have a way to avoid "risky" plans?
Date: 2011-03-24 08:44:33
Message-ID: AANLkTim-SKShZUxD55ZuHHHLgYuY_MzSSPm6GNYR_fYW@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-performance
2011/3/23 Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>

> Claudio Freire <klaussfreire(at)gmail(dot)com> writes:
> > On Wed, Mar 23, 2011 at 5:29 PM, Josh Berkus <josh(at)agliodbs(dot)com> wrote:
> >> On 3/23/11 10:35 AM, Claudio Freire wrote:
> >>>  *  consider plan bailout: execute a tempting plan, if it takes too
> >>> long or its effective cost raises well above the expected cost, bail
> >>> to a safer plan
>
> >> That would actually solve this particular case.  It would still require
> >> us to have some definition of "safer" though.
>
> > In my head, safer = better worst-case performance.
>
> If the planner starts operating on the basis of worst case rather than
> expected-case performance, the complaints will be far more numerous than
> they are today.
>
> This can se GUC-controllable. Like plan_safety=0..1 with low default value.
This can influence costs of plans where cost changes dramatically with small
table changes and/or statistics is uncertain. Also this can be used as
direct "hint" for such dangerous queries by changing GUC for session/single
query.


-- 
Best regards,
 Vitalii Tymchyshyn

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