From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com> |
Cc: | Jim Finnerty <jfinnert(at)amazon(dot)com>, pgsql-hackers(at)postgresql(dot)org |
Subject: | Re: On disable_cost |
Date: | 2019-11-02 15:04:58 |
Message-ID: | 30025.1572707098@sss.pgh.pa.us |
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Lists: | pgsql-hackers |
Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com> writes:
> On Fri, Nov 01, 2019 at 09:30:52AM -0700, Jim Finnerty wrote:
>> re: coping with adding disable_cost more than once
>>
>> Another option would be to have a 2-part Cost structure. If disable_cost is
>> ever added to the Cost, then you set a flag recording this. If any plans
>> exist that have no disable_costs added to them, then the planner chooses the
>> minimum cost among those, otherwise you choose the minimum cost path.
> Yeah, I agree having is_disabled flag, and treat all paths with 'true'
> as more expensive than paths with 'false' (and when both paths have the
> same value then actually compare the cost) is probably the way forward.
It would have to be a count, not a boolean --- for example, you want to
prefer a path that uses one disabled SeqScan over a path that uses two.
I'm with Andres in being pretty worried about the extra burden imposed
on add_path comparisons.
regards, tom lane
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