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help / color / mirror / Atom feedFrom: Tom Lane <[email protected]>
To: Tomas Vondra <[email protected]>
Cc: Jim Finnerty <[email protected]>
Cc: [email protected]
Subject: Re: On disable_cost
Date: Sat, 02 Nov 2019 11:04:58 -0400
Message-ID: <[email protected]> (raw)
In-Reply-To: <20191101170406.w6ybnafyvpxv2ltg@development>
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Tomas Vondra <[email protected]> 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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