Horton

Re: On disable_cost

Tom Lane <tgl@sss.pgh.pa.us>

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Cc: Jim Finnerty <jfinnert@amazon.com>, pgsql-hackers@postgresql.org
Date: 2019-11-02T15:04:58Z
Lists: pgsql-hackers 
Tomas Vondra <tomas.vondra@2ndquadrant.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

Commits

  1. Doc: add detail about EXPLAIN's "Disabled" property

  2. Adjust EXPLAIN's output for disabled nodes

  3. Fix order of parameters in a cost_sort call

  4. Show number of disabled nodes in EXPLAIN ANALYZE output.

  5. Treat number of disabled nodes in a path as a separate cost metric.

  6. Remove grotty use of disable_cost for TID scan plans.