Re: Removing unneeded self joins

On 17 May 2018 at 10:55, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> David Rowley <david(dot)rowley(at)2ndquadrant(dot)com> writes:
>> There's probably some argument for delaying obtaining the relation
>> size until after join removal and probably partition pruning too, but
>> it's currently done well before that in build_simple_rel, where the
>> RelOptInfo is built.
>
> Yeah, but that's something we ought to fix someday; IMO it's an artifact
> of having wedged in remove_useless_joins without doing the extensive
> refactoring that'd be needed to do it at a more desirable time. I don't
> want to build user-visible behavior that's dependent on doing that wrong.
>
> (But wait a second ... we could improve this without quite that much work:
> instead of doing estimate_rel_size immediately during get_relation_info,
> couldn't it be left until the set_base_rel_sizes pass? Since
> RelationGetNumberOfBlocks involves kernel calls, skipping it for removed
> rels seems worth doing.)

I did mean just obtaining the sizes, not delaying building the
RelOptInfo. I see nothing that needs RelOptInfo->pages before
set_base_rel_size apart from the code which I mentioned about moving a
couple of days ago in [1].

[1] https://www.postgresql.org/message-id/CAKJS1f_eUz0_h5_vU1rqE7wuxMcoENcWK2FTODz0pOyxp3_Uig%40mail.gmail.com

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David Rowley http://www.2ndQuadrant.com/
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