Re: Problems with plan estimates in postgres_fdw

Etsuro Fujita <fujita.etsuro@lab.ntt.co.jp> wrote:

> (2019/02/08 21:35), Etsuro Fujita wrote:
> > (2019/02/08 2:04), Antonin Houska wrote:
> >> * add_foreign_ordered_paths()
> >>
> >> Exprssion root->upper_targets[UPPERREL_FINAL] is used to access the
> >> target.
> >>
> >> I think create_ordered_paths() should actually set the target so that
> >> postgresGetForeignUpperPaths() can simply find it in
> >> output_rel->reltarget. This is how postgres_fdw already retrieves the
> >> target
> >> for grouped paths. Small patch is attached that makes this possible.
> >
> > Seems like a good idea. Thanks for the patch! Will review.
> 
> Here are my review comments:
> 
>  		root->upper_targets[UPPERREL_FINAL] = final_target;
> +		root->upper_targets[UPPERREL_ORDERED] = final_target;
>  		root->upper_targets[UPPERREL_WINDOW] = sort_input_target;
>  		root->upper_targets[UPPERREL_GROUP_AGG] = grouping_target;
> 
> I think that's a good idea.  I think we'll soon need the PathTarget for
> UPPERREL_DISTINCT, so how about saving that as well like this?
> 
>                 root->upper_targets[UPPERREL_FINAL] = final_target;
> +               root->upper_targets[UPPERREL_ORDERED] = final_target;
> +               root->upper_targets[UPPERREL_DISTINCT] = sort_input_target;
>                 root->upper_targets[UPPERREL_WINDOW] = sort_input_target;
>                 root->upper_targets[UPPERREL_GROUP_AGG] = grouping_target;

I see nothing wrong about this.

> Another is about this:
> 
>  	/*
> +	 * Set reltarget so that it's consistent with the paths. Also it's more
> +	 * convenient for FDW to find the target here.
> +	 */
> +	ordered_rel->reltarget = target;
> 
> I think this is correct if there are no SRFs in the targetlist, but otherwise
> I'm not sure this is really correct because the relation's reltarget would not
> match the target of paths added to the relation after adjust_paths_for_srfs().

I wouldn't expect problem here. create_grouping_paths() also sets the
reltarget although adjust_paths_for_srfs() can be alled on grouped_rel
afterwards.

-- 
Antonin Houska
Cybertec Schönig & Schönig GmbH
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