Re: Allow DISTINCT to use Incremental Sort

>         /* For explicit-sort case, always use the more rigorous clause */
>         if (list_length(root->distinct_pathkeys) <
>             list_length(root->sort_pathkeys))
>         {
>             needed_pathkeys = root->sort_pathkeys;
>             /* Assert checks that parser didn't mess up... */
>             Assert(pathkeys_contained_in(root->distinct_pathkeys,
>                                          needed_pathkeys));
>         }
>         else
>             needed_pathkeys = root->distinct_pathkeys;
>
> I'm not sure if this is necessary, as AFAIU the parser should have
> ensured that the sortClause is always a prefix of distinctClause.

I think that's true for standard DISTINCT, but it's not for DISTINCT ON.

> In the patch this code has been removed.  I think we should also remove
> the related comments in create_final_distinct_paths.
>
>        * When we have DISTINCT ON, we must sort by the more rigorous of
>        * DISTINCT and ORDER BY, else it won't have the desired behavior.
> -      * Also, if we do have to do an explicit sort, we might as well use
> -      * the more rigorous ordering to avoid a second sort later.  (Note
> -      * that the parser will have ensured that one clause is a prefix of
> -      * the other.)

I'm not quite following what you think has changed here.

I've attached an updated patch