Re: Suggestion. Optional local ORDER BY clause for DISTINCT ON

Stefan Stefanov <stefanov(dot)sm(at)abv(dot)bg> writes:
> Gents, I have a suggestion for DISTINCT ON clause syntax.
> DISTINCT ON (expression(s) [ORDER BY expression(s)])
> Determines the precedence within each DISTINCT ON group (i.e. the ‘first’ row to be picked)

> Motivation
> • Using the query-wide ORDER BY clause to determine which record to pick mixes two unrelated concerns, ‘first’ row selection and result-set ordering. This may be confusing;
> • The DISTINCT ON expression(s) must match the leftmost ORDER BY expression(s). This may cause inconvenience and require nesting as a sub-query to order the result-set.

Since you can get the desired behavior with a sub-select, I'm
not especially excited about extending DISTINCT ON. If it weren't
such a nonstandard kluge, I might feel differently; but it's not
an area that I think we ought to put more effort into.

regards, tom lane