Stefan Stefanov <stefanov...@abv.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
