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

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.

Pros
• Backward compatibility. If the local ORDER BY clause is missing then the current rules shall apply;
• Familiar and consistent syntax and semantics, the same as in *_agg functions;
• Clear distinction of first row selection and result-set ordering;
• Good readability;
• The DISTINCT ON expression(s) do not have to match the leftmost ORDER BY expression(s).

Cons

Stefan Stefanov <stefanov.sm@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
selectionand 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
requirenesting 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