Suggestion. Optional local ORDER BY clause for DISTINCT ON

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.

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
• Possible extra verbosity
Best regards, Stefan
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