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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