DISTINCT ON

Hi all,

It looks like Postgres has a restriction in DISTINCT ON queries where the DISTINCT ON expressions must match the left side of the ORDER BY list. The issue is that if a DISTINCT ON ... has multiple instances of a particular expression, this check doesn't seem to fire correctly.

For example, this query returns an error (but I guess it shouldn't):

SELECT DISTINCT ON ('1'::varchar, '1'::varchar) a FROM (SELECT 1 AS a) AS a ORDER BY '1'::varchar, '1'::varchar, '2'::varchar;

And this query doesn't return an error (but I guess it should):

SELECT DISTINCT ON ('1'::varchar, '2'::varchar, '1'::varchar) a FROM (SELECT 1 AS a) AS a ORDER BY '1'::varchar, '2'::varchar, '2'::varchar;

Am I misunderstanding something or is there a bug?

Thanks for the help
Emmanuel

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Emmanuel Cecchet
Aster Data
Web: http://www.asterdata.com