On Nov 3, 2009, at 10:17 PM, Emmanuel Cecchet <m...@asterdata.com>
wrote:
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?
I'm guessing this is the result of some subtly flakey equivalence
class handling. On first glance ISTM that discarding duplicates is
legit and therefore both examples ought to work...
...Robert
--
Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-hackers
