A recent thread in pgsql-general shows yet another user who's befuddled by the need to add a USING clause to an ALTER TABLE ... ALTER COLUMN TYPE command:
http://www.postgresql.org/message-id/flat/CAD25zGDiaqCG=eqXp=byvzcdgvcqubh7kbpjpjqsespowfv...@mail.gmail.com Specifically, it's not clear why you can change the type of a uuid[] column with alter table t alter u type text[]; but then you can't change it back with alter table t alter u type uuid[]; The reason of course is that uuid-to-text is considered an assignment-grade coercion while text-to-uuid is not. I've lost count of the number of times we've had to tell someone to use a USING clause for this. Maybe it's time to be a little bit less rigid about this situation, and do what the user obviously wants rather than make him spell out a rather pointless USING. Specifically, after a bit of thought, I suggest that (1) If there's no USING, attempt to coerce the column value as though an *explicit* coercion were used. (2) If there is a USING, maintain the current behavior that the result has to be assignment-coercible to the new column type. We could use explicit-coercion semantics here too, but I think that might be throwing away a bit too much error checking, in a case where the odds of a typo are measurably higher than for the default situation. This could be documented as "if there is no USING, the default behavior is as if you'd written USING column::newtype". Thoughts? In any case, we oughta use two different error messages for the two cases, as per my comment in the above thread. That seems like a back-patchable bug fix, though of course any semantics change should only be in HEAD. regards, tom lane -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
