Jeff Davis <pg...@j-davis.com> writes:
> Section 4.18 of SQL200n, "Functional Dependencies", shows some
> interesting ways that the DBMS can make the proper inferences (I think
> this is an optional feature, so I don't think PostgreSQL violates the
> standard here).

Just for the record, this is something that was added in SQL:99 ---
our behavior conforms to the letter of earlier SQL versions.  I think
we have a TODO item to add at least some support for allowing implicit
GROUP BY using functional dependencies, but it's kind of a worrisome
thing.  I don't know of any other part of the SQL spec whereby ALTER
TABLE DROP CONSTRAINT could turn a formerly semantically legal query
into an illegal query.  Could have some unpleasant implications for the
behavior of prepared statements.

(Also, I'll bet a lot of money that mysql has not implemented this
feature according to spec.  The last I heard, as long as you have a
GROUP BY they just blithely assume you know what you're doing and
didn't write a query whose results are ambiguous.)

                        regards, tom lane

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