On Tue, Feb 10, 2009 at 5:03 PM, Tom Lane <t...@sss.pgh.pa.us> wrote: > Robert Haas <robertmh...@gmail.com> writes: >> I don't understand why antijoins need to null-extend the tuple at all. > > Well, we are talking theoretical definition here, not implementation. > But if you need an example where the column values can be referenced: > > select * from a left join b on (a.id = b.id) > where b.id is null > > 8.4 does recognize this as an antijoin, if the join operator is strict.
Oh, I see. Hmm. >> In the case of a semijoin, it's theoretically possible that there >> could be syntax which allows access to the attributes of the outer >> side of the relation, though IN and EXISTS do not. > > Actually, that makes less sense than the antijoin case. For antijoin > there is a well-defined value for the extended columns, ie null. For > a semijoin the RHS values might come from any of the rows that happen > to join to the current LHS row, so I'm just as happy that it's > syntactically impossible to reference them. You might some day want to optimize this case as a semijoin, or something similar to a semijoin: SELECT foo.a, (SELECT bar.b FROM bar WHERE bar.a = foo.a) FROM foo; ...Robert -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
