On Thu, Feb 19, 2009 at 1:38 PM, Tom Lane <t...@sss.pgh.pa.us> wrote: > [ after re-reading the code a bit ] > > Robert Haas <robertmh...@gmail.com> writes: >> Cool. On the topic of documentation, I find the following comment in >> joinrels.c rather impenetrable: > >> /* >> * Do these steps only if we actually have a >> regular semijoin, >> * as opposed to a case where we should >> unique-ify the RHS. >> */ > > The point here is that a semijoin ordinarily requires forming the whole > LHS relation (ie, min_lefthand) before applying the special join rule. > However, if you unique-ify the RHS then it's a regular inner join and > you don't have to obey that restriction, ie, you can join to just part > of min_lefthand. Now ordinarily that's not an amazingly good idea but > there are important special cases where it is. IIRC the case that > motivated this was > > SELECT FROM a, b WHERE (a.x, b.y) IN (SELECT c1, c2 FROM c) > > If you do this as a semijoin then you are forced to form the cartesian > product of a*b before you can semijoin to c. If you uniqueify c > then you can join it to a first and then b using regular joins (possibly > indexscans on a.x and then b.y), or b and then a. > > So join_is_legal allows such a join order, and the code in make_join_rel > has to be careful not to claim that "a semijoin c" is a valid way of > forming that join.
Gotcha. > I'll change the comment. Does this help? > > /* > * We might have a normal semijoin, or a case where we don't have > * enough rels to do the semijoin but can unique-ify the RHS and > * then do an innerjoin. In the latter case we can't apply > * JOIN_SEMI joining. > */ It's an improvement, but your example above is so helpful in understanding what is going on here that it might be worth explicitly mentioning it in the comment, maybe something like this: /* * In a case like the following, we don't have enough rels to plan this as a semijoin, * but we don't give up completely, because it might be possible to unique-ify the * RHS and perform part of the join at this level. * * SELECT FROM a, b WHERE (a.x, b.y) IN (SELECT c1, c2 FROM c) */ ...Robert -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers