[ after re-reading the code a bit ]
Robert Haas <robertmhaas@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.
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
dothe semijoin but can unique-ify the RHS and * then do an innerjoin. In the latter case we can't apply
* JOIN_SEMI joining. */
regards, tom lane