On Fri, Feb 19, 2010 at 11:33 PM, Tom Lane <t...@sss.pgh.pa.us> wrote: > Robert Haas <robertmh...@gmail.com> writes: >> On Fri, Feb 19, 2010 at 10:22 PM, Tom Lane <t...@sss.pgh.pa.us> wrote: >>> Maybe, but the only reasonable place to put it would be within the >>> (SubPlan N) reference, > >> I thought maybe it could do something like this: > >> SubPlan 1 >> Parameters: $0 := b.oid >> -> Index Scan etc. > > No, that's the wrong end of the stick --- that's like trying to annotate > a function definition with the actual parameter values being passed to > it from somewhere else. You haven't got the info there, and even if you > did, it's assuming that there is exactly one call site for any subplan.
OK. Will have to think this one over. >> I am under the (perhaps faulty) impression that when evaluating an >> expression there can only ever be three tuples in score: inner, outer, >> and scan. So when we go to evaluate the expression whose result will >> be assigned to $0, where do we get those inner and/or outer and/or >> scan tuples from? IOW, I understand where the subplan is putting its >> OUTPUT, what I don't understand is what context is being used to set >> its input parameters. > > Consider this small mod on your example: > > regression=# explain (verbose) select oid::int + 1,(select oid from pg_class > a where a.oid = b.relfilenode) from pg_class b; > QUERY PLAN > -------------------------------------------------------------------------------------------------------- > Seq Scan on pg_catalog.pg_class b (cost=0.00..5573.04 rows=671 width=8) > Output: ((b.oid)::integer + 1), (SubPlan 1) > SubPlan 1 > -> Index Scan using pg_class_oid_index on pg_catalog.pg_class a > (cost=0.00..8.27 rows=1 width=4) > Output: a.oid > Index Cond: (a.oid = $0) > (6 rows) > > When we are evaluating the output targetlist of the seqscan node, we > have a scan tuple of pg_class b in scope. We can fetch that tuple's > oid and use it in the first expression. OK. > We can also fetch that tuple's > relfilenode and pass it to the subplan, which we do by setting the $0 > Param value before invoking the subplan. Are the same tuples in scope when evaluating the expression that sets $0 as were in scope when evaluating ((b.oid)::integer + 1)? > The subplan runs an indexscan > and returns a single scalar value (to wit, a.oid from some row of > pg_class a), which becomes the value of the (SubPlan 1) reference > back at the evaluation of the seqscan's targetlist. This part I get, 100%. ...Robert -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
