Re: Clamping reulst row number of joins. - Mailing list pgsql-hackers

Tom Lane <tgl@sss.pgh.pa.us> writes:
> Stephen Frost <sfrost@snowman.net> writes:
>> * Tom Lane (tgl@sss.pgh.pa.us) wrote:
>>> BTW, is that JOIN (VALUES(...)) thing common in applications, or did you
>>> just use it to make a compact example?  If it were something worth
>>> optimizing, it seems like we could teach the planner to "pull up VALUES"
>>> in the same way that it flattens sub-selects.  I'm not sure if this is
>>> worth the trouble or not, though.

>> I've certainly seen and used values() constructs in joins for a variety
>> of reasons and I do think it'd be worthwhile for the planner to know how
>> to pull up a VALUES construct.

>> Would that be a lot of effort, either code-wise or runtime-wise?  My gut
>> feeling is that it wouldn't be, but you're clearly in a much better
>> position to determine that.

> My guess is that it'd be pretty simple to do if we want to do it.
> I've not looked at the code yet though.

I spent a bit of time looking at this, and realized that the blocker
is the same as the reason why we don't pull up sub-selects with empty
rangetables ("SELECT expression(s)").  Namely, that the upper query's
jointree can't handle a null subtree.  (This is not only a matter of
the jointree data structure, but the fact that the whole planner
identifies relations by bitmapsets of RTE indexes, and subtrees with
empty RTE sets couldn't be told apart.)

We could probably fix both cases for order-of-a-hundred lines of new code
in prepjointree.  The plan I'm thinking about is to allow such vacuous
subquery jointrees to be pulled up, but only if they are in a place in
the upper query's jointree where it's okay to delete the subtree.  This
would basically be two cases: (1) the immediate parent is a FromExpr that
would have at least one remaining child, or (2) the immediate parent is
an INNER JOIN whose other child isn't also being deleted (so that we can
convert the JoinExpr to a nonempty FromExpr, or just use the other child
as-is if the JoinExpr has no quals).

More generally, it occurs to me that maybe Oracle wasn't being totally
silly when they invented DUAL.  If we had a jointree representation for
"dummy relation with exactly one row" then we could substitute that in
all vacuous-jointree cases.  However, it's not clear that there's any
functional advantage to replacing a VALUES Scan with a "DUAL Scan",
which is basically what would happen if we flattened a VALUES and then
had to put one of these things in instead.  And having such things
propagate all through the planner, executor, EXPLAIN, etc is way more
code churn than I want to contemplate right now.  The plan proposed in
the preceding para is a bit more ugly logically, but it would limit the
code effects to basically pull_up_subqueries() and its child routines.
        regards, tom lane