Adjust definition of cheapest_total_path to work better with LATERAL.

In the initial cut at LATERAL, I kept the rule that cheapest_total_path
was always unparameterized, which meant it had to be NULL if the relation
has no unparameterized paths.  It turns out to work much more nicely if
we always have *some* path nominated as cheapest-total for each relation.
In particular, let's still say it's the cheapest unparameterized path if
there is one; if not, take the cheapest-total-cost path among those of
the minimum available parameterization.  (The first rule is actually
a special case of the second.)

This allows reversion of some temporary lobotomizations I'd put in place.
In particular, the planner can now consider hash and merge joins for
joins below a parameter-supplying nestloop, even if there aren't any
unparameterized paths available.  This should bring planning of
LATERAL-containing queries to the same level as queries not using that
feature.

Along the way, simplify management of parameterized paths in add_path()
and friends.  In the original coding for parameterized paths in 9.2,
I tried to minimize the logic changes in add_path(), so it just treated
parameterization as yet another dimension of comparison for paths.
We later made it ignore pathkeys (sort ordering) of parameterized paths,
on the grounds that ordering isn't a useful property for the path on the
inside of a nestloop, so we might as well get rid of useless parameterized
paths as quickly as possible.  But we didn't take that reasoning as far as
we should have.  Startup cost isn't a useful property inside a nestloop
either, so add_path() ought to discount startup cost of parameterized paths
as well.  Having done that, the secondary sorting I'd implemented (in
add_parameterized_path) is no longer needed --- any parameterized path that
survives add_path() at all is worth considering at higher levels.  So this
should be a bit faster as well as simpler.

1 files changed, 2 insertions, 1 deletions

diff --git a/src/backend/optimizer/plan/planmain.c b/src/backend/optimizer/plan/planmain.c
index 04acc006f53..c2488a4923b 100644
--- a/src/backend/optimizer/plan/planmain.c
+++ b/src/backend/optimizer/plan/planmain.c
@@ -267,7 +267,8 @@ query_planner(PlannerInfo *root, List *tlist,
 	 */
 	final_rel = make_one_rel(root, joinlist);
 
-	if (!final_rel || !final_rel->cheapest_total_path)
+	if (!final_rel || !final_rel->cheapest_total_path ||
+		final_rel->cheapest_total_path->param_info != NULL)
 		elog(ERROR, "failed to construct the join relation");
 
 	/*