Treat number of disabled nodes in a path as a separate cost metric.

Previously, when a path type was disabled by e.g. enable_seqscan=false,
we either avoided generating that path type in the first place, or
more commonly, we added a large constant, called disable_cost, to the
estimated startup cost of that path. This latter approach can distort
planning. For instance, an extremely expensive non-disabled path
could seem to be worse than a disabled path, especially if the full
cost of that path node need not be paid (e.g. due to a Limit).
Or, as in the regression test whose expected output changes with this
commit, the addition of disable_cost can make two paths that would
normally be distinguishible in cost seem to have fuzzily the same cost.

To fix that, we now count the number of disabled path nodes and
consider that a high-order component of both the startup cost and the
total cost. Hence, the path list is now sorted by disabled_nodes and
then by total_cost, instead of just by the latter, and likewise for
the partial path list.  It is important that this number is a count
and not simply a Boolean; else, as soon as we're unable to respect
disabled path types in all portions of the path, we stop trying to
avoid them where we can.

Because the path list is now sorted by the number of disabled nodes,
the join prechecks must compute the count of disabled nodes during
the initial cost phase instead of postponing it to final cost time.

Counts of disabled nodes do not cross subquery levels; at present,
there is no reason for them to do so, since the we do not postpone
path selection across subquery boundaries (see make_subplan).

Reviewed by Andres Freund, Heikki Linnakangas, and David Rowley.

Discussion: http://postgr.es/m/CA+TgmoZ_+MS+o6NeGK2xyBv-xM+w1AfFVuHE4f_aq6ekHv7YSQ@mail.gmail.com

1 files changed, 8 insertions, 4 deletions

diff --git a/src/include/optimizer/pathnode.h b/src/include/optimizer/pathnode.h
index f00bd55f393..1035e6560c1 100644
--- a/src/include/optimizer/pathnode.h
+++ b/src/include/optimizer/pathnode.h
@@ -27,11 +27,12 @@ extern int	compare_fractional_path_costs(Path *path1, Path *path2,
 										  double fraction);
 extern void set_cheapest(RelOptInfo *parent_rel);
 extern void add_path(RelOptInfo *parent_rel, Path *new_path);
-extern bool add_path_precheck(RelOptInfo *parent_rel,
+extern bool add_path_precheck(RelOptInfo *parent_rel, int disabled_nodes,
 							  Cost startup_cost, Cost total_cost,
 							  List *pathkeys, Relids required_outer);
 extern void add_partial_path(RelOptInfo *parent_rel, Path *new_path);
 extern bool add_partial_path_precheck(RelOptInfo *parent_rel,
+									  int disabled_nodes,
 									  Cost total_cost, List *pathkeys);
 
 extern Path *create_seqscan_path(PlannerInfo *root, RelOptInfo *rel,
@@ -124,7 +125,8 @@ extern Path *create_worktablescan_path(PlannerInfo *root, RelOptInfo *rel,
 									   Relids required_outer);
 extern ForeignPath *create_foreignscan_path(PlannerInfo *root, RelOptInfo *rel,
 											PathTarget *target,
-											double rows, Cost startup_cost, Cost total_cost,
+											double rows, int disabled_nodes,
+											Cost startup_cost, Cost total_cost,
 											List *pathkeys,
 											Relids required_outer,
 											Path *fdw_outerpath,
@@ -132,7 +134,8 @@ extern ForeignPath *create_foreignscan_path(PlannerInfo *root, RelOptInfo *rel,
 											List *fdw_private);
 extern ForeignPath *create_foreign_join_path(PlannerInfo *root, RelOptInfo *rel,
 											 PathTarget *target,
-											 double rows, Cost startup_cost, Cost total_cost,
+											 double rows, int disabled_nodes,
+											 Cost startup_cost, Cost total_cost,
 											 List *pathkeys,
 											 Relids required_outer,
 											 Path *fdw_outerpath,
@@ -140,7 +143,8 @@ extern ForeignPath *create_foreign_join_path(PlannerInfo *root, RelOptInfo *rel,
 											 List *fdw_private);
 extern ForeignPath *create_foreign_upper_path(PlannerInfo *root, RelOptInfo *rel,
 											  PathTarget *target,
-											  double rows, Cost startup_cost, Cost total_cost,
+											  double rows, int disabled_nodes,
+											  Cost startup_cost, Cost total_cost,
 											  List *pathkeys,
 											  Path *fdw_outerpath,
 											  List *fdw_restrictinfo,