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

3 files changed, 19 insertions, 5 deletions

diff --git a/src/include/nodes/pathnodes.h b/src/include/nodes/pathnodes.h
index 14ccfc1ac1c..540d021592e 100644
--- a/src/include/nodes/pathnodes.h
+++ b/src/include/nodes/pathnodes.h
@@ -1658,6 +1658,7 @@ typedef struct Path
 
 	/* estimated size/costs for path (see costsize.c for more info) */
 	Cardinality rows;			/* estimated number of result tuples */
+	int			disabled_nodes; /* count of disabled nodes */
 	Cost		startup_cost;	/* cost expended before fetching any tuples */
 	Cost		total_cost;		/* total cost (assuming all tuples fetched) */
 
@@ -3333,6 +3334,7 @@ typedef struct
 typedef struct JoinCostWorkspace
 {
 	/* Preliminary cost estimates --- must not be larger than final ones! */
+	int			disabled_nodes;
 	Cost		startup_cost;	/* cost expended before fetching any tuples */
 	Cost		total_cost;		/* total cost (assuming all tuples fetched) */
 
diff --git a/src/include/optimizer/cost.h b/src/include/optimizer/cost.h
index 57861bfb446..854a782944a 100644
--- a/src/include/optimizer/cost.h
+++ b/src/include/optimizer/cost.h
@@ -108,35 +108,42 @@ extern void cost_resultscan(Path *path, PlannerInfo *root,
 							RelOptInfo *baserel, ParamPathInfo *param_info);
 extern void cost_recursive_union(Path *runion, Path *nrterm, Path *rterm);
 extern void cost_sort(Path *path, PlannerInfo *root,
-					  List *pathkeys, Cost input_cost, double tuples, int width,
+					  List *pathkeys, int disabled_nodes,
+					  Cost input_cost, double tuples, int width,
 					  Cost comparison_cost, int sort_mem,
 					  double limit_tuples);
 extern void cost_incremental_sort(Path *path,
 								  PlannerInfo *root, List *pathkeys, int presorted_keys,
+								  int input_disabled_nodes,
 								  Cost input_startup_cost, Cost input_total_cost,
 								  double input_tuples, int width, Cost comparison_cost, int sort_mem,
 								  double limit_tuples);
 extern void cost_append(AppendPath *apath);
 extern void cost_merge_append(Path *path, PlannerInfo *root,
 							  List *pathkeys, int n_streams,
+							  int input_disabled_nodes,
 							  Cost input_startup_cost, Cost input_total_cost,
 							  double tuples);
 extern void cost_material(Path *path,
+						  int input_disabled_nodes,
 						  Cost input_startup_cost, Cost input_total_cost,
 						  double tuples, int width);
 extern void cost_agg(Path *path, PlannerInfo *root,
 					 AggStrategy aggstrategy, const AggClauseCosts *aggcosts,
 					 int numGroupCols, double numGroups,
 					 List *quals,
+					 int input_disabled_nodes,
 					 Cost input_startup_cost, Cost input_total_cost,
 					 double input_tuples, double input_width);
 extern void cost_windowagg(Path *path, PlannerInfo *root,
 						   List *windowFuncs, WindowClause *winclause,
+						   int input_disabled_nodes,
 						   Cost input_startup_cost, Cost input_total_cost,
 						   double input_tuples);
 extern void cost_group(Path *path, PlannerInfo *root,
 					   int numGroupCols, double numGroups,
 					   List *quals,
+					   int input_disabled_nodes,
 					   Cost input_startup_cost, Cost input_total_cost,
 					   double input_tuples);
 extern void initial_cost_nestloop(PlannerInfo *root,
@@ -171,6 +178,7 @@ extern void cost_gather(GatherPath *path, PlannerInfo *root,
 						RelOptInfo *rel, ParamPathInfo *param_info, double *rows);
 extern void cost_gather_merge(GatherMergePath *path, PlannerInfo *root,
 							  RelOptInfo *rel, ParamPathInfo *param_info,
+							  int input_disabled_nodes,
 							  Cost input_startup_cost, Cost input_total_cost,
 							  double *rows);
 extern void cost_subplan(PlannerInfo *root, SubPlan *subplan, Plan *plan);
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,