Account better for planning cost when choosing whether to use custom plans.

The previous coding in plancache.c essentially used 10% of the estimated
runtime as its cost estimate for planning.  This can be pretty bogus,
especially when the estimated runtime is very small, such as in a simple
expression plan created by plpgsql, or a simple INSERT ... VALUES.

While we don't have a really good handle on how planning time compares
to runtime, it seems reasonable to use an estimate based on the number of
relations referenced in the query, with a rather large multiplier.  This
patch uses 1000 * cpu_operator_cost * (nrelations + 1), so that even a
trivial query will be charged 1000 * cpu_operator_cost for planning.
This should address the problem reported by Marc Cousin and others that
9.2 and up prefer custom plans in cases where the planning time greatly
exceeds what can be saved.

src/backend/utils/cache/plancache.c

@@ -54,6 +54,7 @@
 #include "executor/executor.h"
 #include "executor/spi.h"
 #include "nodes/nodeFuncs.h"
+#include "optimizer/cost.h"
 #include "optimizer/planmain.h"
 #include "optimizer/prep.h"
 #include "parser/analyze.h"
@@ -91,7 +92,7 @@ static CachedPlan *BuildCachedPlan(CachedPlanSource *plansource, List *qlist,
 				ParamListInfo boundParams);
 static bool choose_custom_plan(CachedPlanSource *plansource,
 				   ParamListInfo boundParams);
-static double cached_plan_cost(CachedPlan *plan);
+static double cached_plan_cost(CachedPlan *plan, bool include_planner);
 static void AcquireExecutorLocks(List *stmt_list, bool acquire);
 static void AcquirePlannerLocks(List *stmt_list, bool acquire);
 static void ScanQueryForLocks(Query *parsetree, bool acquire);
@@ -998,15 +999,16 @@ choose_custom_plan(CachedPlanSource *plansource, ParamListInfo boundParams)
 	avg_custom_cost = plansource->total_custom_cost / plansource->num_custom_plans;
 
 	/*
-	 * Prefer generic plan if it's less than 10% more expensive than average
-	 * custom plan.  This threshold is a bit arbitrary; it'd be better if we
-	 * had some means of comparing planning time to the estimated runtime cost
-	 * differential.
+	 * Prefer generic plan if it's less expensive than the average custom
+	 * plan.  (Because we include a charge for cost of planning in the
+	 * custom-plan costs, this means the generic plan only has to be less
+	 * expensive than the execution cost plus replan cost of the custom
+	 * plans.)
 	 *
 	 * Note that if generic_cost is -1 (indicating we've not yet determined
 	 * the generic plan cost), we'll always prefer generic at this point.
 	 */
-	if (plansource->generic_cost < avg_custom_cost * 1.1)
+	if (plansource->generic_cost < avg_custom_cost)
 		return false;
 
 	return true;
@@ -1014,9 +1016,13 @@ choose_custom_plan(CachedPlanSource *plansource, ParamListInfo boundParams)
 
 /*
  * cached_plan_cost: calculate estimated cost of a plan
+ *
+ * If include_planner is true, also include the estimated cost of constructing
+ * the plan.  (We must factor that into the cost of using a custom plan, but
+ * we don't count it for a generic plan.)
  */
 static double
-cached_plan_cost(CachedPlan *plan)
+cached_plan_cost(CachedPlan *plan, bool include_planner)
 {
 	double		result = 0;
 	ListCell   *lc;
@@ -1029,6 +1035,34 @@ cached_plan_cost(CachedPlan *plan)
 			continue;			/* Ignore utility statements */
 
 		result += plannedstmt->planTree->total_cost;
+
+		if (include_planner)
+		{
+			/*
+			 * Currently we use a very crude estimate of planning effort based
+			 * on the number of relations in the finished plan's rangetable.
+			 * Join planning effort actually scales much worse than linearly
+			 * in the number of relations --- but only until the join collapse
+			 * limits kick in.	Also, while inheritance child relations surely
+			 * add to planning effort, they don't make the join situation
+			 * worse.  So the actual shape of the planning cost curve versus
+			 * number of relations isn't all that obvious.  It will take
+			 * considerable work to arrive at a less crude estimate, and for
+			 * now it's not clear that's worth doing.
+			 *
+			 * The other big difficulty here is that we don't have any very
+			 * good model of how planning cost compares to execution costs.
+			 * The current multiplier of 1000 * cpu_operator_cost is probably
+			 * on the low side, but we'll try this for awhile before making a
+			 * more aggressive correction.
+			 *
+			 * If we ever do write a more complicated estimator, it should
+			 * probably live in src/backend/optimizer/ not here.
+			 */
+			int			nrelations = list_length(plannedstmt->rtable);
+
+			result += 1000.0 * cpu_operator_cost * (nrelations + 1);
+		}
 	}
 
 	return result;
@@ -1104,7 +1138,7 @@ GetCachedPlan(CachedPlanSource *plansource, ParamListInfo boundParams,
 								MemoryContextGetParent(plansource->context));
 			}
 			/* Update generic_cost whenever we make a new generic plan */
-			plansource->generic_cost = cached_plan_cost(plan);
+			plansource->generic_cost = cached_plan_cost(plan, false);
 
 			/*
 			 * If, based on the now-known value of generic_cost, we'd not have
@@ -1133,7 +1167,7 @@ GetCachedPlan(CachedPlanSource *plansource, ParamListInfo boundParams,
 		/* Accumulate total costs of custom plans, but 'ware overflow */
 		if (plansource->num_custom_plans < INT_MAX)
 		{
-			plansource->total_custom_cost += cached_plan_cost(plan);
+			plansource->total_custom_cost += cached_plan_cost(plan, true);
 			plansource->num_custom_plans++;
 		}
 	}