Drop cheap-startup-cost paths during add_path() if we don't need them.

We can detect whether the planner top level is going to care at all about
cheap startup cost (it will only do so if query_planner's tuple_fraction
argument is greater than zero).  If it isn't, we might as well discard
paths immediately whose only advantage over others is cheap startup cost.
This turns out to get rid of quite a lot of paths in complex queries ---
I saw planner runtime reduction of more than a third on one large query.

Since add_path isn't currently passed the PlannerInfo "root", the easiest
way to tell it whether to do this was to add a bool flag to RelOptInfo.
That's a bit redundant, since all relations in a given query level will
have the same setting.  But in the future it's possible that we'd refine
the control decision to work on a per-relation basis, so this seems like
a good arrangement anyway.

Per my suggestion of a few months ago.

1 files changed, 19 insertions, 6 deletions

diff --git a/src/backend/optimizer/util/pathnode.c b/src/backend/optimizer/util/pathnode.c
index 8677ff20fd5..d827b4959b6 100644
--- a/src/backend/optimizer/util/pathnode.c
+++ b/src/backend/optimizer/util/pathnode.c
@@ -139,6 +139,9 @@ compare_fractional_path_costs(Path *path1, Path *path2,
  * total cost, we just say that their costs are "different", since neither
  * dominates the other across the whole performance spectrum.
  *
+ * If consider_startup is false, then we don't care about keeping paths with
+ * good startup cost, so we'll never return COSTS_DIFFERENT.
+ *
  * This function also includes special hacks to support a policy enforced
  * by its sole caller, add_path(): paths that have any parameterization
  * cannot win comparisons on the grounds of having cheaper startup cost,
@@ -146,7 +149,8 @@ compare_fractional_path_costs(Path *path1, Path *path2,
  * (Unparameterized paths are more common, so we check for this case last.)
  */
 static PathCostComparison
-compare_path_costs_fuzzily(Path *path1, Path *path2, double fuzz_factor)
+compare_path_costs_fuzzily(Path *path1, Path *path2, double fuzz_factor,
+						   bool consider_startup)
 {
 	/*
 	 * Check total cost first since it's more likely to be different; many
@@ -155,7 +159,8 @@ compare_path_costs_fuzzily(Path *path1, Path *path2, double fuzz_factor)
 	if (path1->total_cost > path2->total_cost * fuzz_factor)
 	{
 		/* path1 fuzzily worse on total cost */
-		if (path2->startup_cost > path1->startup_cost * fuzz_factor &&
+		if (consider_startup &&
+			path2->startup_cost > path1->startup_cost * fuzz_factor &&
 			path1->param_info == NULL)
 		{
 			/* ... but path2 fuzzily worse on startup, so DIFFERENT */
@@ -167,7 +172,8 @@ compare_path_costs_fuzzily(Path *path1, Path *path2, double fuzz_factor)
 	if (path2->total_cost > path1->total_cost * fuzz_factor)
 	{
 		/* path2 fuzzily worse on total cost */
-		if (path1->startup_cost > path2->startup_cost * fuzz_factor &&
+		if (consider_startup &&
+			path1->startup_cost > path2->startup_cost * fuzz_factor &&
 			path2->param_info == NULL)
 		{
 			/* ... but path1 fuzzily worse on startup, so DIFFERENT */
@@ -177,6 +183,7 @@ compare_path_costs_fuzzily(Path *path1, Path *path2, double fuzz_factor)
 		return COSTS_BETTER1;
 	}
 	/* fuzzily the same on total cost */
+	/* (so we may as well compare startup cost, even if !consider_startup) */
 	if (path1->startup_cost > path2->startup_cost * fuzz_factor &&
 		path2->param_info == NULL)
 	{
@@ -360,6 +367,9 @@ set_cheapest(RelOptInfo *parent_rel)
  *	  reduce the number of parameterized paths that are kept.  See discussion
  *	  in src/backend/optimizer/README.
  *
+ *	  Another policy that is enforced here is that we only consider cheap
+ *	  startup cost to be interesting if parent_rel->consider_startup is true.
+ *
  *	  The pathlist is kept sorted by total_cost, with cheaper paths
  *	  at the front.  Within this routine, that's simply a speed hack:
  *	  doing it that way makes it more likely that we will reject an inferior
@@ -423,7 +433,8 @@ add_path(RelOptInfo *parent_rel, Path *new_path)
 		 * Do a fuzzy cost comparison with 1% fuzziness limit.	(XXX does this
 		 * percentage need to be user-configurable?)
 		 */
-		costcmp = compare_path_costs_fuzzily(new_path, old_path, 1.01);
+		costcmp = compare_path_costs_fuzzily(new_path, old_path, 1.01,
+											 parent_rel->consider_startup);
 
 		/*
 		 * If the two paths compare differently for startup and total cost,
@@ -488,8 +499,10 @@ add_path(RelOptInfo *parent_rel, Path *new_path)
 									remove_old = true;	/* new dominates old */
 								else if (new_path->rows > old_path->rows)
 									accept_new = false; /* old dominates new */
-								else if (compare_path_costs_fuzzily(new_path, old_path,
-											  1.0000000001) == COSTS_BETTER1)
+								else if (compare_path_costs_fuzzily(new_path,
+																	old_path,
+																	1.0000000001,
+																	parent_rel->consider_startup) == COSTS_BETTER1)
 									remove_old = true;	/* new dominates old */
 								else
 									accept_new = false; /* old equals or