Extract the multiplier for CPU process cost of index page into a macro

B-tree, GiST and SP-GiST all charge 50.0 * cpu_operator_cost for processing
an index page.  Extract this to a macro to avoid repeated magic numbers.

Discussion: https://mail.google.com/mail/u/0/?ik=a20b091faa&view=om&permmsgid=msg-f%3A1751459697261369543
Author: Ronan Dunklau

src/backend/utils/adt/selfuncs.c

@@ -140,6 +140,7 @@
 #include "utils/timestamp.h"
 #include "utils/typcache.h"
 
+#define DEFAULT_PAGE_CPU_MULTIPLIER 50.0
 
 /* Hooks for plugins to get control when we ask for stats */
 get_relation_stats_hook_type get_relation_stats_hook = NULL;
@@ -6865,7 +6866,7 @@ btcostestimate(PlannerInfo *root, IndexPath *path, double loop_count,
 	 * touched.  The number of such pages is btree tree height plus one (ie,
 	 * we charge for the leaf page too).  As above, charge once per SA scan.
 	 */
-	descentCost = (index->tree_height + 1) * 50.0 * cpu_operator_cost;
+	descentCost = (index->tree_height + 1) * DEFAULT_PAGE_CPU_MULTIPLIER * cpu_operator_cost;
 	costs.indexStartupCost += descentCost;
 	costs.indexTotalCost += costs.num_sa_scans * descentCost;
 
@@ -7060,7 +7061,7 @@ gistcostestimate(PlannerInfo *root, IndexPath *path, double loop_count,
 	/*
 	 * Likewise add a per-page charge, calculated the same as for btrees.
 	 */
-	descentCost = (index->tree_height + 1) * 50.0 * cpu_operator_cost;
+	descentCost = (index->tree_height + 1) * DEFAULT_PAGE_CPU_MULTIPLIER * cpu_operator_cost;
 	costs.indexStartupCost += descentCost;
 	costs.indexTotalCost += costs.num_sa_scans * descentCost;
 
@@ -7115,7 +7116,7 @@ spgcostestimate(PlannerInfo *root, IndexPath *path, double loop_count,
 	/*
 	 * Likewise add a per-page charge, calculated the same as for btrees.
 	 */
-	descentCost = (index->tree_height + 1) * 50.0 * cpu_operator_cost;
+	descentCost = (index->tree_height + 1) * DEFAULT_PAGE_CPU_MULTIPLIER * cpu_operator_cost;
 	costs.indexStartupCost += descentCost;
 	costs.indexTotalCost += costs.num_sa_scans * descentCost;