Add amgettreeheight index AM API routine

The only current implementation is for btree where it calls
_bt_getrootheight().  Other index types can now also use this to pass
information to their amcostestimate routine.  Previously, btree was
hardcoded and other index types could not hook into the optimizer at
this point.

Author: Mark Dilger <mark.dilger@enterprisedb.com>
Discussion: https://www.postgresql.org/message-id/flat/E72EAA49-354D-4C2E-8EB9-255197F55330@enterprisedb.com

src/include/access/amapi.h

@@ -140,6 +140,13 @@ typedef void (*amcostestimate_function) (struct PlannerInfo *root,
 										 double *indexCorrelation,
 										 double *indexPages);
 
+/* estimate height of a tree-structured index
+ *
+ * XXX This just computes a value that is later used by amcostestimate.  This
+ * API could be expanded to support passing more values if the need arises.
+ */
+typedef int (*amgettreeheight_function) (Relation rel);
+
 /* parse index reloptions */
 typedef bytea *(*amoptions_function) (Datum reloptions,
 									  bool validate);
@@ -272,6 +279,7 @@ typedef struct IndexAmRoutine
 	amvacuumcleanup_function amvacuumcleanup;
 	amcanreturn_function amcanreturn;	/* can be NULL */
 	amcostestimate_function amcostestimate;
+	amgettreeheight_function amgettreeheight;	/* can be NULL */
 	amoptions_function amoptions;
 	amproperty_function amproperty; /* can be NULL */
 	ambuildphasename_function ambuildphasename; /* can be NULL */

src/include/access/nbtree.h

@@ -1186,6 +1186,7 @@ extern IndexBulkDeleteResult *btbulkdelete(IndexVacuumInfo *info,
 extern IndexBulkDeleteResult *btvacuumcleanup(IndexVacuumInfo *info,
 											  IndexBulkDeleteResult *stats);
 extern bool btcanreturn(Relation index, int attno);
+extern int	btgettreeheight(Relation rel);
 
 /*
  * prototypes for internal functions in nbtree.c