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