Add pg_partition_root to display top-most parent of a partition tree

This is useful when looking at partition trees with multiple layers, and
combined with pg_partition_tree, it provides the possibility to show up
an entire tree by just knowing one member at any level.

Author: Michael Paquier
Reviewed-by: Álvaro Herrera, Amit Langote
Discussion: https://postgr.es/m/20181207014015.GP2407@paquier.xyz

src/include/catalog/catversion.h

@@ -53,6 +53,6 @@
  */
 
 /*							yyyymmddN */
-#define CATALOG_VERSION_NO	201902071
+#define CATALOG_VERSION_NO	201902081
 
 #endif

src/include/catalog/pg_proc.dat

@@ -10509,4 +10509,9 @@
   proargnames => '{rootrelid,relid,parentrelid,isleaf,level}',
   prosrc => 'pg_partition_tree' },
 
+# function to get the top-most partition root parent
+{ oid => '3424', descr => 'get top-most partition root parent',
+  proname => 'pg_partition_root', prorettype => 'regclass',
+  proargtypes => 'regclass', prosrc => 'pg_partition_root' },
+
 ]