Add pg_partition_tree to display information about partitions

This new function is useful to display a full tree of partitions with a
partitioned table given in output, and avoids the need of any complex
WITH RECURSIVE query when looking at partition trees which are
deep multiple levels.

It returns a set of records, one for each partition, containing the
partition's name, its immediate parent's name, a boolean value telling
if the relation is a leaf in the tree and an integer telling its level
in the partition tree with given table considered as root, beginning at
zero for the root, and incrementing by one each time the scan goes one
level down.

Author: Amit Langote
Reviewed-by: Jesper Pedersen, Michael Paquier, Robert Haas
Discussion: https://postgr.es/m/8d00e51a-9a51-ad02-d53e-ba6bf50b2e52@lab.ntt.co.jp

doc/src/sgml/func.sgml

@@ -20216,6 +20216,49 @@ postgres=# SELECT * FROM pg_walfile_name_offset(pg_stop_backup());
     The function returns the number of new collation objects it created.
    </para>
 
+   <table id="functions-info-partition">
+    <title>Partitioning Information Functions</title>
+    <tgroup cols="3">
+     <thead>
+      <row><entry>Name</entry> <entry>Return Type</entry> <entry>Description</entry></row>
+     </thead>
+
+     <tbody> 
+      <row>
+       <entry><literal><function>pg_partition_tree(<type>regclass</type>)</function></literal></entry>
+       <entry><type>setof record</type></entry>
+       <entry>
+        List information about tables or indexes in a partition tree for a
+        given partitioned table or partitioned index, with one row for each
+        partition.  Information provided includes the name of the partition,
+        the name of its immediate parent, a boolean value telling if the
+        partition is a leaf, and an integer telling its level in the hierarchy.
+        The value of level begins at <literal>0</literal> for the input table
+        or index in its role as the root of the partition tree,
+        <literal>1</literal> for its partitions, <literal>2</literal> for
+        their partitions, and so on.
+       </entry>
+      </row>
+     </tbody>
+    </tgroup>
+   </table>
+
+   <para>
+    To check the total size of the data contained in
+    <structname>measurement</structname> table described in
+    <xref linkend="ddl-partitioning-declarative-example"/>, one could use the
+    following query:
+   </para>
+
+<programlisting>
+=# SELECT pg_size_pretty(sum(pg_relation_size(relid))) AS total_size
+     FROM pg_partition_tree('measurement');
+ total_size 
+------------
+ 24 kB
+(1 row)
+</programlisting>
+
   </sect2>
 
   <sect2 id="functions-admin-index">