partition tree inspection functions

Hi.

As discussed a little while back [1] and also recently mentioned [2], here
is a patch that adds a set of functions to inspect the details of a
partition tree.  There are three functions:

pg_partition_parent(regclass) returns regclass
pg_partition_root_parent(regclass) returns regclass
pg_partition_tree_tables(regclass) returns setof regclass

Here is an example showing how one may want to use them.

create table p (a int, b int) partition by range (a);
create table p0 partition of p for values from (minvalue) to (0) partition
by hash (b);
create table p00 partition of p0 for values with (modulus 2, remainder 0);
create table p01 partition of p0 for values with (modulus 2, remainder 1);
create table p1 partition of p for values from (0) to (maxvalue) partition
by hash (b);
create table p10 partition of p1 for values with (modulus 2, remainder 0);
create table p11 partition of p1 for values with (modulus 2, remainder 1);
insert into p select i, i from generate_series(-5, 5) i;

select pg_partition_parent('p0') as parent;
 parent
--------
 p
(1 row)

Time: 1.469 ms
select pg_partition_parent('p01') as parent;
 parent
--------
 p0
(1 row)

Time: 1.330 ms
select pg_partition_root_parent('p01') as root_parent;
 root_parent
-------------
 p
(1 row)

select    p as relname,
          pg_partition_parent(p) as parent,
          pg_partition_root_parent(p) as root_parent
from      pg_partition_tree_tables('p') p;
 relname | parent | root_parent
---------+--------+-------------
 p       |        | p
 p0      | p      | p
 p1      | p      | p
 p00     | p0     | p
 p01     | p0     | p
 p10     | p1     | p
 p11     | p1     | p
(7 rows)

select    p as relname,
          pg_partition_parent(p) as parent,
          pg_partition_root_parent(p) as root_parent,
          pg_relation_size(p) as size
from      pg_partition_tree_tables('p') p;
 relname | parent | root_parent | size
---------+--------+-------------+------
 p       |        | p           |    0
 p0      | p      | p           |    0
 p1      | p      | p           |    0
 p00     | p0     | p           | 8192
 p01     | p0     | p           | 8192
 p10     | p1     | p           | 8192
 p11     | p1     | p           | 8192
(7 rows)


select    sum(pg_relation_size(p)) as total_size
from      pg_partition_tree_tables('p') p;
 total_size
-------------
       32768
(1 row)

Feedback is welcome!

Thanks,
Amit

[1]
https://www.postgresql.org/message-id/flat/495cec7e-f8d9-7e13-4807-90dbf4eec4ea%40lab.ntt.co.jp

[2]
https://www.postgresql.org/message-id/18e000e8-9bcc-1bb5-2f50-56d434c8be1f%40lab.ntt.co.jp