表的分区关系存储在pg_inherits中，其定义如下：

                               Table "pg_catalog.pg_inherits"
  Column   |  Type   | Collation | Nullable | Default | Storage | Stats target | Description
-----------+---------+-----------+----------+---------+---------+--------------+-------------
 inhrelid  | oid     |           | not null |         | plain   |              | 子表的 OID
 inhparent | oid     |           | not null |         | plain   |              | 父表的 OID
 inhseqno  | integer |           | not null |         | plain   |              | 直接继承多个父表时指定被继承column 的顺序
Indexes:
    "pg_inherits_relid_seqno_index" UNIQUE, btree (inhrelid, inhseqno)
    "pg_inherits_parent_index" btree (inhparent)

inhrelid和inhparent都是 pg_class 表中的隐藏列 oid。
那么查询 parent_table_name 的分区表名字的SQL如下：

select
	c.relname
from
	pg_class c
	join pg_inherits i on i.inhrelid = c. oid
	join pg_class d on d.oid = i.inhparent
where
	d.relname = 'parent_table_name';

或者使用下面的SQL：

SELECT
	relname
from
	pg_class
where
	oid in (
		SELECT inhrelid FROM pg_inherits WHERE inhparent in (
			SELECT oid FROM pg_class WHERE relname = 'parent_table_name'
			)
		)