What if the partitions aren't all rationally named? There must be a pg_* table out there which contains the partition boundaries...
The pg_class column relpartbound contains an internal representation of the partition boundary, when applicable.
You can decompile it into the canonical text format with pg_get_expr( expr pg_node_tree, relation oid [, pretty boolean ] ) → text.
So:
create table t(x int primary key) partition by list(x);
create table u partition of t for values in (0,1);
create table v partition of t for values in (2,3,4,5,6,7,8,9);
select oid::regclass,pg_get_expr(relpartbound,oid) from pg_class where relkind='r' and relispartition;
oid | pg_get_expr
-----+----------------------------------------
u | FOR VALUES IN (0, 1)
v | FOR VALUES IN (2, 3, 4, 5, 6, 7, 8, 9)
(2 rows)
