Check if table inherits from other table in PostgreSQL

There is a catalog table for that: pg_inherits.

The catalog pg_inherits records information about table inheritance hierarchies. There is one entry for each direct child table in the database. (Indirect inheritance can be determined by following chains of entries.)

Here's a query that fits your question:

SELECT EXISTS (   SELECT 1   FROM   pg_catalog.pg_inherits   WHERE  inhrelid = 'public.cities_capitals'::regclass   );

TRUE if table cities_capitals inherits from somewhere, else FALSE.
Schema-qualify the name to be sure.

The following statement will retrieve the tables that cities inherits from. If the table does not inherit from another table, the result will be empty:

select bt.relname as table_name, bns.nspname as table_schema from pg_class ct     join pg_namespace cns on ct.relnamespace = cns.oid and cns.nspname = 'public'     join pg_inherits i on i.inhrelid = ct.oid and ct.relname = 'cities '     join pg_class bt on i.inhparent = bt.oid     join pg_namespace bns on bt.relnamespace = bns.oid

From Postgresql AutoDoc I found this SQL:

SELECT parnsp.nspname AS par_schemaname    , parcla.relname AS par_tablename    , chlnsp.nspname AS chl_schemaname    , chlcla.relname AS chl_tablename FROM pg_catalog.pg_inherits JOIN pg_catalog.pg_class AS chlcla ON (chlcla.oid = inhrelid) JOIN pg_catalog.pg_namespace AS chlnsp ON (chlnsp.oid = chlcla.relnamespace) JOIN pg_catalog.pg_class AS parcla ON (parcla.oid = inhparent) JOIN pg_catalog.pg_namespace AS parnsp ON (parnsp.oid = parcla.relnamespace)

This is helpful as you can test in both directions with one query.