If you start to experiment with cstore_fdw extension you may need this small query to show you details about data files which cstore_fdw creates.

with pgdir as (select setting as _dir from pg_settings where name = 'data_directory'),
dir as (select setting||'/cstore_fdw/'||(select oid from pg_database where datname= current_database())||'/'as _dir from pg_settingsĀ  where name like 'data_directory'),
objects as (select ftrelid, _dir||ftrelid as _file, ftrelid::regclass::text as tablename, ftoptions from pg_foreign_table, dir ),
pgfiles as (select ftrelid, tablename as relname, 'foreign table'::text as relkind, _file,
string_to_array(replace(replace(pg_stat_file(_file)::text,'(',''),')',''),',') as _detail
from objects
where _file is not null)
ftrelid, relname, relkind,
_detail[1]::bigint as _size,
_detail[2]::timestamp as _last_accessed,
_detail[3]::timestamp as _last_modified,
_detail[4]::timestamp as _last_file_status_change_unix_only,
_detail[5]::text as _file_creation_windows_only
from pgfiles
order by ftrelid

If will show you for every foreign table OID , tablename, file location and details – presumes you have foreign tables based only on cstore_fdw.