Descobrir chave primária de uma tabela

SELECT              
  pg_attribute.attname,
  format_type(pg_attribute.atttypid, pg_attribute.atttypmod)
FROM pg_index, pg_class, pg_attribute
WHERE
  pg_class.oid = 'nome_da_tabela'::regclass AND
  indrelid = pg_class.oid AND
  pg_attribute.attrelid = pg_class.oid AND
  pg_attribute.attnum = any(pg_index.indkey)
  AND indisprimary