Given a table MYTABLE in namespace MYNAMESPACE, you can use this SQL to retrieve the primary key column name(s) for the table:

SELECT
  attname::text
FROM
  pg_attribute
JOIN
  pg_class ON pg_attribute.attrelid = pg_class.oid
JOIN
  pg_namespace ON pg_namespace.oid = pg_class.relnamespace
LEFT JOIN
  pg_constraint ON conrelid = pg_class.oid AND pg_constraint.contype = 'p'
WHERE
  pg_namespace.nspname = 'MYNAMESPACE' AND
  pg_class.relname = 'MYTABLE' AND
  pg_attribute.attnum = ANY (pg_constraint.conkey)
ORDER BY
  pg_attribute.attnum;