Execute o comando SELECT abaixo para obter os nomes dos 
campos da chave-primaria de uma tabela do InterBase ou
FireBird.

SELECT RDB$FIELD_NAME
FROM
  RDB$RELATION_CONSTRAINTS C,
  RDB$INDEX_SEGMENTS S
WHERE
  C.RDB$RELATION_NAME = 'NomeDaTabela' AND
  C.RDB$CONSTRAINT_TYPE = 'PRIMARY KEY' AND
  S.RDB$INDEX_NAME = C.RDB$INDEX_NAME
ORDER BY RDB$FIELD_POSITION