SELECT S.RDB$FIELD_NAME
  FROM RDB$INDICES I
  LEFT JOIN RDB$INDEX_SEGMENTS S
    ON I.RDB$INDEX_NAME = S.RDB$INDEX_NAME
  LEFT JOIN RDB$RELATION_CONSTRAINTS C
    ON C.RDB$INDEX_NAME = I.RDB$INDEX_NAME
 WHERE C.RDB$CONSTRAINT_TYPE = 'PRIMARY KEY'
   AND C.RDB$RELATION_NAME = 'VENDA'