Columns in tables that are joined should have identical attributes: the same column length, same data type (character, numeric), and so forth. Nonidentical attributes result in temporary indexes being built, even though indexes over corresponding columns may exist.
In the following example, join will build a temporary index and ignore an existing one:
SELECT EMPNO, LASTNAME, DEPTNAME
FROM TEMPL, TDEPT
WHERE TEMPL.DEPTNO = TDEPT.DEPTNO
