After you create a join index, its use is determined by the criteria of the SELECT statement.
If a join index exists that joins the tables in the FROM clause by the relationship specified in the WHERE clause, or if a join index exists that is based on ANSI join syntax for natural or key joins, the join index is used to speed up queries. Otherwise, ad hoc joins between indexes on the individual tables are performed at query time. If there is a join index for a subset of tables in the SELECT, Sybase IQ uses it to speed up the resulting ad hoc join.