Indexing

Hello all wizards, Could someone tell me how Informix pick and choose which index to use if I have a table with 2 indexes as following: table t_a ( f_1 integer, f_2 integer, f_3 integer, ..... ) index i_1(f_1, f_2) index i_2(f_3) and the where clause is something like this: where f_1 = 10 and f_3 = 15. In other words, what kind of information that Informix based on to determine which index to use when a table have two separate indexes and both are used in the where clause? thanx in advance, Duc CSC