At the time the SP is compiled (command is prepared and plan is created)
the values of its parameters are not known to engine, i.e. optimizer does not know
whether SP will be called with 'GLAX%' paramter or '%GLAX%'.
For this reason it can't use index.

Index can be used only for
LIKE 'GLAX%'
but not
LIKE '%GLAX%'
nor
LIKE :param

Is using STARTING WITH an option for you ?

Ivan
http://www.volny.cz/iprenosil/interbase