Good point, Simon! Thanks!
Just found exactly what you said in SAP KBA 2112252 : Why 2 different query plans can be generated by ASE depending on the Statement Cache is used or not.
My parameters are currently configured that way:
Parameter Name Default Memory Used Config Value Run Value Unit Type
------------------------------ ----------- ----------- ------------ ------------ -------------------- --------------------
optimization timeout limit 10 0 10 10 percent dynamic
sproc optimize timeout limit 40 0 80 80 percent dynamic
the non-default "sproc optimize timeout limit" is recommended by our software vendor.
Thanks for your time. Vincent