i need to overwrite main query in maintain session. Can someone tell me where should i write the new query and in which event so that it dosen't go to main query??

You can try the
before.program:
query.extension = "QUERY"

More details are needed. On a maintain session in baan 4 it usually built off the main table listed on the session. What exactly are you trying to accomplish, that might change the answer or suggested solutions? For me it would just be easier to build a new maintain session for a different query using a different table. As royspk mentioned you can use query.extension to filter the records on the main table.