Manage dimensions in BTable

Hi all,
I need to change dimensions at runtime of a BTable.
In pre-execution I was able to replace in the array of dimensions the dimension I need doing

var btObj = this.getBTable();
if(btObj.hasOwnProperty( "query" )){
btObj.query.getDimensions()[0]=dimension;
//btObj.query.synchronizeFiltersWithParameters()
//btObj.query.reset();
}

The problem is that underlyng query isn't refreshed with this values. Do you have any idea on how to accomplish this task?

Thank you

regards