what are the drawbacks by using join index table? - response (1) by Raja_KT

You could have read the doc :)
 

Join indexes can be expensive to maintain. 

Each time a join-indexed base table column is updated or deleted, the corresponding join index table column must also be updated or deleted. Each time a new row is added to a join-indexed base table, the corresponding join index table columns must also be inserted as a new join index subtable row.