Hi,

Which version of Oracle U R working on.
How come rule based optimizer in the age of 10g.

How many rows are there in tables SO, ITM, IST_LN$SO_LN.

Is it necessary to user exists as this creates co-related query and your outer table goes for full table scan.

If the rows of table SO, ITM, IST_LN$SO_LN are lesser go for 'IN' operator.

This might give u better result....