I want little bit clarification regarding opaque view and materialized view in OBIEE physical layer
-> what is the main difference between opaque view and materialized view in the RPD point of view we have to create this in RPD level or DB level.
An opaque view is a SQL statement defined in the physical layer, that OBIEE will use whole in building a SQL query. In a way it's no different from a database view, except it is defined in OBIEE only. Optionally, you can deploy an Opaque view, which pushes its definition into the database. Read more about it here: Working with Physical Tables, Cubes, and Joins - 11g Release 1 (11.1.1)
A materialized view (MV) is a database concept where the results of a SQL statement are physicalised as if a table, and so as far as OBIEE is concerned a MV is just another physical table
-> If i want to list out the opaque view's and materialized view's in the RPD how i have to do.
To find opaque views, use the Admin Tool "Query Repository" function. Search for Physical Tables, and set up a filter on Type = Select
To find materialized views you'll need to query your database's catalog (eg select object_name, object_type from user_objects where object_type = 'MATERIALIZED VIEW';) and compare it to the list of tables in your RPD
-> how an opaque view will degrade the performance and how an materialized view will improve the performance in report level.
A materialized view is materialized, that is, the results are already stored by the database. That generally means it will perform better because all the joins and any aggregations etc will have been resolved in advance.