Calculate the computation used for Complex Query in Oracle

I am working on a sample database schema consisting of big fact table and relatively small dimension tables.  While querying this database, the results generated necessarily represent the optimal approach for this kind of data warehouse in real productive environments; and such a design would be driven more by business requirements than by the star itself but the data being fetched from big tables generally increase the computation. In order to optimize and improvise I need to check the compute details for the complex queries. What are the ways to do that using ORACLE RDBMS ?