Clear buffer cache

When you are working with performance issues you need to have an empty database cache to get a "real" result from your query. If you run a query which runs for 2 minutes the first time it would run very much faster the second time. To prevent the database to fetch the result from the cache you need to clear the buffer cache.

Run the command below to flush the cache (you need a DB-user which has sufficient privileges, for example system)

alter system flush buffer_cache;

Note: this should not be done in any production enviroment.