Usage of db_keep_cache_size

I want to test the usage of db_keep_cache_size.

I executed a query and wrote down the time it needed to execute.

Then I issued.

alter system set db_keep_cache_size=100m scope=both;

alter table schema.table storage( buffer_pool keep)

Then I executed the query again and noticed no difference in execution time.

Now I want to make sure that the table is on the keep pool. How can I make sure that the table's data are stored in the keep_pool?

Our database version is 11.2.0.4

Thanks