fix for the performance problem of mysql memory tables

In our project, a very high database performance is needed. Thus we create our critical tables as memory engine tables but although our queries are using corresponding indices we still can't achieve the needed performance. After few research in google I see that tables of memory type are having "hash" indices by default. After we drop the "hash" indices and replace them with "b-tree" ones, we had a much better performance in our project. So for those of you with performance problems using memory tables, I recommend trying b-tree indices.