title: mongodb: force in-memory
body: <p>After using a myisam for years now with 3 indexes + around 500 columns for Mio of rows, I wonder how to "force" mongodb to store indexes in memory for fast-read performance.</p>
  
  <p>In general, it is a simply structured table and all queries are WHERE index1=.. or index2=... or index3=.. (myisam) and pretty simple in mongodb as well.
  It's nice if mongodb is managing the index and ram on its own.
  However, I am not sure if it does and about the way mongodb can speed up these queries on indexs-only best.</p>
  
  <p>Thanks</p>