title: nls_session_parameters vs nls_instance_parameters vs nls_database_parameters
body: <p>Can somebody please explain clearly the difference between values in these tables. (mentioned in title)</p>
  
  <p>I do understand that:</p>
  
  <p><code>nls_session_parameters</code>: sets values at session level (do these overide instance and database level)</p>
  
  <p><code>nls_instance_parameters</code>: sets values at instance level (what do we mean by instance level)</p>
  
  <p><code>nls_database_parameters</code>: sets values at whole database level.</p>
  
  <p>Now if the database is of type RAC can multiple SID's have different values in above tables?</p>
  
  <p>I am currently facing a problem where '<code>NLS_NUMERIC_CHARACTERS</code>' gets set to wrong value in many sessions, and I cannot alter session every time. the behavior is random and very confusing</p>