title: How to decide when use index on table column
body: <p>When should I use index on table?</p>
  
  <ol>
  <li>From how many rows index makes sense?</li>
  <li>If I have table with constant rows, just edited come columns (not in 'where' clause), makes index sense even if table has just about 15 rows? EDIT: Can be in such case non-index selecting/reading more effective than index read?</li>
  </ol>
  
  <p>EDIT:
  Now I am working with firebird 2.5, but most of the time I am using SQL Server 2005/2008. </p>