title: Indexing a column having duplicate values
body: <p>Consider these tables:</p>
  
  <pre><code>seed (seedid, seedname) # PK-(seedid) 
  stock (seedid, stk, storeid)  # PK-(seedid,storeid), FK-(storeid)
  #InnoDB
  
  stock: 
        seedid, stk, storeid
           1    12     81
           2    13     81
           3    14     81
  
           1    12     82
           2    11     82
           3    13     82
           4    12     82
  
  Query -&gt; select stk from stock where seedid = 'aaa' and storeid = 'yyy'.
  </code></pre>
  
  <p>Table <code>stock</code> represent stocks of several stores, hence <code>storeid</code> will be repeated over.</p>
  
  <p>How do I index, table <code>stock</code>, given that it will be queried frequently using <code>storeid</code> ?</p>
  
  <p>Primary keys as automatically indexed, and since <code>(seedid,storeid)</code> is already a Primary key, is there no need to index it further?</p>