title: Where clause on tag keys in influxdb does not work
body: <p>I see some strange behavior in 0.9.6.1. The issue is when i query a field without a where clause, it works but when i add "WHERE" in the statement for a tag key, it gives me empty results.</p>
  
  <p>for ex, </p>
  
  <pre><code>select successful, merchant_id from session_metrics_new limit 5
  
  name: session_metrics_new
  
  time                    successful      merchant_id
  1453975732000000000     1               bms
  1453975733000000000     1               snp
  1453975735000000000     1               bms
  1453975735000000000     1               snp
  1453975739000000000     1               bms
  </code></pre>
  
  <p>but this does not work</p>
  
  <pre><code>select successful, merchant_id from session_metrics_new where merchant_id =~ /bms/ limit 5
  </code></pre>
  
  <p>Here, successful is a field key while merchant_id is a tag key. I do not know if this a bug or the way i have stored data. Please help</p>