title: Is NULL cardinality in an index a problem - MySQL 5.x
body: <p>I'm having performance problems on the live version of a system which I can't reproduce locally. </p>
  
  <p>In comparing a few EXPLAIN results on my local copies of the database with the live one, I note that multi-field indexes aren't used in some places on the live but are locally, and further investigation shows that these indexes have a cardinality of NULL on the live. </p>
  
  <p>I'm guessing this is the problem, but what does NULL cardinality mean and will it cause an index not to be used? Will an Optimize fix this and is there a means of preventing it recurring? I don't have full access to the live MySQL database so Analyze and Optimize are outside my normal capabilities.</p>
  
  <p>Many thanks for any replies!</p>