title: which hash function to choose for indexing uuid string on mysql? md5，crc64 or fnv
body: <p>I have a column store the uuid string. I add a new column to store its integer(64bits) hash value for indexing. which hash function to choose?</p>
  
  <pre><code>1. int(md5('a306d9cb-4d75-4673-ae43-700470692521').hexdigest()[-16:], 16)
  2. crc64('a306d9cb-4d75-4673-ae43-700470692521')
  3. fnv.hash('a306d9cb-4d75-4673-ae43-700470692521', bits=64)
  </code></pre>
  
  <p>it`s a mapping table for third party account.</p>
  
  <p>table example:</p>
  
  <pre><code>openid: o00HW1KP16EvXs5whqPH2qyx667s
  appid: wxb25520ae9512cabb
  uid: 135435123
  </code></pre>
  
  <p>(openid, appid) --> uid</p>
  
  <p>used to create a compound index on (openid, appid).
  Can make it faster if add a integer column like hash(openid, appid)?</p>