title: Why does MySQL Innodb "Creating sort index" when unique index exists?
body: <p>On a simple but very large Innodb table, I have a unique index on column A and I want to get a list of (integer) column B in order of (integer) column A</p>
  
  <p>Very simple query, I am paging through millions of records.</p>
  
  <p><code>SELECT B FROM hugeTable ORDER BY A LIMIT 10000 OFFSET 500000</code></p>
  
  <p>This takes 10 seconds per query on a very fast server?</p>
  
  <p><code>Filesort: Yes   Filesort_on_disk: Yes    Merge_passes: 9</code></p>
  
  <p>This makes no sense to me, why can it not use Index A ?</p>
  
  <p>Explain shows simple, no possible keys and filesort.</p>