title: innodb data structure
body: <p>I believe I understand how the INNODB structures the table (by using clustered btree indexes=PK and the leafs containing the rows themselves). Secondary indexes using the same principle (btree clustered index=secondary index) and the leaf contains the PK that is used as a pointer(that is the reason why second index lookup may be need).</p>
  
  <p><a href="http://www.chenyajun.com/wp-content/uploads/2008/12/3-9.jpg" rel="nofollow">http://www.chenyajun.com/wp-content/uploads/2008/12/3-9.jpg</a>
  so sorting is based on the index in INNODB.</p>
  
  <p>But I really cannot understand how the clustering btree index principle is used to sort and store physically the covering/composite indexes in INNODB.</p>