title: mysql join is very slow
body: <p>I have two tables (MYISAM)</p>
  
  <pre><code>create table A (email varchar(50));
  create table B( email varchar(50) key 'email' (email));
  </code></pre>
  
  <p>Table A has 130K records
  Table B has 20K records</p>
  
  <p>why does this sql statement take very long time (more than two minutes, then i aborted query by Ctrl+C)
  Statement is: </p>
  
  <pre><code>select count(*) from user A, tmp B where A.email=B.email;
  </code></pre>
  
  <p>Thanks</p>