title: How to make an inner join efficient mysql
body: <p>I'm having two tables called 
  1. Table A - with multiple columns, having primary key as user_id (900,000+ records)
  2. Table B - with single column called user_id (900+ records)</p>
  
  <p>The table A contains all user_id 's of table B and more. I need to get a list of * columns of matching A,B rows by user_id. I'm using the below query, it has already run for 1 hour in my p4 machine with 100% CPU with no result in sight. </p>
  
  <pre><code>SELECT a.* FROM tableA AS a INNER JOIN tableB AS b ON a.user_id = b.user_id;
  </code></pre>
  
  <p>Is there a way to make the query execution fast?</p>