title: Speed of IN keyword in MySQL/PostgreSQL
body: <p>I've heard lots of people saying that the <code>IN</code> keyword in most relational databases is slow. How true is this? An example query would be this, off the top of my head:</p>
  
  <pre><code>SELECT * FROM someTable WHERE someColumn IN (value1, value2, value3)
  </code></pre>
  
  <p>I've heard that is much slower than doing this:</p>
  
  <pre><code>SELECT * FROM someTable WHERE
    someColumn = value1 OR
    someColumn = value2 OR
    someColumn = value3
  </code></pre>
  
  <p>Is this true? Or is the speed difference negligible? If it matters, I'm using PostgreSQL, but I'd also like to know how MySQL fares (and if it's any different). Thanks in advance.</p>