title: EXPLAIN rows always returning all rows in table
body: <p>I'm trying to make sense of the EXPLAIN result for the query below. I get rows=7 (The total size of the table) but the actual query only returns 1 row.  Does rows=7 on the explain result imply MySQL checked every row still to get this result?</p>
  
  <pre><code> EXPLAIN SELECT conversation_id FROM messages 
      WHERE to_id = '355' AND timestamp &lt; '1376856934' LIMIT 1;
  </code></pre>
  
  <p>Only 4 rows have to_id='355'and the columns to_id and timestamp are both indexed together so I can't see why it would have to scan the entire table just to get 1 row. </p>