title: PostgreSQL Extract() DOW : why not Date datatype in addition to timestamp?
body: <p>According to the PostgreSQL version 8.1 date-time function <a href="http://www.postgresql.org/docs/8.1/static/functions-datetime.html" rel="nofollow noreferrer">docs</a>:</p>
  <blockquote>
  <p>dow
  The day of the week (0 - 6; Sunday is 0) (<em>for timestamp values only</em>)</p>
  <pre><code>SELECT EXTRACT(DOW FROM TIMESTAMP '2001-02-16 20:38:40');
  Result: 5
  </code></pre>
  </blockquote>
  <p>Why is not a <strong>Date</strong> datatype also a valid argument for this function?  If the sequence of the days of the week does not change by locale:</p>
  <pre><code>  0 - 6; Sunday is 0 
  </code></pre>
  <p>why would the time-component of a combined date-type value be needed to determine the ordinal of the day in the week? Wouldn't the date-chunk alone be sufficient?</p>