   0.000 |
|   MSFT | 2000-02-01 | 36.35 |     39.81 |         -3.460 |
|   MSFT | 2000-03-01 | 43.22 |     36.35 |          6.870 |
|   MSFT | 2000-04-01 | 28.37 |     43.22 |         -14.85 |
|   MSFT | 2000-05-01 | 25.45 |     28.37 |         -2.920 |
</code></pre>
<p>All these operations are intrinsically lazy, so values are only calculated when
requested.  This is usually fine but in some cases it may be desired to force
the calculation of a particular column completely (like in the instance where
the 