Joe Celko's SQL for Smarties, 3rd Edition

33.8. Avoid Expressions on Indexed Columns

If a column appears in a mathematical or string expression, then the optimizer cannot use its indexes. For example, given a table of tasks and their start and finish dates, to find the tasks that took three days to complete in 1994 we could write:

SELECT task_nbr
  FROM Tasks
 WHERE (finish_date - start_date) = INTERVAL '3' DAY
   AND start_date >= CAST ('2005-01-01' AS DATE);

But since most of the reports deal with the finish dates, we have an index on that column. This means that the query will run faster if it is rewritten as:

SELECT task_nbr
  FROM Tasks
 WHERE finish_date = (start_date + INTERVAL '3' DAY)
  AND start_date >= ('2005-01-01' AS DATE);

This same principle applies to columns in string functions ...