Question

Suppose I have a table ‘Tasks’ with a DATETIME column approve_time. I have an index on said column.

If I were to write a query of the form:

SELECT task_id, task_desc, task_owner, approve_time 
  FROM Tasks
 WHERE DATE(approve_time) = '2011-08-31'

My question is about the performance of such a query:

Does MYSQL index DATETIME columns in a way that allows constraining by the date component to be fast?

Or does MYSQL know how to optimize the query into something like the following?

WHERE approve_time >= '2011-08-31 00:00:00'
  AND approve_time <  '2011-09-01 00:00:00'

Or does the query incur a tablescan?

Answer 1

Does MYSQL index DATETIME columns in a way that allows constraining by the date component to be fast?

NO

Or does MYSQL know how to optimize the query into something like the following?

YES

---

the second query will lead to range filter,
try

explain extended query_1; <--- number of rows sent for scan is more, 
                               which should be aLL rows

vs

explain extended query_2;

the value of DATE(approve_time) only can determined after the function applied to column approve_time in all the row, which mean there is not going to make use on index