How does one create an index on the date part of DATETIME field in MySql

Question

How do I create an index on the date part of DATETIME field?

mysql> SHOW COLUMNS FROM transactionlist; +-------------------+------------------+------+-----+---------+----------------+ | Field             | Type             | Null | Key | Default | Extra          | +-------------------+------------------+------+-----+---------+----------------+ | TransactionNumber | int(10) unsigned | NO   | PRI | NULL    | auto_increment | | WagerId           | int(11)          | YES  | MUL | 0       |                | | TranNum           | int(11)          | YES  | MUL | 0       |                | | TranDateTime      | datetime         | NO   |     | NULL    |                | | Amount            | double           | YES  |     | 0       |                | | Action            | smallint(6)      | YES  |     | 0       |                | | Uid               | int(11)          | YES  |     | 1       |                | | AuthId            | int(11)          | YES  |     | 1       |                | +-------------------+------------------+------+-----+---------+----------------+ 8 rows in set (0.00 sec) 

TranDateTime is used to save the date and time of a transaction as it happens

My Table has over 1,000,000 records in it and the statement

SELECT * FROM transactionlist where date(TranDateTime) = '2008-08-17'  

takes a long time.

EDIT:

Have a look at this blog post on "Why MySQL’s DATETIME can and should be avoided"

Answer 1

If I remember correctly, that will run a whole table scan because you're passing the column through a function. MySQL will obediently run the function for each and every column, bypassing the index since the query optimizer can't really know the results of the function.

What I would do is something like:

SELECT * FROM transactionlist  WHERE TranDateTime BETWEEN '2008-08-17' AND '2008-08-17 23:59:59.999999'; 

That should give you everything that happened on 2008-08-17.