Strange behaviour with MySQL 5.1 TO_DAYS partitioning schema

Question

I have this table definition:

CREATE TABLE `va_ventas` (
`ID` INT(10) UNSIGNED NOT NULL AUTO_INCREMENT,
`Date` DATE NOT NULL,
`Time` TIME NULL DEFAULT NULL,
`Country` VARCHAR(3) NULL DEFAULT NULL,
`Type` TINYINT(3) UNSIGNED NOT NULL DEFAULT '0',
`Result` TINYINT(3) UNSIGNED NULL DEFAULT NULL,
`Carrier` TINYINT(3) UNSIGNED NULL DEFAULT NULL,
`WM_Alias` VARCHAR(75) NOT NULL,
`Product_Code` VARCHAR(75) NOT NULL,
`Platform_Code` VARCHAR(40) NOT NULL,
`AltaID` MEDIUMINT(8) UNSIGNED NULL DEFAULT NULL,
`ClienteID` MEDIUMINT(8) UNSIGNED NULL DEFAULT NULL,
`Commission_Code` VARCHAR(40) NULL DEFAULT NULL,
INDEX `Date_WM_Alias_Product_Code` (`Date`, `WM_Alias`, `Product_Code`),
INDEX `Date_WM_Alias` (`Date`, `WM_Alias`),
INDEX `Date_Carrier_WM_Alias` (`Date`, `Carrier`, `WM_Alias`),
INDEX `AltaID` (`AltaID`),
INDEX `ClienteID` (`ClienteID`),
INDEX `Country` (`Country`),
PRIMARY KEY (`ID`, `Date`)
)
COLLATE='utf8_general_ci' ENGINE=MyISAM;

With the next partition schema:

PARTITION BY RANGE(TO_DAYS(Date)) 
(PARTITION p_2012_01 VALUES LESS THAN (TO_DAYS('2012-01-01')),
PARTITION p_2012_02 VALUES LESS THAN (TO_DAYS('2012-02-01')),
PARTITION p_2012_03 VALUES LESS THAN (TO_DAYS('2012-03-01')),
PARTITION p_2012_04 VALUES LESS THAN (TO_DAYS('2012-04-01')),
PARTITION p_2012_05 VALUES LESS THAN (TO_DAYS('2012-05-01')),
PARTITION p_2012_06 VALUES LESS THAN (TO_DAYS('2012-06-01')),
PARTITION p_2012_07 VALUES LESS THAN (TO_DAYS('2012-07-01')),
PARTITION p_2012_08 VALUES LESS THAN (TO_DAYS('2012-08-01')),
PARTITION p_2012_09 VALUES LESS THAN (TO_DAYS('2012-09-01')),
PARTITION p_2012_10 VALUES LESS THAN (TO_DAYS('2012-10-01')),
PARTITION p_2012_11 VALUES LESS THAN (TO_DAYS('2012-11-01')),
PARTITION p_2012_12 VALUES LESS THAN (TO_DAYS('2012-12-01')),
PARTITION p_2013_01 VALUES LESS THAN (TO_DAYS('2013-01-01'))
);

I filled the table with 10.000 dummy data entries and when I use EXPLAIN in a sample query I get the next result:

mysql> EXPLAIN PARTITIONS SELECT COUNT(*) FROM va_ventas WHERE Date BETWEEN '2012-08-01' AND '2012-10-20'\G;
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: va_ventas
   partitions: p_2012_01,p_2012_09,p_2012_10,p_2012_11
         type: index
possible_keys: Date_WM_Alias_Product_Code,Date_WM_Alias,Date_Carrier_WM_Alias
          key: PRIMARY
      key_len: 7
          ref: NULL
         rows: 2591
        Extra: Using where; Using index

I don't know why it's using the p_2012_01 partition where it's not between the date specified in the BETWEEN statement. Also, shouldn't this select the partition p_2012_08 to match the statement? The data in the table is well distributed, so each date gets a similar amount of entries.

Thanks in advance.

EDIT

More information, I changed my partition schema to the next one, that makes more sense:

PARTITION BY RANGE(TO_DAYS(Date)) 
(PARTITION pNULL VALUES LESS THAN (0),
PARTITION p201201 VALUES LESS THAN (TO_DAYS('2012-02-01')),
PARTITION p201202 VALUES LESS THAN (TO_DAYS('2012-03-01')),
PARTITION p201203 VALUES LESS THAN (TO_DAYS('2012-04-01')),
PARTITION p201204 VALUES LESS THAN (TO_DAYS('2012-05-01')),
PARTITION p201205 VALUES LESS THAN (TO_DAYS('2012-06-01')),
PARTITION p201206 VALUES LESS THAN (TO_DAYS('2012-07-01')),
PARTITION p201207 VALUES LESS THAN (TO_DAYS('2012-08-01')),
PARTITION p201208 VALUES LESS THAN (TO_DAYS('2012-09-01')),
PARTITION p201209 VALUES LESS THAN (TO_DAYS('2012-10-01')),
PARTITION p201210 VALUES LESS THAN (TO_DAYS('2012-11-01')),
PARTITION p201211 VALUES LESS THAN (TO_DAYS('2012-12-01')),
PARTITION p201212 VALUES LESS THAN (TO_DAYS('2013-01-01')),
PARTITION p201301 VALUES LESS THAN (TO_DAYS('2013-02-01')),
PARTITION pMAX VALUES LESS THAN MAXVALUE
);

After filling, this is the status of the table:

SELECT PARTITION_NAME,TABLE_ROWS FROM 
INFORMATION_SCHEMA.PARTITIONS WHERE TABLE_NAME = 'va_ventas';

+----------------+------------+
| PARTITION_NAME | TABLE_ROWS |
+----------------+------------+
| pNULL          |          0 |
| p201201        |        846 |
| p201202        |        793 |
| p201203        |        847 |
| p201204        |        820 |
| p201205        |        847 |
| p201206        |        819 |
| p201207        |        847 |
| p201208        |        847 |
| p201209        |        820 |
| p201210        |        847 |
| p201211        |        820 |
| p201212        |        847 |
| p201301        |          0 |
| pMAX           |          0 |
+----------------+------------+
15 rows in set (0.11 sec)


EXPLAIN PARTITIONS SELECT SQL_NO_CACHE 
COUNT(*) FROM va_ventas WHERE Date >= '2012-05-01' AND Date IS NOT NULL\G;
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: va_ventas
   partitions: pNULL,p201205,p201206,p201207,p201208,p201209,p201210,p201211,p201212,p201301,pMAX
         type: index
possible_keys: PRIMARY,Date_WM_Alias
          key: PRIMARY
      key_len: 7
          ref: NULL
         rows: 6694
        Extra: Using where; Using index
1 row in set (0.00 sec)

It's still selecting the pNULL partition althought it's clearly that have 0 rows in it. Also, selection a date directly does not select the pNULL partition:

EXPLAIN PARTITIONS SELECT SQL_NO_CACHE COUNT(*) FROM va_ventas WHERE Date = '2012-05-01' AND Date IS NOT NULL\G;
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: va_ventas
   partitions: p201205

Any thoughts?

Answer 1

sorry, the comment of @phil aroused my curiosity so I decided to cheat and to read the manual:

18.2.7. How MySQL Partitioning Handles NULL

Partitioning in MySQL does nothing to disallow NULL as the value of a partitioning expression, whether it is a column value or the value of a user-supplied expression. Even though it is permitted to use NULL as the value of an expression that must otherwise yield an integer, it is important to keep in mind that NULL is not a number. MySQL's partitioning implementation treats NULL as being less than any non-NULL value, just as ORDER BY does.