select from partitioned table

[janota.m]

Hi,
I have the following problem:
I have partitioned table
CREATE TABLE tdataa_idx1_part (
tm datetime NOT NULL,
address char(6) NOT NULL,
flag char(1),
val float,
PRIMARY KEY (address,tm),
INDEX tm_addr_idx (tm,address)
) engine=myisam
PARTITION BY RANGE ( unix_timestamp(tm) ) (
PARTITION p_2006031900 VALUES LESS THAN ( unix_timestamp('2006-03-19 00:00') ),
PARTITION p_2006031901 VALUES LESS THAN ( unix_timestamp('2006-03-19 01:00') ),
PARTITION p_2006031901 VALUES LESS THAN ( unix_timestamp('2006-03-19 02:00') ),
.
.
.
PARTITION p_2006032122 VALUES LESS THAN ( unix_timestamp('2006-03-21 22:00') ),
PARTITION p_2006032123 VALUES LESS THAN ( unix_timestamp('2006-03-21 23:00') ),
PARTITION p_2006032200 VALUES LESS THAN ( unix_timestamp('2006-03-22 00:00') )
);

I inserted 48 milion rows with times between 2006-03-18 12:00 and 2006-03-21 13:00

Then I created some selects and I got unexpected results

SELECT max(tm) from tdataa_idx1_part;
RESULT: 2006-03-21 12:49:25

SELECT max(tm) from tdataa_idx1_part where tm < '2006-03-22';
RESULT: 2006-03-18 22:59:58

SELECT tm from tdataa_idx1_part order by tm desc limit 1;
RESULT: 2006-03-21 12:49:25

SELECT tm from tdataa_idx1_part where tm < '2006-03-22' order by tm desc limit 1;
RESULT: 2006-03-18 22:59:58


SELECT address,tm from tdataa_idx1_part where address='A00004' order by tm;
RESULT 29378 rows
FIRST tm= 2006-03-18 15:56:34
LAST tm = 2006-03-21 12:49:25

SELECT address,tm from tdataa_idx1_part whree address='A00004' AND tm > '2006-03-20' order by tm;
RESULT 18722 rows

SELECT address,tm from tdataa_idx1_part whree address='A00004' AND tm > '2006-03-20' order by tm desc;
RESULT empty

SELECT address,tm from tdataa_idx1_part whree address='A00004' AND tm < '2006-03-20' order by tm;
RESULT 18722 rows

SELECT address,tm from tdataa_idx1_part whree address='A00004' AND tm < '2006-03-20' order by tm desc;
RESULT empty


May I use desc sorting on partitioned table? May I use group by on partitioned table?
Thanks for any sugestions.
Martin