merge performance problem

[Thiago Dias Fascin]

I'm having a problem when using two merge table in a join, the select became very slow.

Tables a_1, a_2... has each one something like 1 million records.
Tables b_1, b_2... has each one something like five hundred records.

This select is taking 300 seconds:
SELECT
SQL_NO_CACHE count( * )
FROM
temp.a_merge AS a
JOIN temp.b_merge AS b ON b.uniqueid = a.uniqueid
WHERE
a.calldate >= '2007-11-20'
AND a.calldate <= '2007-12-03 23:59:59' ;



Table Structure:

CREATE TABLE `a_1` (
`uniqueID` varchar(32) NOT NULL,
`date_added` datetime NOT NULL,
PRIMARY KEY (`uniqueID`),
KEY `date_added` (`date_added`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1;

CREATE TABLE `a_2` (
`uniqueID` varchar(32) NOT NULL,
`date_added` datetime NOT NULL,
PRIMARY KEY (`uniqueID`),
KEY `date_added` (`date_added`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1;

CREATE TABLE `a_3`...
CREATE TABLE `a_4`...
CREATE TABLE `a_5`...
CREATE TABLE `a_6`...

CREATE TABLE `a_merge` (
`uniqueID` varchar(32) NOT NULL,
`date_added` datetime NOT NULL,
PRIMARY KEY (`uniqueID`),
KEY `date_added` (`date_added`)
) ENGINE=MRG_MyISAM DEFAULT CHARSET=latin1 INSERT_METHOD=FIRST UNION=(`a_6`,`a_5`,`a_4`,`a_3`,`a_2`,`a_1`);

CREATE TABLE `b_1` (
`uniqueid` varchar(32) NOT NULL,
PRIMARY KEY (`uniqueid`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1;

CREATE TABLE `b_2` (
`uniqueid` varchar(32) NOT NULL,
PRIMARY KEY (`uniqueid`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1;

/*Table structure for table `b_merge` */


CREATE TABLE `b_3`...
CREATE TABLE `b_4`...
CREATE TABLE `b_5`...
CREATE TABLE `b_6`...

CREATE TABLE `b_merge` (
`uniqueid` varchar(32) NOT NULL,
PRIMARY KEY (`uniqueid`)
) ENGINE=MRG_MyISAM DEFAULT CHARSET=latin1 INSERT_METHOD=FIRST UNION=(`b_6`,`b_5`,`b_4`,`b_3`,`b_2`,`b_1`);

Somebody can help me?
Tks.