mySQL performance problem on subqueries

[Falcon Meriva]

Hi,

I have table like this;

-----
C001 BIGINT(22) UNSIGNED ZEROFILL AUTO_INCREMENT INDEX
C002 BIGINT(22) UNSIGNED ZEROFILL INDEX
C003 BIGINT(22) UNSIGNED ZEROFILL INDEX
C004 BIGINT(22) UNSIGNED ZEROFILL INDEX
C005 BIGINT(22) UNSIGNED ZEROFILL INDEX
-----

I simply write a query like this,

-----
SELECT
T2.*,
(
SELECT
COUNT(*)-1 AS count
FROM
`myTable`
WHERE
C005 = 121 AND C003 BETWEEN T2.C003 AND T2.C004
) AS depth
FROM
`myTable` AS `T2`
WHERE
T2.C005 = 121
ORDER BY T2.C003;
-----

The query works fine for the table format. It sends the records as wanted. ( In this query it'll send me about 2377 rows )

But there was a big problem about the query. The problem is queries time. It took about 25 seconds to end!

I don't understand why subselecting using count values by fetched rows make this query's time long. In normal life counting a lots of rows takes about 0.00001 seconds.

Any idea?