I have an doubt about design

I have a history table:

CREATE TABLE HISTORY(

CHANNEL INT NOT NUL DEFAULT 0,

HISTORY_DATE DECIMAL(22,6) NOT NULL DEFAULT '0' COMMENT 'UNIX TIME',

VALUE DECIMAL (22,6) NOT NULL DEFAULT 0,

PRIMARY KEY (CHANNEL,HISTORY_DATE)

)

my question is...

i need a history of only 50000 rows for each channel

what's the best method to allow a good history without many deletes, analyse tables, and others stuffs to allow a good table/disk usage?

i was considering a counter, for example, after 1000 inserts, run a

SELECT COUNT(*) FROM HISTORY WHERE CHANNEL={channel_id}

get this number of rows, if it's > 50000 run:

DELETE FROM HISTORY WHERE CHANNEL={channel_id} ORDER BY HISTORY_DATE ASC  LIMIT (number_of_rows - 50000)

any other idea?

maybe a 'batch' operation?

i was think about something like (don't work, just a idea)

DELETE FROM HISTORY WHERE 

CHANNEL IN (channel numbers)

AND xxxxx? LIMIT 50000 GROUP BY CHANNEL

i don't know anything to group by channel id and delete rows using only one sql command , any idea?
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