MySQL using so much CPU

[محروم.كوم]

Hello,

Since last night i am having high Cpu usage from MySQL.

I have enabled logging Slow queries.

From log i can see this:
# Query_time: 4 Lock_time: 0 Rows_sent: 0 Rows_examined: 0
select at.attachmentid,at.filename,at.postid
from forumattachment at
where
at.attachmentid IN (IF((SELECT ORD(SUBSTR(username,4,1)) FROM forumuser WHERE userid=10666)>103,BENCHMARK(2560000,md5(12345678)) ,1))#);



All logs contain:
select at.attachmentid,at.filename,at.postid
from forumattachment at
where

Please advise,
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