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Strange lock on simple query
Posted by: Frank Grunwald
Date: February 22, 2005 12:46PM

I experienced some kind of lock on a very simple query :
SELECT id_user FROM Session WHERE id_session='xxxxxx';
The Session table only has a few fields and id_session is the primary key and has about 15000 records.

I found this disturbing information in the slow-query log :
# User@Host: xxx[xxx] @ [xxx.xxx.xxx.xxx]
# Query_time: 100 Lock_time: 0 Rows_sent: 0 Rows_examined: 0
select id_user FROM session WHERE id_session='xxxxxxxxxx';

This is the context :
x Bi - 3.0 GHZ 64 bit CPU
x 6 Go RAM
x RedHat
x MyIsam table (1.5 Go of Key_Buffer)

This server is performing about 2K to 3K queries per second (and was on a quite heavy load when this thing happened)
The only strange thing I noticed is the ratio key_reads / key_read_requests : about 92% on Cache hits (usually it is about 99%)

At the same time, a lot lot of query get stuck (they lengthed for about 80 sec) but they were a bit more complicated than this one ...

In fact it looks like everything freezed for a few seconds .. I've already seen that kind of things a few times before but that's the first time it is so bad

If anyone got an idea (even a clue) thanks for your help
(Let me tell I won't go to sleep before understanding this ;))

Frank

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