[SOLVED] Why doesn't LOCK TABLES [table] WRITE prevent table reads?

Why doesn't LOCK TABLES [table] WRITE prevent table reads?

According to http://dev.mysql.com/doc/refman/5.0/en/lock-tables.html if I lock a table for writing in mysql, no-one else should have access until it's unlocked. I wrote this script, loaded either as script.php or script.php?l=1 depending on what you want to do:

if ($_GET['l'])
{
    mysqli_query("LOCK TABLES mytable WRITE");
    sleep(10);
    mysqli_query("UNLOCK TABLES");
}
else
{
    $res=mysqli_query("SELECT * FROM mytable");
    // Print Result
}

If I load script.php?l=1 in one browser window then, while it's sleeping, I should be able to load script.php in another window and it should wait until script.php?l=1 is finished, right?

Thing is, script.php loads right away, even though script.php?l=1 has a write lock. If I try to insert in script.php then it does wait, but why is the SELECT allowed?

Note: I am not looking for a discussion on whether to use LOCK TABLES or not. In fact I am probably going to go with a transaction, I am investigating that now, right now I just want to understand why the above doesn't work.

Solution

This happens because of query caching. There is a cache result available that doesn't 'affect' the lock, so the results are returned.

This can be avoided by adding the "SQL_NO_CACHE" keyword to the select:

SELECT SQL_NO_CACHE * FROM mytable