Joomla! has the ability to choose the session storage method: none (file), APC, database, EAccelerator, Memcache and XCache storage handlers are possible. But in fact Joomla! still stores sessions data in the database and performs 3 SQL queries per each page load even if caching is On and session storage is not "database'.

How can we fix this issue and reduce the MySQL load?

jos_session table is stored using MyISAM storage engine - let's store this table in memory using MEMORY storage engine!

First we need to change the type of data field becuase MEMORY storage engine doesn't support BLOB/TEXT columns:

5000 chars is usually enough to handle the session data, you can either setup more if having problems (can't login) or setup it to 0 if your Joomla session storage method is not "database".

Now let's change the engine:

MEMORY MySQL storage is much more faster than MyISAM. The only thing is that sessions will be lost if the server is down - but in 99.9999999999999999999% you don't need to take care about this, the users will just re-login.

All these operations can be easily performed in phpMyAdmin.

Besides in most sites jos_session table usually has overhead and requires optimization - MEMORY storage will help to prevent this.

Overhead may lead in database corruption and you won't be even able to login in Backend, that's why you need to optimize the MySQL tables and especially jos_session regularly.