Joomla is setting the MySQL query_cache_type server system variable when Site Debug is set to Yes. However, this server system variable is deprecated in MySQL 5.7 and removed as of MySQL 8.0.

When using Joomla with the PDOMySQL driver and the Site Debug feature is enabled and an extension tries to close the system database connection (which forces it to reconnect) you get an uncatchable Fatal Error:

Fatal error: Uncaught PDOException: SQLSTATE[HY000]: General error: 1193 Unknown system variable 'query_cache_type' in /.../libraries/joomla/database/driver/pdomysql.php on line 160

Wrapping the code that recycles the MySQL connection in a try-catch doesn't work, even if the caught exception type is Throwable.

The only solution is to fix Joomla's database driver.

On Joomla 3 this is found in the following code locations:

• joomla-cms/libraries/joomla/database/driver/mysql.php

  Lines 100 to 108 in 4172b68

  	if ($this->debug)
  	{
  	mysql_query('SET query_cache_type = 0;', $this->connection);
  	if ($this->hasProfiling())
  	{
  	mysql_query('SET profiling_history_size = 100, profiling = 1;', $this->connection);
  	}
  	}

• joomla-cms/libraries/joomla/database/driver/mysqli.php

  Lines 192 to 200 in 4172b68

  	if ($this->debug)
  	{
  	mysqli_query($this->connection, 'SET query_cache_type = 0;');
  	if ($this->hasProfiling())
  	{
  	mysqli_query($this->connection, 'SET profiling_history_size = 100, profiling = 1;');
  	}
  	}

• joomla-cms/libraries/joomla/database/driver/pdomysql.php

  Lines 158 to 166 in 4172b68

  	if ($this->debug)
  	{
  	$this->connection->query('SET query_cache_type = 0;');
  	if ($this->hasProfiling())
  	{
  	$this->connection->query('SET profiling_history_size = 100, profiling = 1;');
  	}
  	}

This has already been fixed on Joomla 4.

I consider this a very high priority issue since there is no viable workaround in third party code (as I said, try/catch fails to catch the exceptions thrown by PDO). Not closing the connection is not an alternative, e.g. when we need to share the PDOMySQL resource with a third party database driver. Sharing the internal resource is desirable instead of using the connection information from JConfig to open a new connection because opening yet another MySQL connection from the same PHP process to the same server is disallowed on many live hosts.

I know that probably nobody will care enough to fix it. I am filing this issue just to have public proof that the Joomla project was made aware of the issue, its nature and its solution.