Transaction isolation level REPEATABLE-READ The recommended level for Drupal is "READ COMMITTED". (Drupal Status Report)

Problem (Drupal Status Report) "/admin/reports/status":

Transaction isolation level
REPEATABLE-READ
The recommended level for Drupal is "READ COMMITTED". 
See the setting MySQL transaction isolation level page for more information.

Solution (in sites/default/settings.php):

$databases['default']['default'] = array(
  'database' => 'mydb',
  'username' => 'mydbu',
  'password' => 'mydbpasswd',
  'host' => 'localhost',
  'driver' => 'mysql',
  'prefix' => '',
  'port' => '3306',
  'init_commands' => [
    'isolation_level' => 'SET SESSION transaction_isolation=\'READ-COMMITTED\'',
  ],
);

Sometimes in this form:

$databases['default']['default'] = array (
  ...
  'driver' => 'mysql',
    'init_commands' => [
        'isolation_level' => 'SET SESSION TRANSACTION ISOLATION LEVEL READ COMMITTED',
    ],
);