Definition of prefixInfo array structure.
Rather than redefining DatabaseSchema::getPrefixInfo() for each driver, by defining the defaultSchema variable only MySQL has to re-write the method.
DatabaseSchema::getPrefixInfo()
protected $defaultSchema = 'public';
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https://api.drupal.org/api/drupal/core!lib!Drupal!Core!Database!Schema.php/property/Schema::defaultSchema/8.1.x