How do I run queries against a view configuration Where is all the config for Views stored? I am working on a Drupal site with a very large number of Views. What's the easiest way to find "All the Views which have a Relation of a certain type", or "All the Views which output a specific field?"

It would be nice to do this with some SQL but I'm stumped as to how.
Just expanding a bit on @Clive's answer. Load all views, iterate through them and check what you need. 

Views has a function that will help, views_get_all_views(), https://cgit.drupalcode.org/views/tree/views.module#n1536

From there, you can go through them and get what you need. 

Basically,


  $views_list = views_get_all_views();
  foreach ($views_list  as $view) {
    //Logic operating on the $view object
  }


This will probably be useful as well, https://api.drupal.org/api/views/includes!view.inc/class/view/7.x-3.x