Slow Performance

Thanks for the suggestions, the query could certain do with some optimisations. Can you provide any benchmarks?

Additionally this will allow for fields on the webform_submission table to also be queried

This is something I had hoped to support at some point. I'll need to review the patch in more detail.

I'm looking into this now, specifically querying the webform_submission table.

I chose to use query() over select() for performance reasons. See Introduction to Dynamic Queries:

Note: in 90% of select query use cases you will have a static query. If in a critical performance path, you should use query() instead of select() for performance reasons.

There are a few unrelated changes which make this patch harder to review. Doc block additions and changes to naming conventions mean we're not comparing like for like. I'll need to see if I can extract the bare minimum to including querying the webform_submission table.

1. Retained current execute() method returning the same result type for compatibility
2. Added method processQuery() to return a database statement. This can be used to change the result type. E.g. $query->processQuery()->fetchCol();
3. Added "table" parameter to addCondition() to allow arbitrary tables which have a "sid" column to be queried
4. Stuck with pure SQL for performance
5. Now queries webform_submission and other arbitrary non-EAV tables first for performance
6. All methods are still public at the moment, this could potentially be changed in a separate issue
7. Camel case isn't used internally, this could be changed in a separate issue.
8. Typos not fixed.

The last 3 changes could potentially be handled within the same general tidying up issue.

Example:

$query = \Drupal::service('webform_query');

$query->addCondition('field_name', 'value')    
  ->setWebform('my_webform')
  ->addCondition('uid', 1, '=', 'webform_submission');

$results = $query->processQuery()->fetchCol();

Committed anyway. Try the latest dev.

Let me know if there are any issues.