Optimization - Skip processing non-existing fields in FieldsWillMerge

[francisbeaudoin]

Context

It was observed that if a query includes thousands of non-existing fields, it takes a lot of processing time within the FieldsWillMerge#find_conflicts_within.

What

The iteration loop represents n(n-1^2)/2, where n is the number of duplicated fields being queried, and the underlying call to #find_conflict is using considerable time. For example, a query with 2000 times the same non-existing field i.e.query { a a ... 2000 times } takes ~6000 ms to process and skipping these fields reduces the response time to ~500ms.

Optimization

As far as I can tell, skipping non-existing fields from the FieldsWillMerge rule doesn't alter the business logic.

The only caveat of this optimization, is that if a service using this library was relying on the Schema#validate_timeout to safeguard from a potentially malicious query, it may no longer reach the configured timeout but rather instead return multiple undefinedField errors, which can be mitigated by the Schema#validate_max_errors.