For the AppMod initiative, we are enabling more MQL syntax in SBE. We would like to make sure this newly enabled MQL syntax is well-tested by appending to the QueryTester corpus. Currently there are some gaps in how QueryTester covers certain MQL operators, as documented here.

Decisions like adding a limit to every $lookup, or not allowing stages after $lookup were made because randomly generated queries can have large result sets or reach the max BSON size. If we are careful about what documents to include in the collections or handwrite queries, we can expand QueryTester to cover SBE-AppMod queries more robustly.

A combined approach of handwriting queries and randomly generating them might be best.