It would be nice if there was a way to optionally bypass over a lookup stage based on a value in the document.

Imagine pipeline of

A,
lookup B,
unwind B,
lookup C

Where the relationship is many-to-zero A:B, zero-to-many B:C

However, if there is no B records found, there is no point to do lookup C. In my scenario, the problem is that because there is no B, then the lookup C attempts to do a match on a null value, and the lookup C collection index is sparse. So the lookup is extremely slow due to no index.

Could be avoided if there was a way to bypass the lookup based on the current document value.

Let me know if this is unclear and I can draw up an example.

Another simpler example might just be A lookup B, where A.value is null, and you don't want to bother doing a lookup against B since A.value is null.