In classic runtime planner for SBE when running a trial period for cached plan, we track the amount of documents that are returned by find part of the query, not the whole pipeline.

Consider the pipeline: 

[{$match: ..}, {$lookup: ..}, {$unwind: ..}]

When first we multi-plan the query, we will run only $match part.

After that we will extend the plan with the pipeline and run it.

In SBE plan cache, we will only have the whole plan.

When recovering a plan from cache, we want to run a trial period to make sure plan is still effective enough. So we set a read budget and run the plan until it returns enough documents.

We can't just look at the amount of documents, returned by the whole plan, as $unwind may affect it randomly.

So we track via TrialRunTracker the amount of documents, returned by the part of the plan that correspond to $match stage only.

However, we can't run only a part of the plan, so we run the whole plan. 

And when the whole plan returns a document, we need to stash it.

However, $lookup + $unwind can inflate the amount of memory needed for this stash.

Currently we just cap the stash at 101 * max document size.

We want to come up with more memory efficient way to do with.