In  SERVER-113732, we decided that when a query is not eligible for join optimization, but the join optimization flag is enabled, we should include "usedJoinOptimization":false in the explain output. This does not occur if the base collection uses an express plan.

For example

db.foo.explain().aggregate([{$match: {_id: 0}}, {$lookup: {from: "bar", localField: "a", foreignField: "a", as: "res" }}])

This query is not eligible for join optimization because it’s missing an $unwind, so when the join opt flag is enabled, we would expect to see "usedJoinOptimization":false in the explain output, but it is missing.