Clarity required around use of indexes in uncorrelated sub-query using $lookup

The MongoDB docs at https://www.mongodb.com/docs/manual/reference/operator/aggregation/lookup/#perform-multiple-joins-and-a-correlated-subquery-with--lookup make the following statement (for 6.0):

  • Indexes are not used for comparisons with more than one field path operand.

We have the following $lookup query in our pipeline:

{
    $lookup: {
      from: "metadata",
      let: {
        a_local: "$a",
        b_local: "$b",
      },
      pipeline: [
        {
          $match: {
            $expr: {
              $and: [
                { $eq: ["$aEqv", "$$a_local"] },
                { $eq: ["$bEqv", "$$b_local"] },
              ],
            },
          },
        },
      ],
      as: "metadata",
    },
  },

along with a compound index,

{"metadata.bEqv": 1, "metadata.aEqv": 1, metadata.fieldOfInterest: 1}

This lookup will be used over a massive number of Documents so it must use an index. In this particular case it is not strictly critical that it uses the complete compound index - just the metadata.bEqv part of it would be ok as that has high cardinality, though if it could cover the whole query using the complete compound index that would be ideal.

Unfortunately, the documentation does not provide any examples of more than 1 field path, so it is unclear whether an index will be used in the above query. Enlightenment would be much appreciated.