// Figure out whether the $lookup with foreign field index is pushed down to the SBE as INLJ.
//
// This function assumes that  'pipeline' has a $lookup stage.
//
// 'checkSBEEnabled(..., featureFlag) does not work very well for a mixed version cluster because
// when it detects the first shard that the 'featureFlag' is turned on, it just returns true but
// in a mixed version cluster, some shard may not support such 'featureFlag'. So, need to check
// whether the $lookup is pushed down or not based on 'explain' result.
var isLookupPushedDownToSBEAsINLJ = function(coll, pipeline) {
    const explainDoc =
        db.runCommand({aggregate: coll.getName(), pipeline: pipeline, explain: true});
    // If $lookup is pushed down to the SBE, 'explainVersion' must be "2".
    if (explainDoc.explainVersion != "2") {
        return false;
    }
    const eqLookupStage = getPlanStage(explainDoc, "EQ_LOOKUP");
    // If $lookup is pushed down to the SBE, EQ_LOOKUP stage must exist.
    if (!eqLookupStage) {
        return false;
    }
    return eqLookupStage.strategy == "IndexedLoopJoin";
};

if (!isLookupPushedDownToSBEAsINLJ(col, pipeline)) {
    assert.eq(2,
              getUsageCount("_id_", foreignCollection),
              "Expected each lookup to be tracked as an index use");
} else {
    assert.eq(1,
              getUsageCount("_id_", foreignCollection),
              "Expected the index join lookup to be tracked as a single index use");
}