[6.0] Check whether $lookup pushdown feature availability on primary shard in mixed version cluster

// 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");

}