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  1. Core Server
  2. SERVER-67377

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

    • Type: Icon: Bug Bug
    • Resolution: Fixed
    • Priority: Icon: Major - P3 Major - P3
    • 6.0.1
    • Affects Version/s: None
    • Component/s: None
    • Labels:
    • Fully Compatible
    • ALL
    • v6.0
    • QE 2022-06-27, QE 2022-07-11, QE 2022-07-25, QE 2022-08-08
    • 10

      index_stats.js is failing on sharding_jscore_passthrough_last_continuous and sharding_jscore_passthrough_last_lts test suites because of unreliable feature detection logic of checkSBEEnabled().

      Instead of relying on checkSBEEnabled(), better to detect feature availability based on explain result for index_stats.js.

      Proposed changes:

      // 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)) {
                    getUsageCount("_id_", foreignCollection),
                    "Expected each lookup to be tracked as an index use");
      } else {
                    getUsageCount("_id_", foreignCollection),
                    "Expected the index join lookup to be tracked as a single index use");

            rui.liu@mongodb.com Rui Liu
            yoonsoo.kim@mongodb.com Yoon Soo Kim
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