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  1. Core Server
  2. SERVER-76102

handleRIDRangeScan() sets hasCompatibleCollation wrong in recursive cases

    • Type: Icon: Bug Bug
    • Resolution: Fixed
    • Priority: Icon: Major - P3 Major - P3
    • 7.1.0-rc0, 7.0.0-rc1, 6.3.2, 6.0.7
    • Affects Version/s: 6.0.0-rc0, 6.0.6
    • Component/s: None
    • Labels:
    • Query Optimization
    • Fully Compatible
    • ALL
    • v7.0, v6.3, v6.0
    • QO 2023-05-01

      Found by code inspection.

      planner_access.h handleRIDRangeScan() calls itself recursively on the branches of AndMatchExpressions, but in setting the output value of collScan->hasCompatibleCollation it neglects to consider the recursions, so the value from whichever recursion happened last is the final value.

      The recursions should each have a chance to conjunctively falsify it, i.e. updates should be something like

      collScan->hasCompatibleCollation =
          collScan->hasCompatibleCollation && allEltsCollationCompatible;

      but instead the first occurrence of this updates it incorrectly (introduced in 7.0.0-rc0 by SERVER-67105 commit https://github.com/mongodb/mongo/commit/9f391ea93fa62f57d6b4576130283d98eec69456 and subsequently backported to 6.0.6):

      collScan->hasCompatibleCollation = allEltsCollationCompatible;

      and toward the bottom of the function (which is not always reached) there is another incorrect update (introduced in 6.0.0-rc0 by SERVER-60824 commit https://github.com/10gen/mongo/commit/8e4d331e8c20eff3ed50e3d6bb7a1b4a58aa1c16):

      // Even if the collations don't match at this point, it's fine,
      // because the bounds exclude values that use it
      collScan->hasCompatibleCollation = true;

      The fix might be to change the first existing update to use && as I suggested above and delete the second existing update, but this suggestion needs validation from a QO expert.

      I confirmed with hana.pearlman@mongodb.com that the current code is incorrect. 

            hana.pearlman@mongodb.com Hana Pearlman
            kevin.cherkauer@mongodb.com Kevin Cherkauer
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