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  1. MariaDB Server
  2. MDEV-6214

After BKA has filled join buffer, it should check again whether to use MRR

        Details

        • Type: Task
        • Status: Open
        • Priority: Minor
        • Resolution: Unresolved
        • Fix Version/s: 10.1
        • Component/s: None

          Description

          (filing based on discussions with Igor)

          The problem is: if one uses mrr_cost_based=ON, it is possible for the following
          to occur:

          1. The optimizer decides to use DS-MRR (or whatever MRR implementation the engine has)
          2. Execution starts
            1. The join buffer is filled, it turns out that it has fewer records then expected
            2. DS-MRR scan is used (according to the decision), but use of DS-MRR only adds overhead.

          The idea to address this is as follows: after BKA code has filled the join buffer, we should note the amount of data we have in the join buffer and check again whether to use DS-MRR.

          Tasks to be done

          • Fix the current DS-MRR cost choice formulas (or decide to use current ones)
          • Implement "the idea" described above.

          Unresolved questions

          • DS-MRR cost formulas (use current? take 5.6's? something else?)
          • Do we make the late-decision every time we've filled the join buffer? or just the first time?

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                  • Assignee:
                    Unassigned
                    Reporter:
                    psergey Sergei Petrunia
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                    Dates

                    • Created:
                      Updated: