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  1. Core Server
  2. SERVER-21064

It's possible to fail on sort memory overflow even if there is an alternative plan that requires no sort.

    • Type: Icon: Bug Bug
    • Resolution: Duplicate
    • Priority: Icon: Major - P3 Major - P3
    • None
    • Affects Version/s: 3.0.3
    • Component/s: Querying
    • Labels:
    • QuInt B (11/02/15)

      With a collection that has two indexes like:
      1) a_1_b_1_c_1
      2) c_1_b_1_a_1

      If you run a query like: {a: { "$in": [1]}, b:1, c: { "$in": [1,2,3]}}).sort({c:1}), a query shape will be cached with the associated plans that do no require sort and it is likely that the plan using the a_1_b_1_c_1 will get the top score.

      If you now run query like: {a: { "$in": [1,2,3]}, b:1, c: { "$in": [1,2,3]}}).sort({c:1}), the cached plan with top score will be used. As the query requires SORT stage, the plan will be re-adjusted, however if at the sort stage we hit the 32Mb limit, the query will fail despite the fact that there is an alternative plan cached that uses the c_1_b_1_a_1 index, which if used, will require no sort and re-adjustment.

            david.storch@mongodb.com David Storch
            dmitry.ryabtsev@mongodb.com Dmitry Ryabtsev
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