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  1. Core Server
  2. SERVER-38021

【query plans issue】inappropriate candidate plans are generated

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    • Type: Icon: Bug Bug
    • Resolution: Duplicate
    • Priority: Icon: Major - P3 Major - P3
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    • Affects Version/s: 3.6.8
    • Component/s: Querying
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      1、create index: 

      { f1:1 , f2: 1 }

      2、aggregate explain：

      db.data.aggregate([{
$match: {
    f1:1,
    $or: [
        { f2: 1 },
        {
           w: 1,
           f2: { $ne: 1 }
        }
    ]
}}], {
        explain: true
});

       when I add more elements to $or, the number of candidate plans is growing too, and it takes more time to evaluate them. 

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      1、create index:  { f1:1 , f2: 1 } 2、aggregate explain： db.data.aggregate([{ $match: { f1:1, $or: [ { f2: 1 }, { w: 1, f2: { $ne: 1 } } ] }}], { explain: true });  when I add more elements to $or, the number of candidate plans is growing too, and it takes more time to evaluate them. 
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      when we update our mongodb from 3.4 to 3.6, some query plans take much more time than 3.4. some aggregation took 10 seconds in 3.6 while 100ms in 3.4.

      explain(): "rejectedPlans" is empty in 3.4, but it contains one element in 3.6.

      I found the variable "_candidates" in "multi_plan.cpp" are inappropriate. In this case, it may take much more time to evaluate these candidate plans.

            Assignee:
            daniel.hatcher@mongodb.com Danny Hatcher (Inactive)
            Reporter:
            z572505258 zhang mengzhi
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              Created:
              Updated:
              Resolved: