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  1. Core Server
  2. SERVER-99025

histogramCE: Estimate for inequality over an array column is inaccurate

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    • Type: Icon: Bug Bug
    • Resolution: Unresolved
    • Priority: Icon: Major - P3 Major - P3
    • None
    • Affects Version/s: None
    • Component/s: None
    • None
    • Query Optimization
    • ALL
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      db.foo.drop();

let array_docs = [];
let array = [];
for (let i = 0; i < 50; i++) {
    array_docs.push({a: array.slice()});
    array.push(i);
}


db.foo.insertMany(array_docs);
db.foo.runCommand({analyze: "foo", key: "a"});
db.adminCommand({setParameter: 1, planRankerMode: "histogramCE"});
db.foo.find({a: {$gt: 40}}).count();
db.foo.find({a: {$gt: 40}}).explain().queryPlanner.winningPlan.cardinalityEstimate;
      Show
      db.foo.drop(); let array_docs = []; let array = []; for (let i = 0; i < 50; i++) { array_docs.push({a: array.slice()}); array.push(i); } db.foo.insertMany(array_docs); db.foo.runCommand({analyze: "foo" , key: "a" }); db.adminCommand({setParameter: 1, planRankerMode: "histogramCE" }); db.foo.find({a: {$gt: 40}}).count(); db.foo.find({a: {$gt: 40}}).explain().queryPlanner.winningPlan.cardinalityEstimate;

      The estimates for inequality predicates over array fields are inaccurate, sometimes overestimating by a factor of 10x

      Enterprise test> db.foo.find({a: {$gt: 40}}).count();
8
Enterprise test> db.foo.find({a: {$gt: 40}}).explain().queryPlanner.winningPlan.cardinalityEstimate;
18.911000191707124

      I think the reason is that one document can contribute multiple entries into the histogram, thus inflating all estimates involving inequalities.

            Assignee:
            Unassigned Unassigned
            Reporter:
            philip.stoev@mongodb.com Philip Stoev
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              Created:
              Updated: