Pushdown stand-alone LIMIT into streaming operators while estimating CE

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    • Type: Improvement
    • Resolution: Unresolved
    • Priority: Blocker - P1
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    • Affects Version/s: None
    • Component/s: None
    • Query Optimization
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      Currently, the cost of the stand-alone `LIMIT` does not take into account the fact that:

      • the input stage will be prematurely terminated on LIMIT values that are smaller than collection size
      • limit values larger than the collection size should be disregarded
      Enterprise test> db.foo.find({a: {$gte: 0}}).hint({$natural: 1}).limit(1).explain().queryPlanner.winningPlan;
{
  isCached: false,
  stage: 'LIMIT',
  costEstimate: 0.6246551 <- Fixed value that is incorrect 
  estimatesMetadata: { ceSource: 'Metadata' },
  limitAmount: 1,
  inputStage: {
    stage: 'COLLSCAN',
    costEstimate: 4.3291855,
    cardinalityEstimate: 10000,
    numDocsEstimate: 10000,
    estimatesMetadata: { ceSource: 'Histogram' },
    filter: { a: { '$gte': 0 } },
    direction: 'forward'
  }


Enterprise test> db.foo.find({a: {$gte: 0}}).hint({$natural: 1}).limit(10000000000000000000000).explain().queryPlanner.winningPlan;
{
  isCached: false,
  stage: 'LIMIT',
  costEstimate: 0.6246551,
  cardinalityEstimate: 10000,
  estimatesMetadata: { ceSource: 'Histogram' },
  limitAmount: Long('9223372036854775807'),
  inputStage: {
    stage: 'COLLSCAN',
    costEstimate: 4.3291855,
    cardinalityEstimate: 10000,
    numDocsEstimate: 10000,
    estimatesMetadata: { ceSource: 'Histogram' },
    filter: { a: { '$gte': 0 } },
    direction: 'forward'
  }
}

            Assignee:
            James Harrison
            Reporter:
            Philip Stoev
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              Created:
              Updated: