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Type: Improvement
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Resolution: Unresolved
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Priority: Major - P3
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None
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Affects Version/s: None
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Component/s: None
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Query Optimization
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QO 2021-10-04, QO 2021-10-18
MongoDB 4.2 introduced the ability to avoid full index scans for aggregation pipelines that include a specific type of $group stage - those that logically request distinct information which can be obtained by scanning a single document. This was initially implemented via SERVER-9507.
As confirmed by this comment from a related enhancement in SERVER-40090, this optimization currently only applies when the aggregation is grouping on a single field. We should consider expanding this optimization, or a similar one, to reduce index scanning when the _id specified for the $group is on multiple fields.
- depends on
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SERVER-21992 Inconsistent results when grouping by possibly missing values
- Backlog
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SERVER-14832 Distinct command result set may include or exclude null/undefined depending on presence of index
- Needs Scheduling
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SERVER-60977 Make $group _id behavior with null and missing more consistent
- Closed
- is duplicated by
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SERVER-56294 Extend group to distinct optimization to compound keys
- Closed
- related to
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SERVER-40090 DISTINCT_SCAN in agg is only used when certain format of _id is specified
- Closed