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Type: Improvement
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Resolution: Duplicate
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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
if we are using $expr, we don’t produce covered projections.
If index on a is present,
This will have fetch + projection simple
db.test.explain().aggregate([{$match: {$expr: {$eq: ["$a", 1]}}}, {$project: {_id: 0, a: 1}}])
But this will have projection covered:
db.test.explain().aggregate([\{$match: {a: 1}}, \{$project: {_id: 0, a: 1}}])
It looks like a problem with dependency analysis of $expr that can’t determine that we only need field “a” that can come from the index.
In some cases we have to use $expr syntax. For example, in $lookup with pipelines.
- duplicates
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SERVER-41252 cannot get covered query with $expr with $eq only
- In Progress
- is depended on by
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SERVER-84094 Implement initial drain policy for unsharded collections when the user specifies a destination shard
- Backlog