Details
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Improvement
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Status: Backlog
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Major - P3
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Resolution: Unresolved
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None
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None
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Query Optimization
Description
Description
Suppose we have the following documents in the emails collection:
[{_id: ObjectId(), subject: 'Hello there', to: ['address1@example.com', 'address2@example.com']}] |
With the index:
{ { "v" : 2, "key" : { "to" : -1 }, "name" : "to_-1", "ns" : "test.emails" } |
And the following documents in the collection users:
[{_id: ObjectId(), name: 'John Doe', address: 'address1@example.com'}] |
Then, if we use the following pipeline on 'users' in order to insert all emails sent to our users into 'sentEmails':
db.users.aggregate([
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{$lookup: {
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from: 'emails', |
let: {userAddress: '$address'}, |
as: 'sentEmails', |
pipeline: [
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{$match: {$expr: {
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$in: ['$$userAdress', '$to'] |
}}}
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]
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}}
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])
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Or, even if we extracted the emails sub-pipeline into an equivalent pipeline for our first user (John Doe):
db.emails.aggregate([
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{$match: {$expr: {
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$in: ['address1@example.com', '$to'] |
}}}
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])
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Its not using the index on the emails collection.
However, if you do the same without using the $expr, the index is used properly:
db.emails.aggregate([{$match: {to: 'address1@example.com'}}], {explain: true}) |
...
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"winningPlan" : { "winningPlan" : { "stage" : "FETCH", "inputStage" : { "stage" : "IXSCAN", |
...
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Observed results
The index is not used when the $in operator is used inside an $expr on a $match
Expected results
The index should be used to perform the $match stage
Attachments
Issue Links
- is related to
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- Backlog
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- related to
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SERVER-45326 Poor Aggregation Framework performance in relational queries
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- Closed
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