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Assigned Teams:

Query Optimization
Operating System:
ALL
Steps To Reproduce:
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Create test data:

db.C.insertMany([{ _id : 1, Name : "John Doe" }, { _id : 2, Name : "Jane Doe" }])

Execute command:

db.C.aggregate([{ $match : { $expr : { $eq : ['$$ROOT', { _id : 2, Name : 'Jane Doe' }] } } }])

Expected result:

> db.C.aggregate([{ $match : { $expr : { $eq : ['$$ROOT', { _id : 2, Name : 'Jane Doe' }] } } }]) [ { _id: 2, Name: 'Jane Doe' } ]

Result in 5.0:

> db.C.aggregate([{ $match : { $expr : { $eq : ['$$ROOT', { _id : 2, Name : 'Jane Doe' }] } } }]) MongoServerError: Failed to optimize pipeline :: caused by :: Failed to optimize expression :: caused by :: FieldPath::tail() called on single element path
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Create test data: db.C.insertMany([{ _id : 1, Name : "John Doe" }, { _id : 2, Name : "Jane Doe" }]) Execute command: db.C.aggregate([{ $match : { $expr : { $eq : [ '$$ROOT' , { _id : 2, Name : 'Jane Doe' }] } } }]) Expected result: > db.C.aggregate([{ $match : { $expr : { $eq : [ '$$ROOT' , { _id : 2, Name : 'Jane Doe' }] } } }]) [ { _id: 2, Name: 'Jane Doe' } ] Result in 5.0: > db.C.aggregate([{ $match : { $expr : { $eq : [ '$$ROOT' , { _id : 2, Name : 'Jane Doe' }] } } }]) MongoServerError: Failed to optimize pipeline :: caused by :: Failed to optimize expression :: caused by :: FieldPath::tail() called on single element path

The following aggregate command works on 6.0 but not on earlier versions:

db.C.aggregate([{ $match : { $expr : { $eq : ['$$ROOT', { _id : 2, Name : 'Jane Doe' }] } } }])

duplicates

SERVER-64764 $expr rewrite is not prepared to handle "$$ROOT"

related to

CSHARP-4244 Support object comparison at root level in LINQ3

Participants:: [DO NOT USE] Backlog - Query Optimization, Charlie Swanson, Robert Stam