Mongodb query does not use prefix on compound index with text field

Question

I\'ve created the following index on my collection:

db.myCollection.createIndex({
  user_id: 1,
  name: \'text\'
})

If I try to see the exe

Answer 1

In general MongoDB can use index prefixes to support queries, however compound indexes including geospatial or text fields are a special case of sparse compound indexes. If a document does not include a value for any of the text index field(s) in a compound index, it will not be included in the index.

In order to ensure correct results for a prefix search, an alternative query plan will be chosen over the sparse compound index:

If a sparse index would result in an incomplete result set for queries and sort operations, MongoDB will not use that index unless a hint() explicitly specifies the index.

Setting up some test data in MongoDB 3.4.5 to demonstrate the potential problem:

db.myCollection.createIndex({ user_id:1, name: 'text' }, { name: 'myIndex'})

// `name` is a string; this document will be included in a text index
db.myCollection.insert({ user_id:123, name:'Banana' })

// `name` is a number; this document will NOT be included in a text index
db.myCollection.insert({ user_id:123, name: 456 })

// `name` is missing; this document will NOT be included in a text index
db.myCollection.insert({ user_id:123 })

Then, forcing the compound text index to be used:

db.myCollection.find({user_id:123}).hint('myIndex')

The result only includes the single document with the indexed text field name, rather than the three documents that would be expected:

{
  "_id": ObjectId("595ab19e799060aee88cb035"),
  "user_id": 123,
  "name": "Banana"
}

This exception should be more clearly highlighted in the MongoDB documentation; watch/upvote DOCS-10322 in the MongoDB issue tracker for updates.