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Compound

Compound Index Sort Order

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  • Use Case
  • Example
  • Supporting Index for the Leaderboard
  • Reverse Results
  • Unsupported Queries
  • Learn More

Indexes store references to fields in either ascending (1) or descending (-1) order. For compound indexes, sort order can determine whether the index can support a sort operation.

Compound indexes support sort operations that match either the sort order of the index, or the reverse sort order of the index.

Use Case

A mobile game has a leaderboard that shows the following information:

  • Highest game scores

  • The user who achieved each score

  • The date each score was achieved

The application sorts the leaderboard first by score in descending order. Then, the username associated with each score is sorted in ascending order (alphabetically).

A compound index can improve performance for the leaderboard if the sort order in the index matches the sort order in the query.

Example

Consider a leaderboard collection with these documents:

db.leaderboard.insertMany( [
   {
      "score": 50,
      "username": "Alex Martin",
      "date": ISODate("2022-03-01T00:00:00Z")
   },
   {
      "score": 55,
      "username": "Laura Garcia",
      "date": ISODate("2022-03-02T00:00:00Z")
   },
   {
      "score": 60,
      "username": "Alex Martin",
      "date": ISODate("2022-03-03T00:00:00Z")
   },
   {
      "score": 60,
      "username": "Riya Patel",
      "date": ISODate("2022-03-04T00:00:00Z")
   },
   {
      "score": 50,
      "username": "Laura Garcia",
      "date": ISODate("2022-03-05T00:00:00Z")
   }
] )

This query returns leaderboard results:

db.leaderboard.find().sort( { score: -1, username: 1 } )

Output:

[
   {
      _id: ObjectId("632235700646eaee87a56a74"),
      score: 60,
      username: 'Alex Martin',
      date: ISODate("2022-03-03T00:00:00.000Z")
   },
   {
      _id: ObjectId("632235700646eaee87a56a75"),
      score: 60,
      username: 'Riya Patel',
      date: ISODate("2022-03-04T00:00:00.000Z")
   },
   {
      _id: ObjectId("632235700646eaee87a56a73"),
      score: 55,
      username: 'Laura Garcia',
      date: ISODate("2022-03-02T00:00:00.000Z")
   },
   {
      _id: ObjectId("632235700646eaee87a56a72"),
      score: 50,
      username: 'Alex Martin',
      date: ISODate("2022-03-01T00:00:00.000Z")
   },
   {
      _id: ObjectId("632235700646eaee87a56a76"),
      score: 50,
      username: 'Laura Garcia',
      date: ISODate("2022-03-05T00:00:00.000Z")
   }
]

The results are sorted first by score in descending order, then by username in ascending order (alphabetically).

Supporting Index for the Leaderboard

The following index improves performance for the leaderboard results because the sort order of the index matches the sort order used in the query:

db.leaderboard.createIndex( { score: -1, username: 1 } )

This compound index stores:

  • score values in descending order.

  • username values in ascending order (alphabetically).

Reverse Results

MongoDB can traverse a compound index in either direction. If the application allows users to view the leaderboard in reverse order, the index supports that query as well.

The following query returns the leaderboard in reverse order, where results are sorted first by ascending score values and then by descending username values (reverse alphabetically):

db.leaderboard.find().sort( { score: 1, username: -1 } )

Output:

[
   {
      _id: ObjectId("632235700646eaee87a56a76"),
      score: 50,
      username: 'Laura Garcia',
      date: ISODate("2022-03-05T00:00:00.000Z")
   },
   {
      _id: ObjectId("632235700646eaee87a56a72"),
      score: 50,
      username: 'Alex Martin',
      date: ISODate("2022-03-01T00:00:00.000Z")
   },
   {
      _id: ObjectId("632235700646eaee87a56a73"),
      score: 55,
      username: 'Laura Garcia',
      date: ISODate("2022-03-02T00:00:00.000Z")
   },
   {
      _id: ObjectId("632235700646eaee87a56a75"),
      score: 60,
      username: 'Riya Patel',
      date: ISODate("2022-03-04T00:00:00.000Z")
   },
   {
      _id: ObjectId("632235700646eaee87a56a74"),
      score: 60,
      username: 'Alex Martin',
      date: ISODate("2022-03-03T00:00:00.000Z")
   }
]

The { score: -1, username: 1 } index supports this query.

Unsupported Queries

Compound indexes cannot support queries where the sort order does not match the index or the reverse of the index. As a result, the { score: -1, username: 1 } index cannot support sorting by ascending score values and then by ascending username values, such as this query:

db.leaderboard.find().sort( { score: 1, username: 1 } )

Additionally, for a sort operation to use an index, the fields specified in the sort must appear in the same order that they appear in an index. As a result, the above index cannot support this query:

db.leaderboard.find().sort( { username: 1, score: -1, } )

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