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Reduce $lookup Operations

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  • Overview
  • Examples
  • Denormalization
  • Learn More

Overview

$lookup operations join data from two collections in the same database based on a specified field. $lookup operations can be useful when your data is structured similarly to a relational database and you need to model large hierarchical datasets. However, these operations can be slow and resource-intensive because they need to read and perform logic on two collections instead of a single collection.

If you frequently run $lookup operations, consider restructuring your schema such that the your application can query a single collection to obtain all of the information it needs. You can utilize MongoDB's flexible schema model with embedded documents and arrays to capture relationships between data in a single document structure. Use this denormalized model to take advantage of MongoDB's rich documents and allow your application to retrieve and manipulate related data in a single query.

Examples

The following examples show two schema structures designed to reduce $lookup operations:

Use Embedded Documents

Consider the following example where a grocery store tracks one-to-one inventory and nutrition information in two separate collections. Each inventory item corresponds to a unique nutrition facts item. The nutrition_id field links the inventory collection to the nutrition_facts collection, similar to a tabular database:

// inventory collection
{
   "name": "Pear",
   "stock": 20,
   "nutrition_id": 123, // reference to a nutrition_fact document
   ...
}
{
   "name": "Candy Bar",
   "stock": 26,
   "nutrition_id": 456,
   ...
}
// nutrition_facts collection
{
   "_id": 123,
   "calories": 100,
   "grams_sugar": 17,
   "grams_protein": 1,
   ...
}
{
   "_id": 456,
   "calories": 250,
   "grams_sugar": 27,
   "grams_protein": 4,
   ...
}

If an application requests the nutrition facts for an inventory item by name, this schema structure requires a $lookup of the nutrition_facts collection to find an entry that matches the inventory item's nutrition_id.

Instead, you can embed the nutrition information inside the inventory collection:

// inventory collection
{
   "name": "Pear",
   "stock": 20,
   "nutrition_facts": {
      "calories": 100,
      "grams_sugar": 17,
      "grams_protein": 1,
      ...
   }
   ...
}
{
   "name": "Candy Bar",
   "stock": 26,
   "nutrition_facts": {
      "calories": 250,
      "grams_sugar": 27,
      "grams_protein": 4,
      ...
   }
   ...
}

This way, when you query for an item in inventory, the nutrition facts are included in the result without the need for another query or a $lookup operation. Consider embedding documents when data across collections has a one-to-one relationship.

Use Arrays

Consider the following example where documents in a baseball league's players collection reference documents in a teams collection, similar to a tabular database:

// players collection
{
   "team_id": 1, // reference to a team document
   "name": "Nick",
   "position": "Pitcher"
   ...
}
{
   "team_id": 1,
   "name": "Anuj",
   "position": "Shortstop"
   ...
}
// teams collection
{
   "_id": 1,
   "name": "Danbury Dolphins"
   ...
}

If an application requests a list of players on a team, this schema structure requires a $lookup of the players collection to find each player that matches a team_id.

Instead, you can list the players in an array on the team document itself:

// teams collection
{
    "_id": 1,
    "name": "Danbury Dolphins",
    "players": [
       {
          "name": "Nick",
          "position": "Pitcher"
          ...
       },
       {
          "name": "Anuj",
          "position": "Shortstop"
          ...
       }
    ]
}

By using arrays to hold related data, an application can retrieve complete team information, including that team's players, without $lookup operations or indexes on other collections. In this case, using arrays is more performant than storing the information in separate collections.

Note

In the example above, the baseball teams have a set number of players and there is no risk of arrays becoming exceedingly large.

Array Considerations

The performance cost of reading and writing to large arrays can outweigh the benefit gained by avoiding $lookup operations. If your arrays are unbounded or exceedingly large, those arrays may degrade read and write performance.

If you create an index on an array, each element in the array is indexed. If you write to that array frequently, the performance cost of indexing or re-indexing a potentially large array field may be significant.

Tip

See also:

Avoid Unbounded Arrays

Denormalization

Denormalizing your schema is the process of duplicating fields or deriving new fields from existing ones. Denormalization can improve read performance in a variety of cases, such as:

  • A recurring query requires a few fields from a large document in another collection. You may choose to maintain a copy of those fields in an embedded document in the collection that the recurring query targets to avoid merging two distinct collections or performing frequent $lookup operations.

  • An average value of some field in a collection is frequently requested. You may choose to create a derived field in a separate collection that is updated as part of your writes and maintains a running average for that field.

While embedding documents or arrays without duplication is preferred for grouping related data, denormalization can improve read performance when separate collections must be maintained.

Note

When you denormalize your schema, it becomes your responsibility to maintain consistent duplicated data.

Learn More

The best structure for your schema depends on your application context. The following resources provide detailed information on data modeling and additional example use cases for embedded documents and arrays:

Data Models

MongoDB.live 2020 Data Modeling Presentations

To learn how to incorporate the flexible data model into your schema, see the following presentations from MongoDB.live 2020:

Arrays

To learn more about how to query arrays in MongoDB, see Query an Array.

Design Patterns

To read about situations in which arrays work well, see the following design patterns:

  • Use The Attribute Pattern for handling data with unique combinations of attributes, such as movie data where each movie is released in a subset of countries.

  • Use The Bucket Pattern for handling tightly grouped or sequential data, such as time span data.

  • Use The Polymorphic Pattern for handling differently shaped documents in the same collection, such as athlete records across several sports.

Denormalized Schemas

To read about a situation in which duplicating data improves your schema, see the following design pattern:

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