Wildcard indexes in MongoDB are designed to index fields in documents that have a dynamic or flexible schema. These indexes are especially useful for collections where documents do not have a consistent structure, and the fields that need to be indexed are not known in advance. Wildcard indexes can index all fields or specific fields that match a pattern, making them highly versatile for indexing diverse data.

Key Points:

• Indexes dynamic fields: Wildcard indexes are ideal for collections with documents that have varying fields.
• Flexible indexing: Supports indexing of all fields or specific fields that match a pattern.
• Simplifies indexing: Reduces the need for creating multiple specific indexes for each possible field.

Syntax

db.collection.createIndex({ "$**": 1 }, { <option1>: <value1>, <option2>: <value2>, ... })

• "$"**: A special wildcard operator indicating that all fields should be indexed.
• option1, option2: Optional parameters to customize the index behavior (e.g., unique, background).

Example Setup

Let's insert some documents into the logs collection:

db.logs.insertMany([
  { timestamp: new Date("2023-06-01T12:00:00Z"), level: "info", message: "User login", user: "user1" },
  { timestamp: new Date("2023-06-01T12:05:00Z"), level: "error", message: "Failed payment", user: "user2", errorCode: 502 },
  { timestamp: new Date("2023-06-01T12:10:00Z"), level: "info", message: "Data export", user: "user3", exportId: "exp123" },
  { timestamp: new Date("2023-06-01T12:15:00Z"), level: "warn", message: "High memory usage", system: "backend1" },
  { timestamp: new Date("2023-06-01T12:20:00Z"), level: "info", message: "User logout", user: "user1" }
])

Index Creation Example

Create a wildcard index on all fields:

db.logs.createIndex({ "$**": 1 })

• db.logs.createIndex({ "$**": 1 }): This command creates a wildcard index that indexes all fields in the documents. This index is beneficial for querying documents based on any field.

Querying Using the Indexes

Example 1: Query by Specific Field

db.logs.find({ level: "error" })

• This query uses the wildcard index to quickly locate documents where the level field is "error".

Example 2: Query by Multiple Fields

db.logs.find({ user: "user1", message: /login/ })

• This query uses the wildcard index to find documents where the user field is "user1" and the message field contains the word "login".

Wildcard indexes in MongoDB provide a powerful solution for indexing collections with dynamic schemas. By indexing all fields or fields that match a pattern, wildcard indexes simplify index management and enhance query performance.