Indexes

Indexes

An index is a special data structure that makes queries fast. Without an index, MongoDB must scan every document in the collection - a full collection scan. With the right index, it only looks at the matching documents.

The trade-off

• Indexes speed up reads.
• They slow down writes - every insert and update must also update the index.
• Only create indexes for queries you actually run.

How they work

Indexes use a B-tree structure. They store a small portion of the data (the indexed field values) in sorted order, making equality checks, range queries, and sorted results fast.

Types of indexes

The _id index

MongoDB automatically creates a unique index on _id when a collection is created. It prevents two documents from sharing an _id, and you cannot drop it.

Creating an index

// Single-field index on title
db.posts.createIndex({ title: 1 })

// Compound index on category (ascending) and likes (descending)
db.posts.createIndex({ category: 1, likes: -1 })

Inspect and drop indexes

db.posts.getIndexes()   // list indexes
db.posts.dropIndex({ title: 1 })   // remove an index

Naming

An index's default name combines the keys and directions, like category_1_likes_-1.

TL;DR

• Indexes speed up reads but slow down writes.
• Types: single-field, compound, multikey, geospatial, text.
• _id always has a unique, un-droppable index.