Introduction to Indexes

Understand why indexes exist, create your first index, and see the dramatic performance difference they make.

The Problem: Collection Scans

Without an index, MongoDB must perform a collection scan — examining every single document to find ones matching a query. On a collection with a few dozen documents this is invisible; on one with millions, it becomes painfully slow.

What an Index Does

An index is a separate, ordered data structure that maps field values to document locations — conceptually similar to a book's index letting you jump straight to a topic instead of reading every page. It lets MongoDB find matching documents directly, without scanning the whole collection.

Query arrives

↓

Without an index: scan every document

↓

With an index: jump directly to matching entries

↓

Return only the matched documents

Creating a Single-Field Index

db.users.createIndex({ email: 1 }); // 1 = ascending order

Once created, a query filtering on email can use this index to jump directly to matching documents instead of scanning the whole collection.

db.users.find({ email: "ada@example.com" }); // now uses the index automatically

The Default _id Index

Every collection automatically gets a unique index on _id when it's created — this is why looking up a document by its _id is always fast, with no extra setup required.

Unique Indexes

Passing { unique: true } enforces that no two documents can share the same value for the indexed field — the standard way to guarantee something like email addresses are never duplicated.

db.users.createIndex({ email: 1 }, { unique: true });

Indexes Aren't Free

Every index speeds up matching reads, but also adds overhead to every write (since the index must be updated too) and consumes additional storage — indexes should be added deliberately, matched to real query patterns, not applied to every field by default.

Common Beginner Mistakes

Creating an index for every field "just in case"

Each unnecessary index slows down writes and wastes storage without a matching read benefit — create indexes based on actual, known query patterns.

Assuming a unique index alone prevents application-level duplicate logic bugs

A unique index is a strong database-level safety net, but your application should still handle the resulting error gracefully (e.g. "this email is already registered") rather than crashing.

FAQs

How many indexes can a collection have?

Up to 64 per collection — in practice, most well-designed collections need far fewer, matched to their actual query patterns.

Do indexes get created instantly on a large collection?

Not necessarily — building an index on an existing large collection can take significant time; MongoDB supports building indexes in the background to avoid blocking other operations.

Summary

An index lets MongoDB jump directly to matching documents instead of scanning an entire collection, at the cost of some write overhead and storage. Next, you'll learn compound indexes, covering queries that filter on multiple fields at once.

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