Mongoose Indexes: Types and When to Use Them

Mongoose Indexes: Types and When to Use Them

Without indexes, MongoDB scans every document in a collection. Indexes make queries fast. Here is which types exist and when to use them.

1. Single Field Index

Index on one field. Use for fields you query often.

userSchema.index({ email: 1 });

2. Unique Index

Enforces uniqueness. Use for fields that must be unique (email, username).

email: { type: String, unique: true }

Or:

userSchema.index({ email: 1 }, { unique: true });

3. Compound Index

Index on multiple fields. Use for queries that filter on multiple fields together.

connectionRequestSchema.index({ fromUserId: 1, toUserId: 1 }, { unique: true });

Order matters: put the field you filter by first, then the field you sort by. Use explain() to verify queries use the index.

4. Text Index

For full-text search. Use sparingly; it is heavy.

postSchema.index({ title: 'text', body: 'text' });

Then Post.find({ $text: { $search: 'nodejs' } }).

5. Geospatial Index

For location queries. Use 2dsphere for geo points.

placeSchema.index({ location: '2dsphere' });

6. TTL Index

Auto-deletes documents after a timeout. Use for sessions, tokens, or logs.

sessionSchema.index({ createdAt: 1 }, { expireAfterSeconds: 3600 });

When to Index

• You query by a field often.
• You sort by a field often.
• You need uniqueness (use unique index).
• You do full-text search (text index).

When Not to Index

• The collection is small (< 1000 docs).
• You rarely query by that field.
• Writes are very frequent (every index slows writes).

How to Verify an Index Is Used

User.find({ email }).explain('executionStats'). Look at executionStats.totalDocsExamined. If it equals the collection size, the index is not used.

Indexes and Write Performance

Every index adds overhead to writes. Indexes are a tradeoff: faster reads, slower writes. Do not index every field. Index the queries you actually run.

The Takeaway

Mongoose indexes: single field, unique, compound, text, geospatial, and TTL. Use them for fields you query and sort by, for uniqueness, for full-text, for geo, and for auto-expiring data. Do not index every field; index the queries you run. Verify with explain().