Why should you care?

find({ email }) on millions of users without an index is a full collection scan — login and password-reset APIs will time out.

See it live — copy this example

Open mongosh or MongoDB Compass, select database nosqlverse, then run the example. Change one field and run again.

db.users.insertMany([
  { email: "a@nosqlverse.dev", city: "Pune" },
  { email: "b@nosqlverse.dev", city: "Goa" }
])
db.users.createIndex({ email: 1 }, { unique: true })
db.users.find({ email: "a@nosqlverse.dev" }).explain("executionStats")
db.users.getIndexes()

Run Example »

Edit the code below and click Run to see the result in Toolliyo’s live editor.

What happened?

• createIndex on email builds an ascending index and enforces uniqueness.
• explain should show IXSCAN.
• getIndexes lists _id_ and email_1.

Practice next

1. Create the unique email index.
2. Find by email and inspect explain.
3. Try inserting a duplicate email — expect error.
4. Index city: 1 and query city ranges alphabetically.
5. Create a sparse index on phone.

Remember

Single-field indexes speed filters on that field. unique prevents duplicates. Verify with explain IXSCAN.

Login by email

Auth service finds users by email on every sign-in.

Outcome: Unique index gives speed and blocks duplicate accounts.