Indexing for Performance
Imagine searching for a book in a library without a catalog system. You'd have to check every single shelf! That's what MongoDB does without indexes – it scans every document (called a collection scan). **Indexes** are like the library's catalog system – they make searches lightning fast.
What are Indexes?
Indexes are special data structures that store a small portion of the collection's data in an easy-to-search form. They support efficient execution of queries by reducing the number of documents that need to be examined.
Think of an index like the index at the back of a book. Instead of reading every page to find a topic, you look in the index and go directly to the right page.
Without Index – Collection Scan
When you run a query without an index, MongoDB scans every document in the collection:
db.users.find({ email: "john@example.com" })If you have 1 million users, MongoDB checks all 1 million documents. This is slow and inefficient.
Creating a Single Field Index
<!-- Create an index on the email field -->db.users.createIndex({ email: 1 })
<!-- 1 means ascending order, -1 would be descending -->Now when you search by email, MongoDB uses the index to find the document instantly.
Checking if an Index is Used
Use `explain()` to see how MongoDB executes a query:
db.users.find({ email: "john@example.com" }).explain("executionStats")Look for `stage: "IXSCAN"` (index scan) vs `stage: "COLLSCAN"` (collection scan).
Compound Indexes
Often you query on multiple fields. A compound index includes multiple fields.
<!-- Create index on (category, price) -->db.products.createIndex({ category: 1, price: -1 })
<!-- This index helps queries like: -->db.products.find({ category: "electronics", price: { $lt: 100 } })Unique Indexes
To prevent duplicate values in a field, create a unique index:
db.users.createIndex({ email: 1 }, { unique: true })Now you cannot insert two users with the same email.
Listing All Indexes
db.users.getIndexes()Dropping an Index
<!-- Drop by name -->db.users.dropIndex("email_1")
<!-- Drop by specification -->db.users.dropIndex({ email: 1 })Practical Example: Performance Comparison
<!-- Insert 10,000 test documents -->for (let i = 0; i < 10000; i++) { db.test.insertOne({ x: i, y: i % 100 });}
<!-- Query without index -->db.test.find({ x: 5000 }).explain("executionStats")<!-- Notice totalDocsExamined: 10000 -->
<!-- Create index -->db.test.createIndex({ x: 1 })
<!-- Query with index -->db.test.find({ x: 5000 }).explain("executionStats")<!-- Notice totalDocsExamined: 1 -->Two Minute Drill
- Indexes make queries faster by avoiding collection scans.
- Create single field indexes with `createIndex({ field: 1 })`.
- Compound indexes support queries on multiple fields.
- Use `explain()` to check if your index is being used.
- Unique indexes prevent duplicate values.
- Indexes speed up reads but slow down writes slightly (because the index must be updated).
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