Multikey Indexes
Multikey Indexes: free step-by-step lesson with examples, common mistakes, and interview tips — part of MongoDB Tutorial on Toolliyo Academy.
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MongoDB Tutorial · Lesson 43 of 100
Multikey Indexes
Foundations & CRUD ✓ → Queries & Schema → Aggregation & Scale → Atlas & Projects
Queries & Schema · 2 — Design · ~6 min · MongoDB — Indexing & Performance
What is this?
A multikey index is created when you index a field that holds arrays. MongoDB indexes each array value so queries can find documents containing a value.
Why should you care?
Posts tagged ["mongodb","atlas"] need fast tag browse. Multikey indexes make tags: "atlas" efficient.
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.posts.insertMany([
{ title: "A", tags: ["mongodb", "indexing"] },
{ title: "B", tags: ["atlas", "mongodb"] }
])
db.posts.createIndex({ tags: 1 })
db.posts.find({ tags: "atlas" })
db.posts.find({ tags: { $all: ["mongodb", "indexing"] } })Run Example »
Edit the code below and click Run to see the result in Toolliyo’s live editor.
What happened?
- Indexing tags creates multikey entries for each tag string.
- find tags: "atlas" uses the index.
- $all still benefits when looking for multiple tags on one post.
Practice next
- Insert posts and create the tags index.
- explain a tags equality query.
- Try indexing an array of subdocuments carefully (index key limits).
- Index roles on users and find role "admin".
- Use $elemMatch with multikey on array of objects.
Remember
Array fields produce multikey indexes. Great for tags and roles. Keep arrays bounded.
Blog tag pages
NoSQLVerse tag page lists every lesson tagged "sharding".
Outcome: Multikey index returns matches without scanning all posts.
Interview prep for this lesson
Practice these questions aloud after reading—each links to a full structured answer.
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