MongoDBmedium

Compound index order — what does the “prefix rule” mean?

Answer

For an index like `{a: 1, b: 1, c: 1}`, queries can efficiently use the leftmost prefix (a), (a,b), (a,b,c). If you skip `a`, the index is much less useful for that query.

db.users.createIndex({ orgId: 1, email: 1 })

// uses prefix (orgId, email)
db.users.find({ orgId: 1, email: "[email protected]" })

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