When would you use $in rather than $or?

Short answer

$in when you are testing one field against several values; $or when the conditions involve different fields. $in is easier to read and the planner handles it better.

Why?

An $or across different fields can only use an index if every branch is indexed; otherwise it degrades toward a collection scan. $in on a single indexed field is served by that one index.

Example

db.orders.find({ status: { $in: ["shipped", "delivered"] } })

db.orders.find({
  $or: [{ status: "shipped" }, { amount: { $gt: 10000 } }]
})

Interview tip

If every branch of an $or is the same field, rewrite it as $in before talking about indexes.

Common mistake

Writing $or: [{ city: 'Bangalore' }, { city: 'Mumbai' }] instead of $in.