Challenge 1: Create a Compound Index — Possible Solution
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db.orders.createIndex({ status: 1, createdAt: -1 })

WHY THIS WORKS AS AN ANSWER
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The query pattern described has two parts: an EQUALITY filter on
status, and a SORT on createdAt in descending order. Following the
Equality-Sort-Range ordering this chapter introduced, the equality
field (status) comes first, and the sort field (createdAt) comes
second — matching the direction the sort actually needs (-1 for
descending, since that's how it's being sorted). There's no range
clause in this particular query, so the index only needs these two
fields to match the described pattern.