Write one production-safe index from an EXPLAIN plan

About this exercise

The account activity page is slow. Your migration rules: production indexes are always CREATE INDEX CONCURRENTLY, and this one must be named idx_events_account_id_created_at. Write exactly one statement in 1 code block (at most 3 lines) that fits the query's filter and sort, as a partial index on rows that aren't deleted. Then exactly one line starting 'Why: ' under 20 words, and nothing else.

Common mistakes when you ask AI for this

• A locking index. A plain CREATE INDEX locks writes on a 38-million-row table while it builds. Try: Add: "Production rule: CREATE INDEX CONCURRENTLY."
• Letting the AI name it. The AI invents a name, and your migration checker rejects it. Try: Add: "Name it idx_events_account_id_created_at."
• Wrong columns or order. Indexing kind, or created_at first, doesn't help this filter and sort. Try: Add: "Fit the WHERE and ORDER BY: (account_id, created_at DESC)."
• Several options and a lecture. The AI offers three indexes and explains query plans. Try: Add: "Exactly one statement, then one 'Why:' line under 20 words."

What a strong prompt includes

• Says it's for production
• Gives the index name
• Asks for a partial index
• Asks for one statement
• Shapes the explanation

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