慢 SQL 索引诊断
给出查询和表结构，让模型提出索引与验证计划。
Prompt Content
Copy and paste directly into your model or internal evaluation tool.
表 orders(id, user_id, status, created_at, amount)，查询：select * from orders where user_id = ? and status = 'paid' order by created_at desc limit 20。数据量 5000 万。请设计索引、说明字段顺序，并给出如何验证索引是否生效。
Use Cases
Reference Output
可建联合索引 (user_id, status, created_at desc)，用 EXPLAIN/ANALYZE 验证过滤、排序和回表成本。
Scoring Rubric
5分：说明顺序和验证；3分：只给索引；1分：建议单列索引。
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