读了多少行才吐出这几行
看了执行计划，走索引了 —— 那它扫了多少行、返回几行？
Copy the prompt, then pick a model. It plays the interviewer and asks you through it scene by scene instead of handing you the answer.
On a phone with the app installed, the system opens the app directly. Links cannot open external apps inside WeChat — open it in a browser first.
The copied text ends with the answer key the model grades you against. Don't read it yourself first — working it out on your own is the point.
这一份不教你背 type 那一串从好到坏的顺序，它只逼一个比值：扫描行数 ÷ 返回行数。这个数说明的问题比耗时多得多。
中间会给你一份真实的计划让你逐字段解释，其中有一问几乎人人第一次都答错：都有 LIMIT 20 了，为什么还要排四万多行？
最后一幕问计划之外的事：执行计划完美但查询还是慢，能是什么。
What this one drills
This prompt in the repo (the answer key is at the end of that file — you'll see it)
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Notes from people who did this one
How to use this
These are learning drills: the model guides, it does not corner you. When you get stuck it first asks which layer you are stuck at (never heard of it / know it but not why / know why but cannot apply it), then decides what to give. It only explains after guiding twice, and then asks you to say it back in your own words.
It assumes MySQL / InnoDB by default — half the traps here are implementation details, and clustered indexes, back-to-the-table lookups and gap locks work differently elsewhere. PostgreSQL or anything else is fine: say so in your first message and it will follow your engine, calling out where the conclusions differ.
How to start
- Pick one, open it, copy the prompt.
- Paste it into a fresh, empty chat, and say which engine and version you use first (say nothing and it assumes MySQL 8).
- Then say nothing else and wait for its first question.
Four commands you can call out any time
跳过Move on to the next scene换个说法Explain this bit from another angle直说吧Give the full answer, then make me say it back复盘Jump straight to the wrap-up card
Three traps
- Do not read the answer key at the end first — the "what people get wrong" section is the answer to every question in the drill.
- Expect to be pushed for numbers. "A bit faster", "saves IO" gets cut off on the spot: how many rows? how many times? how many times faster? A conclusion with no number behind it is memorised.
- The moment it starts praising you, point it back at the "do not praise me" rule. Guiding is not coddling.
Start with the composite index one — it is the foundation here; "locks sit on index records" and "why is it scanning so many rows" both lean on its one-line model. If concurrency is biting you in production, start with read-modify-write instead; if queries are slow, start with the execution plan one.
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