一次查询读了几棵树
一次查询到底读了几棵树、回表几次？
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.
「覆盖索引可以避免回表」这句话谁都背得出，但它省掉的是几次、每次贵在哪，多数人没算过。这一份全程逼数字：命中 100 行，回表 100 次，随机还是顺序。
顺带解决两个日常问题：为什么不该写 SELECT *（比「浪费带宽」实在得多的那个理由），以及用 UUID 当主键除了体积还会多出什么。
最后一幕问代价：这张表写入 QPS 3000，为了覆盖加一个四列索引，每次写多做了什么。
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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