它为什么没走索引
索引明明建了，它就是不走 —— 是写废了，还是它算过了？
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.
「索引失效的十种情况」那种清单背完就忘，因为它没告诉你共同点。这一份要你自己说出那个共同点，然后拿它去判每一条具体的 SQL。
但真正的重点在第三幕：有一类「没走索引」是优化器算过之后的正确选择。分不开这两类，你会一路奔着「加个 hint 强制走索引」去 —— 而它会追你一句：你怎么知道走索引真的更快？拿什么数证明？
最后一幕反过来问：有没有可能走了索引比全表扫还慢。
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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