What is a database index and what is the trade-off?
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“What is a database index and what is the trade-off?”
🧠Mind Map Answer
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An index is like a book's index — instead of scanning every page (row), the DB jumps straight to the data.
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CREATE INDEX idx_users_email ON users(email);
-- now WHERE email = ? uses a B-tree seek, not a full scan🔥What If?
Think Beyond the Expected
Why might the database ignore your index?
If a query returns a large fraction of the table, a full scan is cheaper than many random index seeks — so the optimizer skips the index. Low-selectivity columns rarely benefit.
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Plain-language foundations
I'm preparing for a software engineering interview and want to understand this from scratch, as a beginner. Topic: Indexes (SQL) Interview question: "What is a database index and what is the trade-off?" Please: 1. Explain the core idea in simple, plain language, using an everyday analogy. 2. Define any technical terms you use. 3. Walk through one small, concrete example. 4. Finish with a single sentence I can easily remember. Keep the tone friendly and assume I'm new to this topic.
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