索引与查询优化
了解数据库索引的工作方式和适用场景，并学习如何优化查询以实现快速读取，同时避免严重拖慢写入。
索引与查询优化 是 CoddyKit 上的免费 System Design Basics for Backend Developers 课时。 这是第 4 节课，共 4 节。 你可以在下方免费阅读本课时的完整内容 — 然后在浏览器中使用内置代码编辑器和全天候 AI 导师进行实践。 这是 System Design Basics for Backend Developers 学习路径的一部分，你的进度在网页和 CoddyKit 应用中同步。 System Design Basics for Backend Developers 课程共包含 4 节课。
本课时的部分内容尚未翻译，以英文显示。
Why Indexes Matter
Without an index, finding a row means scanning every row — a full table scan. As tables grow into millions of rows, this becomes painfully slow.
An index is a separate data structure that lets the database jump straight to matching rows.
The B-Tree Index
Most relational indexes use a B-tree: a balanced tree that keeps keys sorted. Lookups, range scans, and ordering all become logarithmic instead of linear.
- Fast equality lookups (
WHERE id = 5) - Fast range queries (
WHERE age > 30) - Supports
ORDER BYwithout re-sorting
Creating an Index
You create an index on the column(s) you frequently filter or sort by.
Here we index the email column so login lookups are instant.
CREATE INDEX idx_users_email
ON users (email);
SELECT * FROM users
WHERE email = 'a@example.com';Composite Indexes
A composite index covers multiple columns. Column order matters: the index helps queries that filter on a left-prefix of the columns.
An index on (country, city) helps WHERE country = ? and WHERE country = ? AND city = ?, but not WHERE city = ? alone.
CREATE INDEX idx_loc
ON customers (country, city);Covering Indexes
If an index contains every column a query needs, the database answers from the index alone and never touches the table. This is a covering index.
It is one of the most powerful read optimizations available.
The Write Cost
Indexes are not free. Every INSERT, UPDATE, or DELETE must also update each affected index.
- More indexes = slower writes
- More indexes = more storage
Index for the queries you actually run, not speculatively.
Reading EXPLAIN
Use EXPLAIN (or EXPLAIN ANALYZE) to see the query plan. Look for Index Scan (good) versus Seq Scan (full table scan).
EXPLAIN ANALYZE
SELECT * FROM orders
WHERE customer_id = 42;Selectivity
An index helps most when the column is highly selective — it filters down to a tiny fraction of rows. Indexing a boolean is_active with a 50/50 split is nearly useless; the planner may ignore it.
Avoiding Index-Defeating Queries
Wrapping an indexed column in a function or doing a leading wildcard defeats the index.
WHERE LOWER(email) = ?— index onemailunusedWHERE name LIKE '%son'— leading wildcard, no index
Store data in the form you query, or use a functional index.
-- Defeats the index:
SELECT * FROM users WHERE LOWER(email) = 'a@x.com';
-- Better: store email already lowercasedIndexing and Sharding Together
In a sharded system each shard maintains its own indexes. A query that includes the shard key hits one shard and one index; a query without it must fan out to every shard. Design indexes and shard keys together.
A Practical Workflow
Optimize iteratively: find slow queries from logs, run EXPLAIN, add a targeted (often composite or covering) index, re-measure, and drop indexes that are never used.
Quick Check
Test your understanding of indexing.
Recap
You learned how to make reads fast with indexes:
- B-tree indexes power equality, range, and ordering
- Composite indexes follow the left-prefix rule
- Covering indexes answer queries without touching the table
- Indexes cost write speed and storage — index deliberately
- Use EXPLAIN and watch for index-defeating patterns
常见问题解答
「索引与查询优化」课时是免费的吗？
是的 — 「索引与查询优化」的完整文本可在网页上免费阅读。要进行交互式练习（内置代码编辑器和全天候 AI 导师）并解锁 System Design Basics for Backend Developers 课程的其余内容，请升级到 CoddyKit PRO。 System Design Basics for Backend Developers 课程共包含 4 节课。
「索引与查询优化」这节课中我会学到什么？
了解数据库索引的工作方式和适用场景，并学习如何优化查询以实现快速读取，同时避免严重拖慢写入。 你通过在浏览器中直接运行的动手代码来练习 System Design Basics for Backend Developers，全天候 AI 导师会在你学习这节课的过程中回答你的问题。
学习 System Design Basics for Backend Developers 需要有经验吗？
无需任何先前经验。CoddyKit 上的 System Design Basics for Backend Developers 课程适合初学者到高级学习者，你可以从这里开始或从头开始，按照自己的节奏学习。 这是第 4 节课，共 4 节。
「索引与查询优化」课时需要多长时间？
大多数 CoddyKit 课程大约需要 5–10 分钟。每节课都很精短且互动，所以你能稳步进步，并在网页和应用中从离开的地方继续。
我能在这节 System Design Basics for Backend Developers 课中编写并运行代码吗？
能。每节 System Design Basics for Backend Developers 课都包含内置代码编辑器，你可以在浏览器中直接编写并运行真实代码，并获得即时 AI 反馈 — 无需本地设置。