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PostgreSQL Performance & Query Optimization · 课时

了解锁与死锁

学习 PostgreSQL 中不同的锁类型，以及如何识别和避免死锁。

了解锁与死锁 是 CoddyKit 上的免费 PostgreSQL Performance & Query Optimization 课时。 这是第 1 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 PostgreSQL Performance & Query Optimization 学习路径的一部分，你的进度在网页和 CoddyKit 应用中同步。 PostgreSQL Performance & Query Optimization 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。

Welcome to Concurrency

When many users or applications access a database at the same time, it's called concurrency. PostgreSQL, like other databases, needs to manage these concurrent operations carefully.

Without proper management, multiple operations could try to modify the same data simultaneously, leading to inconsistencies or errors. This is where locks come in!

What are Database Locks?

A database lock is a mechanism that controls access to data by multiple transactions. It ensures that data remains consistent and prevents conflicts.

  • Data Integrity: Guarantees that data is accurate and reliable.
  • Consistency: Ensures that transactions see a consistent view of the database.
  • Conflict Prevention: Stops different operations from corrupting data by trying to write to the same place at the same time.

PostgreSQL Lock Modes

PostgreSQL uses various lock modes to define the level of access transactions have to a particular resource. These modes determine compatibility: some allow shared access, while others are exclusive.

  • ACCESS SHARE: Acquired by simple SELECT statements. Allows other transactions to also acquire ACCESS SHARE, ROW SHARE, etc.
  • ROW EXCLUSIVE: Acquired by INSERT, UPDATE, DELETE. Allows concurrent reads but prevents other transactions from acquiring ROW EXCLUSIVE on the same row.
  • ACCESS EXCLUSIVE: The most restrictive lock. Acquired by DROP TABLE or TRUNCATE. Prevents all other access to the table.

Implicit Locks in Action

Most of the time, PostgreSQL automatically acquires the necessary locks for you. These are called implicit locks.

For example:

  • When you run a SELECT query, an ACCESS SHARE lock is acquired on the table.
  • When you run an UPDATE, INSERT, or DELETE query, a ROW EXCLUSIVE lock is acquired on the affected rows and a corresponding table-level lock.

This automatic locking ensures data consistency without you needing to explicitly manage it.

Explicit Table Locks

While implicit locks handle most cases, you can also acquire locks explicitly using the LOCK TABLE command. This is useful for specific scenarios where you need to control access more precisely.

You can specify the lock mode, like ACCESS EXCLUSIVE to block all other operations, or SHARE for concurrent index creation.

Demo: Locking a Table

Try running this example. It creates a simple table and then explicitly locks it in ACCESS EXCLUSIVE mode. While this lock is held (for the duration of the transaction), no other transaction can read or write to this table.

CREATE TABLE IF NOT EXISTS inventory (
    item_id SERIAL PRIMARY KEY,
    item_name VARCHAR(100),
    quantity INT
);
TRUNCATE TABLE inventory;
INSERT INTO inventory (item_name, quantity) VALUES ('Widget A', 50);

-- This command acquires an ACCESS EXCLUSIVE lock on the 'inventory' table.
-- In a real scenario, this would block other operations until committed.
LOCK TABLE inventory IN ACCESS EXCLUSIVE MODE;

SELECT 'Table locked successfully!' AS status;

-- The lock is released when this script finishes (transaction commits).

Explicit Row Locks: SELECT FOR UPDATE

For fine-grained control, you can lock specific rows using SELECT FOR UPDATE or SELECT FOR SHARE. This is crucial for preventing race conditions in application logic.

  • SELECT ... FOR UPDATE: Acquires a ROW EXCLUSIVE lock on selected rows. Other transactions can read but cannot update or lock these rows for update until your transaction commits.
  • SELECT ... FOR SHARE: Acquires a ROW SHARE lock. Other transactions can also acquire ROW SHARE locks, but not ROW EXCLUSIVE.

Spotting Active Locks with pg_locks

To see what locks are currently active in your database, you can query the pg_locks system view. This view provides detailed information about each lock.

Key columns to look for:

  • pid: The process ID holding or waiting for the lock.
  • locktype: Type of resource being locked (e.g., relation, transactionid).
  • mode: The lock mode (e.g., ACCESS SHARE, ROW EXCLUSIVE).
  • granted: t if the lock is held, f if waiting.

You can query it like this:

SELECT * FROM pg_locks WHERE NOT granted;

Understanding Deadlocks

A deadlock occurs when two or more transactions are waiting for each other to release a resource, resulting in a standstill. Neither transaction can proceed.

PostgreSQL automatically detects deadlocks and aborts one of the transactions (the 'deadlock victim') to allow the other to complete. This usually results in an error message for the aborted transaction.

A Classic Deadlock Scenario

Imagine two transactions, T1 and T2, trying to update two rows, Row A and Row B:

  • T1: Locks Row A, then tries to lock Row B.
  • T2: Locks Row B, then tries to lock Row A.

At this point, T1 holds A and waits for B, while T2 holds B and waits for A. Neither can progress. PostgreSQL detects this cycle and terminates one transaction.

Lock Mode Check

Which lock mode is typically acquired by a simple SELECT statement?

Recap: Locks & Deadlocks

We've covered the essentials of database locks and deadlocks in PostgreSQL. Locks ensure data consistency and integrity in concurrent environments, operating through various lock modes.

Remember:

  • PostgreSQL uses implicit and explicit locks.
  • pg_locks helps monitor active locks.
  • Deadlocks occur when transactions wait for each other, and PostgreSQL detects and resolves them by aborting one transaction.

Understanding these concepts is key to building robust and performant applications!

常见问题解答

「了解锁与死锁」课时是免费的吗？

是的 — 「了解锁与死锁」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 PostgreSQL Performance & Query Optimization 课程的其余内容，请升级到 CoddyKit PRO。 PostgreSQL Performance & Query Optimization 课程共包含 4 节课。

「了解锁与死锁」这节课中我会学到什么？

学习 PostgreSQL 中不同的锁类型，以及如何识别和避免死锁。 你通过在浏览器中直接运行的动手代码来练习 PostgreSQL Performance & Query Optimization，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 PostgreSQL Performance & Query Optimization 需要有经验吗？

无需任何先前经验。CoddyKit 上的 PostgreSQL Performance & Query Optimization 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 1 节课，共 4 节。

「了解锁与死锁」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 PostgreSQL Performance & Query Optimization 课中编写并运行代码吗？

能。每节 PostgreSQL Performance & Query Optimization 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。

此课程中的所有课时

  1. 了解锁与死锁
  2. 识别并解决锁竞争
  3. 行级锁定策略
  4. 使用建议锁协调应用任务
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