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Advanced PostgreSQL: Indexing, Partitioning, Replication · 课时

逻辑解码与变更数据捕获

使用逻辑解码，将 PostgreSQL 中行级别的变更流式传输出去，用于 CDC 数据管道和事件驱动系统。

逻辑解码与变更数据捕获 是 CoddyKit 上的免费 Advanced PostgreSQL: Indexing, Partitioning, Replication 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 Advanced PostgreSQL: Indexing, Partitioning, Replication 学习路径的一部分，你的进度在网页和 CoddyKit 应用中同步。 Advanced PostgreSQL: Indexing, Partitioning, Replication 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。

What Is Logical Decoding

Logical decoding turns the write-ahead log into a readable stream of logical row changes. It is the foundation of both logical replication and external Change Data Capture (CDC).

Output Plugins

A plugin decides the format of the change stream. Built-in pgoutput powers logical replication; test_decoding emits human-readable text; tools like Debezium use their own.

Prerequisite: wal_level

Logical decoding needs wal_level = logical. Changing it requires a restart.

-- postgresql.conf
wal_level = logical

Creating a Logical Slot

A logical replication slot tracks how far a consumer has read and prevents WAL needed by it from being removed.

SELECT pg_create_logical_replication_slot(
  'cdc_slot', 'test_decoding');

Peeking at Changes

pg_logical_slot_peek_changes shows pending changes without advancing the slot, useful for inspection.

SELECT * FROM pg_logical_slot_peek_changes(
  'cdc_slot', NULL, NULL);

Consuming Changes

pg_logical_slot_get_changes reads and advances the slot, so those changes will not be returned again.

SELECT * FROM pg_logical_slot_get_changes(
  'cdc_slot', NULL, NULL);

Example Change Output

With test_decoding an INSERT appears as readable text describing the table and new column values, wrapped between BEGIN and COMMIT markers.

BEGIN 712
table public.users: INSERT: id[int]:1 name[text]:'Ada'
COMMIT 712

Slots Hold WAL

An inactive slot keeps WAL forever, which can fill the disk. Monitor and drop unused slots.

SELECT slot_name, active FROM pg_replication_slots;
SELECT pg_drop_replication_slot('cdc_slot');

CDC Pipelines

External CDC connectors stream from a slot into systems like Kafka, search indexes, or data warehouses, enabling near-real-time downstream updates.

At-least-once Delivery

Consumers must track the last confirmed LSN and handle possible re-delivery after a crash. Design downstream writes to be idempotent.

Replica Identity for CDC

To capture the full before image on updates and deletes, set REPLICA IDENTITY FULL on tables without a usable primary key.

ALTER TABLE orders REPLICA IDENTITY FULL;

Quick Check

Why must logical replication slots be monitored?

Recap

You learned logical decoding: set wal_level=logical, create a logical slot with an output plugin, peek or consume changes, and feed CDC pipelines. Monitor slots, design idempotent consumers, and set replica identity for full change images.

常见问题解答

「逻辑解码与变更数据捕获」课时是免费的吗？

是的 — 「逻辑解码与变更数据捕获」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 Advanced PostgreSQL: Indexing, Partitioning, Replication 课程的其余内容，请升级到 CoddyKit PRO。 Advanced PostgreSQL: Indexing, Partitioning, Replication 课程共包含 4 节课。

「逻辑解码与变更数据捕获」这节课中我会学到什么？

使用逻辑解码，将 PostgreSQL 中行级别的变更流式传输出去，用于 CDC 数据管道和事件驱动系统。 你通过在浏览器中直接运行的动手代码来练习 Advanced PostgreSQL: Indexing, Partitioning, Replication，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 Advanced PostgreSQL: Indexing, Partitioning, Replication 需要有经验吗？

无需任何先前经验。CoddyKit 上的 Advanced PostgreSQL: Indexing, Partitioning, Replication 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。

「逻辑解码与变更数据捕获」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 Advanced PostgreSQL: Indexing, Partitioning, Replication 课中编写并运行代码吗？

能。每节 Advanced PostgreSQL: Indexing, Partitioning, Replication 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. 级联复制
  2. 多主复制（BDR）
  3. 复制槽与 WAL 管理
  4. 逻辑解码与变更数据捕获
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