Why Logical replication basics in PostgreSQL? - Purpose & Use Cases

The Big Idea

What if your databases could talk and update each other instantly without you lifting a finger?

The Scenario

Imagine you have two separate databases on different servers, and you need to keep their data exactly the same. You try copying data manually by exporting and importing files every day.

The Problem

This manual copying is slow, can miss updates, and if you forget or make a mistake, the databases get out of sync. It's like copying a long list by hand and risking typos or missing lines.

The Solution

Logical replication automatically sends only the changes (like new or updated rows) from one database to another in real time. This keeps both databases synchronized without manual copying.

Before vs After

✗ Before

pg_dump source_db > dump.sql
psql target_db < dump.sql

✓ After

CREATE PUBLICATION mypub FOR ALL TABLES;
CREATE SUBSCRIPTION mysub CONNECTION 'host=source_host dbname=source_db user=replicator password=yourpassword' PUBLICATION mypub;

What It Enables

It enables continuous, automatic syncing of data across databases, making multi-location setups reliable and efficient.

Real Life Example

A company with offices in different cities uses logical replication to keep their sales database updated everywhere instantly, so all teams see the latest orders.

Key Takeaways

Manual data copying is slow and error-prone.

Logical replication sends only data changes automatically.

This keeps multiple databases in sync in real time.

Practice

1. What is the main purpose of logical replication in PostgreSQL?

easy

A. To copy data changes from specific tables between databases

B. To create a full backup of the database

C. To optimize query performance

D. To encrypt data during transfer

Solution

1. Step 1: Understand logical replication concept

   Logical replication copies only data changes from selected tables, not the entire database.

2. Step 2: Compare options with concept

   Encrypting data during transfer, creating a full backup of the database, and optimizing query performance are unrelated to logical replication.

3. Final Answer:

   To copy data changes from specific tables between databases -> Option A

4. Quick Check:

   Logical replication = Copy data changes [OK]

Hint: Logical replication copies changes, not full backups [OK]

Common Mistakes:

• Confusing logical replication with physical backup
• Thinking it copies entire database
• Assuming it improves query speed

2. Which SQL command is used to create a publication for logical replication?

easy

A. CREATE REPLICATION SLOT myslot LOGICAL;

B. CREATE SUBSCRIPTION mysub CONNECTION 'conninfo' PUBLICATION mypub;

C. CREATE PUBLICATION mypub FOR ALL TABLES;

D. CREATE DATABASE mydb WITH REPLICATION;

Solution

1. Step 1: Identify command for publication creation

   The command to create a publication is CREATE PUBLICATION followed by publication name and tables.

2. Step 2: Analyze options

   Only CREATE PUBLICATION mypub FOR ALL TABLES; creates a publication. CREATE SUBSCRIPTION mysub CONNECTION 'conninfo' PUBLICATION mypub; creates a subscription, CREATE REPLICATION SLOT myslot LOGICAL; creates a replication slot, and CREATE DATABASE mydb WITH REPLICATION; creates a database.

3. Final Answer:

   CREATE PUBLICATION mypub FOR ALL TABLES; -> Option C

4. Quick Check:

   Publication creation = CREATE PUBLICATION [OK]

Hint: Publication uses CREATE PUBLICATION, subscription uses CREATE SUBSCRIPTION [OK]

Common Mistakes:

• Mixing publication and subscription commands
• Using replication slot command for publication
• Confusing database creation with replication setup

3. Given the following commands on the publisher:

CREATE PUBLICATION mypub FOR TABLE customers;

And on the subscriber:

CREATE SUBSCRIPTION mysub CONNECTION 'host=source dbname=mydb user=replicator password=secret' PUBLICATION mypub;

What will happen when a new row is inserted into the customers table on the publisher?

medium

A. The new row will be replicated to the subscriber's customers table

B. The new row will not be replicated because subscription is missing

C. The entire customers table will be copied again

D. An error will occur because publications cannot replicate inserts

Solution

1. Step 1: Understand publication and subscription setup

   The publication includes the customers table, and the subscription connects to it, enabling replication of changes.

2. Step 2: Analyze effect of insert on replication

   Inserts on the published table are sent to the subscriber, so the new row will appear there.

3. Final Answer:

   The new row will be replicated to the subscriber's customers table -> Option A

4. Quick Check:

   Insert on published table = replicated row [OK]

Hint: Inserts on published tables replicate if subscription exists [OK]

Common Mistakes:

• Thinking subscription is missing when it is created
• Assuming full table copy on each insert
• Believing inserts are not replicated

4. You created a subscription but notice no data is replicating. Which of the following is a likely cause?

medium

A. Logical replication does not support inserts

B. The subscriber database is offline

C. You forgot to create a replication slot on the subscriber

D. The publication does not include the tables you want to replicate

Solution

1. Step 1: Check publication includes tables

   If the publication does not include the desired tables, no changes will be sent to the subscriber.

2. Step 2: Evaluate other options

   The subscriber database is offline is unlikely because subscription requires the subscriber to be online. Logical replication does not support inserts is false; inserts are supported. You forgot to create a replication slot on the subscriber is incorrect because replication slots are created on the publisher, not subscriber.

3. Final Answer:

   The publication does not include the tables you want to replicate -> Option D

4. Quick Check:

   Missing tables in publication = no replication [OK]

Hint: Ensure publication includes tables to replicate [OK]

Common Mistakes:

• Confusing replication slot location
• Assuming inserts are unsupported
• Ignoring publication table list

5. You want to replicate only changes from the orders table but exclude the order_logs table, which is large and not needed on the subscriber. How should you set up the publication?

hard

A. CREATE PUBLICATION mypub FOR TABLE orders, order_logs;

B. CREATE PUBLICATION mypub FOR TABLE orders;

C. CREATE PUBLICATION mypub FOR ALL TABLES;

D. CREATE PUBLICATION mypub FOR TABLE order_logs;

Solution

1. Step 1: Understand selective table replication

   To replicate only the orders table, the publication must include only that table.

2. Step 2: Analyze options for correct table inclusion

   CREATE PUBLICATION mypub FOR ALL TABLES; replicates all tables, including order_logs which is unwanted. CREATE PUBLICATION mypub FOR TABLE orders, order_logs; includes both tables. CREATE PUBLICATION mypub FOR TABLE order_logs; includes only order_logs, which is unwanted.

3. Final Answer:

   CREATE PUBLICATION mypub FOR TABLE orders; -> Option B

4. Quick Check:

   Include only needed tables in publication [OK]

Hint: List only needed tables in publication to exclude others [OK]

Common Mistakes:

• Using FOR ALL TABLES when exclusion is needed
• Including unwanted tables in publication
• Confusing publication and subscription roles