Logical replication basics in PostgreSQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

What is the primary purpose of logical replication in PostgreSQL?

Logical replication allows data to be copied between PostgreSQL servers. What is its main use?

A To replicate data changes at the table level for selective synchronization

B To compress data for faster network transfer

C To create physical backups of the database files

D To replicate the entire database cluster including configuration files

💡 Hint

Think about what logical replication copies: is it the whole database or parts of it?

Logical replication copies data changes at the table level, allowing selective replication of specific tables. It does not replicate the entire cluster or configuration files.

❓ query_result

intermediate

What rows are replicated by default in logical replication?

Given a publication created with CREATE PUBLICATION pub1 FOR TABLE employees;, which rows will be replicated to the subscriber?

PostgreSQL

CREATE PUBLICATION pub1 FOR TABLE employees;

A All rows existing before and after the publication creation

B All rows inserted, updated, or deleted after the publication is created

C Only rows inserted after the publication is created

D Only rows updated after the publication is created

💡 Hint

Think about whether logical replication copies existing data or just changes.

Logical replication replicates data changes (inserts, updates, deletes) that happen after the publication is created. It does not automatically copy existing rows.

📝 Syntax

advanced

Which command correctly creates a subscription to replicate from a publisher?

Choose the correct SQL command to create a subscription named sub1 connecting to a publisher at host=192.168.1.10 dbname=company for publication pub1.

A CREATE SUBSCRIPTION sub1 WITH CONNECTION 'host=192.168.1.10 dbname=company' PUBLICATION pub1;

B CREATE SUBSCRIPTION sub1 CONNECTION 'host=192.168.1.10 dbname=company' FOR PUBLICATION pub1;

C CREATE SUBSCRIPTION sub1 CONNECTION 'host=192.168.1.10 dbname=company' PUBLICATION pub1;

D CREATE SUBSCRIPTION sub1 CONNECTION 'host=192.168.1.10 dbname=company' TO PUBLICATION pub1;

💡 Hint

Check the exact syntax for the CREATE SUBSCRIPTION command in PostgreSQL.

The correct syntax uses CONNECTION followed by the connection string and PUBLICATION followed by the publication name without extra keywords.

❓ optimization

advanced

How to reduce replication lag in logical replication?

You notice replication lag between publisher and subscriber. Which approach helps reduce lag?

A Reduce the <code>max_replication_slots</code> to 1

B Increase the <code>wal_sender_timeout</code> to a higher value

C Disable the replication slot on the publisher

D Use synchronous_commit = 'remote_apply' on the publisher

💡 Hint

Consider how commit settings affect replication speed and confirmation.

Setting synchronous_commit to remote_apply ensures transactions wait until changes are applied on the subscriber, reducing lag but possibly increasing transaction latency.

🔧 Debug

expert

Why does the subscription fail with error 'could not find replication origin'?

A subscription fails to start with the error: ERROR: could not find replication origin. What is the most likely cause?

A The subscriber's replication slot was dropped or corrupted

B The publication does not exist on the publisher

C The connection string has wrong credentials

D The subscriber database is missing the table being replicated

💡 Hint

Think about what replication origin means and what happens if it is missing.

The error usually means the replication slot or origin on the subscriber side is missing or corrupted, causing the subscription to fail.

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