Top PostgreSQL Interview Questions And Answers

Introduction

Looking for PostgreSQL interview questions? One of the key skills one should know while working with data is to know how to use relational databases. Structured Query Language or SQL is the go to tool for anyone trying to create or manage these databases. SQL is also behind popular systems like Microsoft SQL Server, MySQL, PostgreSQL, and SQLite. 

Since PostgreSQL is so popular, there’s a good chance the job you’re eyeing will expect you to know it. To help you out, we’ve put together some of the most common PostgreSQL interview questions and answers for all experience levels.

When you’re in an interview, it’s a good idea to keep your answers short and sweet for SQL performance tuning questions. Getting straight to the point can help you avoid any follow-up questions that might throw you off. But don’t stress—you’ll have plenty of chances to show off your skills during the interview process.

Ready to dive in our PostgreSQL interview questions? Let’s get started!

PostgreSQL Interview Questions For Beginners

Q1: What is PostgreSQL, and how is it different from other SQL databases?

A: PostgreSQL is an open-source database system that’s great for handling both traditional SQL data and more flexible JSON data. What makes PostgreSQL special is its advanced features like handling complex queries, foreign keys, triggers, and updatable views. It’s also super flexible because you can create custom types, functions, and operators.

Q2: What does a PostgreSQL partitioned table look like?

A: A partitioned table in PostgreSQL is like taking a big table and breaking it into smaller, more manageable pieces called partitions. This makes it easier to work with large amounts of data.

Q3: How can you completely delete a table?

A: To wipe out all the data in a table, you can use the TRUNCATE TABLE command in PostgreSQL. It’s like hitting a reset button for your table.

Q4: What is a Foreign Key in PostgreSQL?

A: A foreign key is a way to link data between two tables. It ensures that a value in one table matches a value in another, helping keep your data consistent. Here’s an example:

 CREATE TABLE departments (

    department_id SERIAL PRIMARY KEY,

    department_name VARCHAR(100)

);

CREATE TABLE employees (

    employee_id SERIAL PRIMARY KEY,

    name VARCHAR(100),

    department_id INT,

    FOREIGN KEY (department_id) REFERENCES departments(department_id)

);

Q5: What are the key advantages and disadvantages of PostgreSQL?

A: 

Advantages:

• PostgreSQL is free and open-source, so you can customize it to fit your needs.
• It’s easy to learn, so you can get started quickly.
• It’s great for running dynamic websites and applications.
• PostgreSQL is very reliable and perfect for large-scale projects.
• It doesn’t require a lot of maintenance, which is a plus for busy teams.
• It’s strong and powerful, making it ideal for big web applications.

Disadvantages:

• PostgreSQL doesn’t work with as many open-source apps as MySQL.
• Setting up replication (copying data to another server) can be tricky.
• It’s not as fast as some other databases.
• PostgreSQL isn’t backed by a single company, which could be a downside for some.
• It’s a bit slower than MySQL, and the installation process can be tough for beginners.

Q6: What is MVCC in PostgreSQL?

A: MVCC stands for Multi-Version Concurrency Control. It’s a method PostgreSQL uses to let multiple transactions happen at the same time without causing issues. It keeps multiple versions of data so that different transactions don’t interfere with each other.

Q7: How can you set up pgAdmin in PostgreSQL?

A: Here’s how to set up pgAdmin:

• Start pgAdmin 4.
• Go to the "Dashboard" tab, find the "Quick Link" section, and click "Add new Server."
• In the "Create-Server" window, select the "Connection" tab.
• Enter your server's IP address in the "Hostname/Address" field.
• Set the "Port" to "5432," which is the default for PostgreSQL.

Q8: What is a non-clustered index?

A: A non-clustered index is a type of index where the order of the index doesn’t match the order of the data in the table. Instead of holding the actual data, it points to where the data is stored, making it quicker to find what you’re looking for.

Q9: How do you query data from a table in PostgreSQL?

A: To get data from a table, use the SELECT statement. For example:

 SELECT * FROM employees;

Q10: What are CRUD operations in PostgreSQL?

A: CRUD stands for Create, Read, Update, and Delete. These are the basic actions you can perform on any database table.

Q11: What is Multi-Version Concurrency Control in PostgreSQL? Why is it used?

A: Multi-Version Concurrency Control (MVCC) is a method PostgreSQL uses to improve performance when many users are accessing the database at the same time. It avoids locking the database by keeping multiple versions of data, ensuring everything runs smoothly.

Q12: What is a schema in PostgreSQL?

A: A schema is like the blueprint of your database. In PostgreSQL, it includes tables, data types, views, indexes, sequences, constraints, and functions.

Q13: What is a Primary Key in PostgreSQL?

A: A primary key is a column (or set of columns) that uniquely identifies each row in a table. It makes sure every entry in the table is unique and not empty. For example:

CREATE TABLE employees (

    employee_id SERIAL PRIMARY KEY,

    name VARCHAR(100)

);

Q14: What is the base directory in PostgreSQL?

A: The base directory in PostgreSQL is data_dir/base. It’s a folder that holds all the subdirectories for your database clusters, storing all your data.

Q15: How do you create an index in PostgreSQL?

A: To create an index, use the CREATE INDEX statement. Indexes help speed up searches by pointing directly to where the data is stored. For example:

CREATE INDEX idx_employee_name ON employees(name);

PostgreSQL Interview Questions For Intermediate

Q16: What are Materialized Views in PostgreSQL?

A: A materialized view in PostgreSQL is like a special kind of table that stores the results of a query. Unlike regular views, which run the query every time you use them, materialized views save the results, making it faster to access the data. You can update these views manually or on a schedule, which is super helpful if you have complex queries that take a lot of time to run.

To create one, you use the CREATE MATERIALIZED VIEW command, specifying the query that defines what data should be in the view. They’re great for speeding up queries that don't need to update all the time.

Q17: What is Multi-Version Concurrency Control (MVCC) in PostgreSQL, and why is it important?

A: Multi-Version Concurrency Control (MVCC) in PostgreSQL lets multiple transactions happen at the same time without messing up the data. It works by creating a new version of a row each time a transaction modifies it, so other transactions can still see the old data. This prevents conflicts and ensures everything runs smoothly, even when lots of people are using the database.

Q18: What are the key differences between Oracle and PostgreSQL?

A: 

Oracle	PostgreSQL
Oracle is an object-relational database management system, known for being the first designed for grid computing. Written in C, C++, and assembly language. Developed in 1977 by Larry Ellison and Bob. Requires a license to use. Compatible with operating systems like OS X, Linux, Windows, z/OS, and more. Supports programming languages like C, C++, Java, Perl, .NET, JavaScript, PHP, and more.	PostgreSQL is a free, open-source database system that follows SQL standards and is highly extensible Written in C language. Developed in 1996 by the PostgreSQL Global Development Group. Free and open-source. Compatible with operating systems like Linux, Windows, Unix, and more. Provides good security support, though not as advanced as Oracle.

Q19: How do you provide security in PostgreSQL?

A: In PostgreSQL, security is handled at multiple levels:

• File Protection: Database files are protected so only the PostgreSQL superuser can access them.
• Client Connections: You can limit access by username or IP address.
• Local Unix Socket: By default, client connections to the server are only allowed through the local unix socket.
• External Authentication: You can authenticate client connections using external packages.
• User and Group Permissions: Each user has a username and password, and you can assign users to groups with specific table access.

Q20: What are tokens in PostgreSQL?

A: Tokens in PostgreSQL are pieces of a SQL query, like keywords, operators, and identifiers. These tokens help PostgreSQL understand what you’re asking it to do.

Q21: How do you back up and restore a PostgreSQL database?

A: To back up a PostgreSQL database, use the pg_dump utility. To restore it, use the psql utility. Here’s how:

pg_dump mydatabase > mydatabase_backup.sql
psql mydatabase < mydatabase>

Q22: What is a Composite Type in PostgreSQL?

A: A Composite Type in PostgreSQL lets you create custom data structures that can hold multiple types of values. It’s useful for grouping related data into a single entity. You create one using the CREATE TYPE statement. For example:

CREATE TYPE country_type AS (
    state VARCHAR,
    city VARCHAR,
    district VARCHAR
);

Q23: What is Write-Ahead Logging in PostgreSQL?

A: Write-ahead logging (WAL) is a technique used to ensure data integrity in PostgreSQL. It logs all changes before they’re applied to the database, which helps recover data in case of a crash. It ensures that no data is lost, and you can resume work from where it left off.

Q24: When should you use the “EXPLAIN ANALYZE” command in PostgreSQL?

A: Use the “EXPLAIN ANALYZE” command when you want to see the actual execution plan of a SQL statement and how long it takes. It’s helpful for identifying and fixing slow or complex queries.

Q25: What’s the downside of using the DROP TABLE command to delete data?

A: While the DROP TABLE command deletes all the data in a table, it also removes the table structure itself from the database. This means you’d need to recreate the table if you want to use it again.

Q26: What are Transactions in PostgreSQL, and how do they work?

A: Transactions in PostgreSQL are a way to group several operations into one unit of work. Either all the operations succeed (commit) or none of them do (rollback). This ensures your data stays consistent. Use the BEGIN, COMMIT, and ROLLBACK commands to manage transactions:

BEGIN;
UPDATE employees SET salary = 90000 WHERE name = 'John Doe';
COMMIT;

Q27: How can you improve query performance in PostgreSQL?

A: Here are some ways to make queries run faster in PostgreSQL:

• Indexing: Create indexes on columns used in WHERE clauses.
• Partitioning: Break large tables into smaller pieces.
• Efficient SQL Statements: Write SQL statements that avoid unnecessary processing, like selecting only the columns you need.
• Memory Tuning: Adjust server settings to make better use of your hardware.

Q28: What are the 4 main properties of a transaction in PostgreSQL? What acronym is used to describe them?

A: The four main properties of a transaction in PostgreSQL are Atomicity, Consistency, Isolation, and Durability. These are often referred to by the acronym ACID.

Q29: What is a string constant in PostgreSQL?

A: A string constant is a sequence of characters surrounded by single quotes, like 'example'. String constants are used when inserting data or passing characters to database objects. In PostgreSQL, you can use single quotes, and if you need to include a backslash, use a C-style backslash.

Q30: How does PostgreSQL handle concurrent updates?

A: PostgreSQL uses Multi-Version Concurrency Control (MVCC) to manage concurrent updates. This means that when two transactions try to update the same data, PostgreSQL creates separate versions for each transaction. This way, each transaction sees a consistent view of the data without interfering with others. Developers can choose the appropriate transaction isolation levels, like REPEATABLE READ or SERIALIZABLE, depending on their needs.

PostgreSQL Interview Questions For Advanced

Q31: What is the CTIDs field in PostgreSQL used for?

A: The CTIDs field helps identify the exact physical location of rows in a table. It points to where each row is stored based on its block and position within that block.

Q32: What are triggers in PostgreSQL?

A: Triggers are like automatic actions that the database performs when certain events happen, such as inserting, updating, or deleting data. They ensure that specific tasks are done at the right time, helping maintain data accuracy and integrity.

Q33: How do we implement replication in PostgreSQL?

A: PostgreSQL offers two main methods for replication to ensure data is always available and backed up:

• Physical Replication: This method creates an exact copy of the main database by continuously streaming changes to multiple backup servers, which can be used if the main server fails.
• Logical Replication: This method only replicates specific changes made to particular databases or tables, providing more flexibility but requiring careful setup and monitoring.

Q34: Can you briefly describe the history of PostgreSQL?

A: PostgreSQL started as part of the POSTGRES project in 1986 at the University of California, Berkeley. It supports various operating systems like macOS, Windows, Linux, and UNIX, and has been continually improved for over 30 years. PostgreSQL is known for its robust features and is widely used in the tech industry.

Q35: How do you implement row-level security in PostgreSQL?

A: Row-level security limits access to specific rows in a table based on set rules. To implement it:

• Enable the feature in the PostgreSQL configuration.
• Define security policies on the table.
• Create rules to determine which rows can be accessed or modified.
• Assign the correct privileges to users so they can work with these secure tables.

Q36: What are the pros and cons of using pg_dump for backups?

A: Pros:

• Portability: Backup files are easy to move and run on other PostgreSQL setups.
• Selective Backup: You can back up specific parts of the database as needed.
• Comprehensive: It backs up both data and the database structure.
• Automation: It integrates well with scripts for automated backups.

Cons:

• Performance Impact: Backups can slow down the database, especially large ones.
• Storage Space: Backups need storage space and time to transfer.
• Restoration Time: Restoring data can be slow, possibly causing downtime.
• Limited Parallelism: It uses a single thread, which can make the process longer.

Q37: How do we implement parallel query execution in PostgreSQL?

A: To speed up queries using parallel execution:

• Set the max_parallel_workers parameter to allow multiple workers for queries.
• Adjust the max_parallel_workers_per_gather setting to control how many workers each query can use.
• Set the minimum table or index sizes that qualify for parallel execution.
• You can also manually control parallel execution for specific queries if needed.

Q38: How do we implement sharding in PostgreSQL?

A: Sharding spreads data across multiple servers to handle large databases more efficiently:

• Set up separate PostgreSQL instances for each shard.
• Decide how to split data across these shards, like using hash-based or range-based methods.
• Update your application to direct data and queries to the correct shard.
• Define clear rules for how data is distributed.
• Make sure data stays consistent across all shards by using tools for replication and synchronization.

Q39: What new features were introduced in PostgreSQL 9.1?

A: PostgreSQL 9.1 brought several new features, including:

• Support for JSON data.
• Synchronous replication.
• Improved geographic search capabilities.
• External data connections (SQL/MED).
• Security labels and index-only access.

Q40: What are the three issues that PostgreSQL prevents between concurrent transactions?

A: PostgreSQL uses transaction isolation to avoid these issues:

• Dirty Read: Reading data from an uncommitted transaction.
• Non-repeatable Read: Re-reading data and finding it has been changed by another transaction.
• Phantom Read: Re-running a query and finding new rows that weren’t there before due to another transaction.

Q41: How many types of replication exist in PostgreSQL?

A: PostgreSQL has four main types of replication:

• Physical Replication: Includes Streaming Replication and File-based Replication.
• Logical Replication: Involves built-in replication tools and third-party solutions.
• Bi-Directional Replication (BDR)
• Custom Replication Solutions

Q42: How do you stop a PostgreSQL server, and can you stop just one database?

A: To stop a PostgreSQL server:

• Windows: Use the pg_ctl command or stop the service via services.msc.
• Linux: Use the command sudo service postgresql stop.
• macOS: Use the pg_ctl command to stop the server.
• Note: You cannot stop just one database in a PostgreSQL cluster; you must stop the entire server.

Q43: What are the differences between PostgreSQL and NoSQL databases?

A: Comparing PostgreSQL with NoSQL databases isn’t straightforward since NoSQL covers many different technologies. When choosing a database, consider features, scalability, reliability, and community support. It's common to use both relational and non-relational databases in large projects.

Q44: How do we create and manage user-defined functions in PostgreSQL?

CREATE FUNCTION function_name(arg1, arg2) RETURNS return_type AS $$

BEGIN
-- Function logic here
END;
$$

Call the function using SELECT function_name(arg1, arg2);.

Q45: Why do companies prefer PostgreSQL?

A: Companies like PostgreSQL for its flexibility, scalability, and cost-effectiveness. It allows for customization and handles large data volumes efficiently. As an open-source database, it’s also free from licensing fees, making it an attractive option for businesses.

Conclusion

Mastering PostgreSQL can significantly enhance your database management skills. Whether it's understanding CTIDs, triggers, or replication methods, these concepts are crucial for efficient database operations. If you're looking to deepen your knowledge and gain hands-on experience, Janbask Training offers comprehensive courses tailored to help you excel in PostgreSQL and other database technologies. With expert guidance and practical exercises, you'll be well-equipped to advance your career in this field.