Top 50 PostgreSQL Interview Questions for Job Seekers

PostgreSQL is a powerful, open-source relational database management system (RDBMS) that is widely used in modern applications. If you’re preparing for a PostgreSQL-related job interview, you’ve come to the right place. This guide provides 50 essential PostgreSQL interview questions along with expert answers to help you succeed.. Here’s a comprehensive list of 50 interview questions and answers for PostgreSQL, ranging from basic to expert level. These questions cover different aspects of PostgreSQL, including setup, configuration, optimisation, SQL queries, and advanced features.

Beginner Level – Top 50 PostgreSQL interview questions

1. What is PostgreSQL?
   • PostgreSQL is an open-source, object-relational database management system (ORDBMS). It is known for its robustness, scalability, and support for advanced data types and indexing.
2. How do you create a database in PostgreSQL?
   • You can create a database using the following SQL command: CREATE DATABASE database_name;
3. What is the difference between SQL and PL/pgSQL?
   • SQL is the standard query language for accessing and managing databases. PL/pgSQL is PostgreSQL’s procedural language, allowing the use of control structures like loops and conditions in functions.
4. What are the different data types supported in PostgreSQL?
   • PostgreSQL supports several data types, including:
     • Integer types (e.g., INTEGER, BIGINT)
     • Character types (e.g., CHAR, TEXT, VARCHAR)
     • Date/Time types (e.g., DATE, TIMESTAMP)
     • Boolean, Arrays, JSON, and more.
5. How do you list all the tables in a PostgreSQL database?
   • Use the following command in psql: \dt
6. How can you connect to a PostgreSQL database using psql?
   • You can connect by running: psql -U username -d database_name
7. What is a primary key in PostgreSQL?
   • A primary key is a column or a set of columns that uniquely identifies each row in a table. It cannot contain NULL values.
8. What are indexes in PostgreSQL?
   • Indexes are database objects that speed up query retrieval. They are created on columns that are frequently used in WHERE clauses or as part of a join condition.
9. What is the difference between DELETE and TRUNCATE in PostgreSQL?
   • DELETE removes rows one by one and can have a WHERE clause. It also logs each row’s removal.
   • TRUNCATE removes all rows at once and is faster because it doesn’t log individual row deletions.
10. What is a foreign key?
    • A foreign key is a column or a group of columns that is used to establish a link between the data in two tables, ensuring referential integrity.

Intermediate Level – Top 50 PostgreSQL interview questions

11. What is the difference between INNER JOIN and OUTER JOIN?
    • INNER JOIN returns rows when there is a match in both tables.
    • OUTER JOIN returns all rows from one table and matching rows from the other. If there is no match, NULL values are returned for non-matching rows.
12. How can you find the size of a PostgreSQL database?
    • Use the following query: SELECT pg_size_pretty(pg_database_size('database_name'));
13. What is the purpose of VACUUM in PostgreSQL?
    • VACUUM reclaims storage occupied by dead tuples (rows) due to updates or deletes. It helps in maintaining performance by cleaning up outdated rows and preventing table bloat.
14. Explain the concept of “normalization” in PostgreSQL.
    • Normalization is the process of organizing data in a database to reduce redundancy and dependency by dividing large tables into smaller ones and defining relationships between them.
15. How do you add a new column to an existing table?
    • Use the following SQL: ALTER TABLE table_name ADD COLUMN column_name data_type;
16. What is a NULL value in PostgreSQL?
    • A NULL value represents the absence of data or an unknown value. It is different from an empty string or a zero value.
17. How do you perform a backup of a PostgreSQL database?
    • You can use the pg_dump command to back up a database: pg_dump -U username -F c database_name > backup_file.dump
18. What is EXPLAIN in PostgreSQL?
    • EXPLAIN provides information about the execution plan of a query, showing how PostgreSQL intends to execute the query, and is used to analyze and optimize queries.
19. What is the difference between VARCHAR and TEXT in PostgreSQL?
    • Both VARCHAR and TEXT can store variable-length strings, but VARCHAR can be limited to a specific length, whereas TEXT has no length limit.
20. What is a schema in PostgreSQL?
    • A schema is a namespace that contains database objects like tables, views, and indexes. It helps organize and manage database objects.

Advanced Level Top 50 PostgreSQL interview questions

21. What is the WITH clause in PostgreSQL?
    • The WITH clause is used to define common table expressions (CTEs), which are temporary result sets that can be referenced within a SELECT, INSERT, UPDATE, or DELETE query.
22. How can you improve the performance of a PostgreSQL database?
    • Performance can be improved by:
      • Proper indexing
      • Optimizing queries
      • Using connection pooling
      • Adjusting database configuration settings
      • Using VACUUM and ANALYZE
23. What are triggers in PostgreSQL?
    • Triggers are functions that are automatically invoked when certain events (INSERT, UPDATE, DELETE) occur on a table or view.
24. Explain table partitioning in PostgreSQL.
    • Table partitioning involves splitting a large table into smaller, more manageable pieces (partitions) based on a specified key (e.g., range, list). It can improve performance and manageability.
25. What is the difference between synchronous and asynchronous replication in PostgreSQL?
    • In synchronous replication, data must be written to the primary and all replicas before the transaction is considered committed.
    • In asynchronous replication, the primary only waits for the transaction to be written to its own disk before committing, with replication to the standby occurring later.
26. What is the pg_stat_activity view used for?
    • The pg_stat_activity view shows the current activity of all database connections, including the queries they are running and the session state.
27. How do you handle concurrency control in PostgreSQL?
    • PostgreSQL uses Multi-Version Concurrency Control (MVCC) to handle concurrency. This allows transactions to work without interfering with each other, ensuring data consistency.
28. What is a materialized view in PostgreSQL?
    • A materialized view is a view whose result is stored physically, and it can be refreshed periodically. Unlike regular views, materialized views do not re-query the database each time they are accessed.
29. What are the advantages of using PostgreSQL over other relational databases?
    • PostgreSQL is highly extensible, supports advanced data types (like JSON and arrays), provides strong ACID compliance, and is known for its performance and reliability.
30. How do you restore a PostgreSQL database from a backup?
    • Use the pg_restore command: pg_restore -U username -d database_name backup_file.dump

Expert Level – top 50 PostgreSQL interview questions

31. What is the role of the PostgreSQL query planner and optimizer?
    • The query planner and optimizer decide the most efficient way to execute a query by choosing the best execution plan based on factors like available indexes, join types, and data distribution.
32. Explain the concept of Serializable isolation level in PostgreSQL.
    • Serializable is the highest isolation level in PostgreSQL, ensuring that transactions are executed in such a way that the result is equivalent to a serial execution, i.e., no transactions will overlap in conflicting ways.
33. How do you implement high availability in PostgreSQL?
    • High availability can be achieved using:
      • Streaming replication
      • Synchronous replication
      • Hot standby
      • Failover clusters (e.g., with tools like Patroni or repmgr)
34. What is pg_bench and how is it used?
    • pg_bench is a benchmarking tool provided by PostgreSQL to measure the performance of a database under load.
35. What are some PostgreSQL extension modules you have worked with?
    • Extensions like PostGIS (for GIS data), pg_partman (for partitioning), and pg_stat_statements (for tracking query statistics) are commonly used to extend PostgreSQL functionality.
36. How do you optimize large join operations in PostgreSQL?
    • Use proper indexing on join columns, apply filters early, and consider breaking large queries into smaller subqueries or using EXPLAIN to analyze and optimize the execution plan.
37. Explain the difference between LO and BYTEA data types in PostgreSQL.
    • LO (Large Object) is used for storing large binary data in PostgreSQL, while BYTEA stores binary data directly in a column, but with size limits.
38. What are postgresql.conf and pg_hba.conf used for?
    • postgresql.conf is the configuration file for setting database parameters (e.g., memory, logging). pg_hba.conf is used for controlling client authentication and connection permissions.
39. How do you handle deadlocks in PostgreSQL?
    • Deadlocks are handled automatically by PostgreSQL. When a deadlock is detected, one of the transactions involved will be rolled back, allowing the others to proceed.
40. What are the advantages of using JSONB in PostgreSQL?
    • JSONB allows efficient storage and querying of JSON data, supporting indexing and full-text search operations, making it a better choice than plain JSON for many use cases.
41. How do you perform an in-place upgrade of PostgreSQL?
    • You can use the pg_upgrade utility for an in-place upgrade. This allows you to upgrade the data directory and binary files without needing to dump and restore data.
42. What are the differences between LEFT JOIN and RIGHT JOIN in PostgreSQL?
    • The LEFT JOIN returns all rows from the left table and matching rows from the right table, while the RIGHT JOIN does the reverse, returning all rows from the right table.
43. How do you implement partition pruning in PostgreSQL?
    • Partition pruning automatically skips partitions when they are not needed based on the query’s filter condition, improving performance by only scanning relevant partitions.
44. How do you handle migrations in PostgreSQL?
    • Migrations can be handled using tools like Flyway or Liquibase for version-controlled schema changes, or manually using ALTER TABLE, CREATE INDEX, etc.
45. What are some common performance tuning strategies in PostgreSQL?
    • Strategies include indexing frequently used columns, using EXPLAIN to analyze queries, optimizing joins, adjusting memory settings (e.g., shared_buffers), and avoiding unnecessary large transactions.
46. Explain the concept of WAL (Write-Ahead Logging).
    • WAL ensures that changes to the database are first written to a log file before being applied, ensuring data integrity and crash recovery.
47. How do you monitor PostgreSQL performance?
    • You can monitor performance using pg_stat_activity, pg_stat_user_indexes, pg_stat_io, and various third-party tools like pgBadger or Prometheus.
48. How do you scale PostgreSQL horizontally?
    • Horizontal scaling can be achieved using replication (master-slave), sharding, or read replicas. This requires distributing data across multiple nodes.
49. What is the role of pg_stat_statements?
    • pg_stat_statements tracks SQL queries that have been executed, helping identify slow or inefficient queries that need optimization.
50. How would you migrate data from MySQL to PostgreSQL?
    • Migration can be done by:
      • Using the pgloader tool or mysqldump for exporting MySQL data.
      • Converting MySQL-specific types to PostgreSQL equivalents.
      • Rewriting any MySQL-specific queries to PostgreSQL-compatible syntax.

These top 50 PostgreSQL interview questions cover key aspects of PostgreSQL, and answers will help you prepare for PostgreSQL-related interviews.

Pub/Sub Architecture in Flask Python