PostgreSQL Interview Questions 2026, 35 Q&A on MVCC, Indexes, and JSONB

PostgreSQL is the rising default for product backends, and candidates report its interviews probe deeper internals than MySQL: MVCC, advanced indexes, JSONB, and VACUUM. This guide compiles 35 questions from candidate-reported rounds and public preparation resources, each with SQL and the trade-off being tested.

The edge: Postgres rewards engineers who understand its concurrency model and its rich index and type system. Surface-level SQL is not enough.

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SQL and Types (Q1 to Q12)

Q1. What is PostgreSQL and what sets it apart?

Q2. What is MVCC in PostgreSQL?

Q3. What are the index types in PostgreSQL?

Knowing GIN for JSONB is candidate-reported as a differentiator.

Q4. What is JSONB and how does it differ from JSON?

SELECT data->>'name' FROM users WHERE data @> '{"active": true}';

Q5. When do you use JSONB vs normalized columns?

Q6. What are arrays in PostgreSQL?

Q7. What is a CTE (Common Table Expression)?

WITH recent AS (
  SELECT * FROM orders WHERE created_at > now() - interval '7 days'
)
SELECT user_id, count(*) FROM recent GROUP BY user_id;

Q8. What is a recursive CTE?

WITH RECURSIVE tree AS (
  SELECT id, parent_id FROM cat WHERE parent_id IS NULL
  UNION ALL
  SELECT c.id, c.parent_id FROM cat c JOIN tree t ON c.parent_id = t.id
)
SELECT * FROM tree;

Q9. What are window functions?

SELECT name, dept, salary,
  RANK() OVER (PARTITION BY dept ORDER BY salary DESC) AS rnk
FROM emp;

Q10. What is the difference between RANK, DENSE_RANK, and ROW_NUMBER?

Q11. What is the difference between UNION and UNION ALL?

Q12. What are the PostgreSQL constraints?

Concurrency and Performance (Q13 to Q24)

Q13. What are the isolation levels in PostgreSQL?

Q14. What is the difference between READ COMMITTED and REPEATABLE READ here?

Q15. What is VACUUM and why is it needed?

Q16. What is the difference between VACUUM and VACUUM FULL?

Q17. What is table bloat?

Q18. How do you read EXPLAIN ANALYZE?

Q19. Why might Postgres choose a sequential scan over an index?

Q20. What is a partial index?

CREATE INDEX idx_active ON users (email) WHERE active;

Q21. What is an expression index?

Q22. How do you paginate efficiently in PostgreSQL?

Q23. What is the difference between a materialized view and a view?

Q24. How does connection pooling work in Postgres?

Architecture and Scaling (Q25 to Q35)

Q25. How does replication work in PostgreSQL?

Q26. What is the WAL?

Q27. What is logical vs physical replication?

Q28. How do you do point-in-time recovery?

Q29. What is table partitioning in Postgres?

Q30. What is the difference between TRUNCATE and DELETE here?

Q31. What are sequences and SERIAL?

Q32. How do you prevent SQL injection in Postgres?

Q33. What causes high CPU or slow queries?

Q34. What is pg_stat_statements?

Q35. Scenario: updates have slowed dramatically over weeks. Diagnose.

PostgreSQL Mock Test, 2026 Edition

5 original questions calibrated to the 2026 PostgreSQL batch by Aditya Sharma, from candidate-reported patterns.

Question 1

The index type for JSONB and arrays is:

a) B-tree b) GIN c) Hash d) BRIN

Solution: GIN indexes inverted structures like JSONB and arrays. Answer: (b)

Question 2

VACUUM is needed because MVCC leaves:

a) locks b) dead tuples (old row versions) c) indexes d) WAL files

Solution: Dead tuples accumulate and must be reclaimed. Answer: (b)

Question 3

The default isolation level in Postgres is:

a) READ UNCOMMITTED b) READ COMMITTED c) REPEATABLE READ d) SERIALIZABLE

Solution: READ COMMITTED is the default. Answer: (b)

Question 4

To index WHERE lower(email) = ... you create:

a) a partial index b) an expression index on lower(email) c) a hash index d) a BRIN index

Solution: Expression indexes handle computed predicates. Answer: (b)

Question 5

DENSE_RANK differs from RANK by:

a) using random order b) not leaving gaps after ties c) being faster d) ignoring partitions

Solution: DENSE_RANK produces consecutive ranks with no gaps. Answer: (b)

FAQ, PostgreSQL Interview Questions

Q: How important is MVCC knowledge? Very. Candidate-reported Postgres rounds frequently probe MVCC, dead tuples, and VACUUM as a depth check.

Q: Do I need JSONB for a 2026 interview? Increasingly yes, especially when to use it and how to index it. Confirm the stack on the official company careers page.

Q: PostgreSQL or MySQL to learn? The SQL transfers; Postgres adds richer types and indexing. Many modern startups default to Postgres.

Q: What is the most-missed Postgres concept? Bloat and autovacuum, and why a seq scan was chosen over an index, per candidate-reported feedback.

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