Intermediate~3h
Query Optimization in SQL & DatabasesInterview Readiness Check
EXPLAIN ANALYZE, N+1 queries, cursor vs offset pagination, hash join vs nested loop.
explainquery plann+1paginationoffsetcursorhash joinnested loop
Learning path
Try a sample question
EXPLAIN ANALYZE on a query shows 'Seq Scan on orders (cost=0.00..18450.00 rows=800000)'. When is a sequential scan NOT a problem?
A.Never — a sequential scan on any table larger than 1000 rows always indicates a missing index
B.Only when the table has no primary key defined
C.When the query reads a large fraction of the table — the planner correctly avoids index overhead when most rows will be returned anyway
D.Sequential scans are always bad; EXPLAIN just shows them when query planning fails
Sign up free to take the full check and see the answer with an explanation.
What this module tests
- Conceptual understanding of query optimization fundamentals
- Trade-offs and design decisions in real system scenarios
- explain and query plan — applied knowledge
- Interview-style reasoning and explanation quality
Free to start
Take the Query Optimization check →
10 questions · Dimension scoring · AI coach feedback · Shareable result
Test your knowledge — free →No credit card · Takes 10–15 min
Frequently asked questions
- What does the Query Optimization check test?
- The check covers explain analyze, n+1 queries, cursor vs offset pagination, hash join vs nested loop. Questions span beginner to advanced difficulty and test both conceptual understanding and practical trade-off reasoning.
- What keywords should I know for Query Optimization?
- Key concepts include: explain, query plan, n+1, pagination, offset, cursor, hash join, nested loop. These frequently appear in sql & databases interviews at product and infrastructure companies.
- How many questions are in the check?
- Each check has 10 questions — a mix of MCQ and short-answer. You get scored by dimension (e.g., scalability, consistency) so you know exactly where to focus.
- How long does it take to study Query Optimization?
- Most engineers spend around 3 hours on this module. The check itself takes about 10–15 minutes. You can retake it up to 3 times per day.
- What prerequisite knowledge do I need before Query Optimization?
- Query Optimization builds on Indexes & Access Patterns. Complete those first for the best results.
- Is explain, query plan, n+1 covered?
- Yes. The Query Optimization module covers explain, query plan, n+1 among other key concepts. The check includes questions that reflect real sql & databases interview patterns.