Tutorials System Design Tutorial
Query Optimization for Heavy Workloads — Complete Guide
Query Optimization for Heavy Workloads — Complete Guide: free step-by-step lesson with examples, common mistakes, and interview tips — part of System Design Tutorial on Toolliyo Academy.
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System Design Tutorial · Lesson 27 of 100
Query Optimization for Heavy Workloads
Basics → Scale → Interview
Basics · 1 — Building blocks · ~6 min · Module 3: Database Systems
What is this?
Query optimization reshapes SQL/access patterns — selective filters, fewer columns, better joins, avoiding N+1 — so the database does less work.
Why should you care?
One bad ShopNest report can steal I/O from checkout.
See it live — copy this example
Sketch the architecture on paper. These lessons focus on concepts and trade-offs.
Bad: SELECT * FROM Orders WHERE YEAR(createdAt)=2026
Good: SELECT orderId, total FROM Orders
WHERE createdAt >= '2026-01-01' AND createdAt < '2027-01-01'
Also: batch status lookups instead of N+1 per order lineRun Example »
This lesson uses terminal or setup steps. Run commands on your computer — the live editor appears on coding lessons.
What happened?
- Sargable date ranges use indexes.
- Narrow selects reduce I/O.
- App-side N+1 multiplies round trips — fix with joins or IN batches.
Practice next
- Rewrite a YEAR() filter to a range.
- Find one N+1 in a ShopNest API and batch it.
- Move heavy reports off the primary.
- Add EXPLAIN/plan review to the PR checklist.
- Paginate with keyset instead of deep OFFSET.
Remember
Make predicates sargable. Kill N+1 patterns. Isolate analytics from OLTP.
Report no longer melts primary
ShopNest moves GMV report to replica + sargable SQL.
Outcome: Checkout CPU stabilizes at noon.
Interview prep for this lesson
Practice these questions aloud after reading—each links to a full structured answer.
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