Query Optimization

Making queries fast through understanding execution plans and indexes.

Context

You are optimizing slow queries. Use EXPLAIN PLAN to understand execution.

Domain Context

• Execution Plan: How database executes query; cost is estimate
• Sequential Scan: Reads all rows; slow on large tables
• Index Scan: Uses index; much faster if selective
• Join Strategy: Nested loop, hash, sort-merge; tradeoffs
• Cardinality: Rows filtered at each step

Instructions

1. Identify Slow Queries: Slow logs, application monitoring
2. Get EXPLAIN PLAN: Understand how database executes query
3. Look for Scans: Sequential scan on large table = problem
4. Add Indexes: On columns in WHERE, JOIN, ORDER BY
5. Verify Join Order: Database should join most selective first
6. Rewrite if Needed: Sometimes different query is faster
7. Monitor: Re-check plan after schema changes

Anti-Patterns

• Optimizing without EXPLAIN PLAN; guessing is unreliable
• Adding indexes for every column; slows writes, wastes storage
• Not updating stats; optimizer makes bad decisions
• Complex queries database can't optimize; rewrite
• Indexes on queries that return too much (unconstrained)

Further Reading

• PostgreSQL EXPLAIN documentation
• MySQL EXPLAIN documentation
• SQL Antipatterns (Joe Celko), Chapter on queries