Query Optimizer

---
name: query-optimizer
description: |
  Analyzes and optimizes slow database queries.
  IMPORTANT: Activate this skill whenever the user says "query is slow",
  "optimize this query", "database performance", or "slow database".
  This skill MUST be used for all query optimization requests.
---

# Query Optimizer

Analyzes slow database queries, identifies the bottleneck, suggests indexes, rewrites inefficient patterns, and explains the optimization. Uses EXPLAIN output to verify improvements.

**Trigger:**

Say any of these to activate this skill:
- "this query is slow"
- "optimize this query"
- "database is slow"
- "Use skill: query optimizer"

**How It Works:**

1. Read the slow query and understand what it's trying to do
2. Check the table schema for existing indexes
3. Identify performance issues: missing indexes, N+1 patterns, full table scans, unnecessary JOINs
4. Run EXPLAIN (or EXPLAIN ANALYZE) on the query to see the execution plan
5. Suggest specific fixes: add indexes, rewrite subqueries as JOINs, add WHERE clauses, use pagination
6. Apply the optimization and re-run EXPLAIN to verify improvement
7. If using an ORM, show both the ORM code change and the resulting SQL

**Guidelines:**

- Always check EXPLAIN output before and after optimization
- Adding indexes speeds up reads but slows down writes; consider the tradeoff
- For N+1 queries, prefer eager loading or batched queries
- Avoid SELECT * in production; select only the columns needed
- Consider query caching for expensive, frequently-run queries

**Works With:**

- Best with: Claude Code CLI (can run queries and EXPLAIN)
- Also works: Cursor, Windsurf (share the query and schema)
- Expects: Access to the database or at minimum the schema and query

**Output Format:**

- Query analysis: what's slow and why
- EXPLAIN output (before)
- Optimization applied: index added, query rewritten, or both
- EXPLAIN output (after) showing the improvement