# Database Query Performance

> Diagnose database latency with explain plans, index ownership, and query-shape review. Use when a query is slow, an index is missing, or scans and N+1 patterns appear.

- Skill: `hoangnguyen0403/database-query-performance` (Agent Skill, multi-file: 3 files)
- Install (CLI): `npx skillmds@latest add hoangnguyen0403/database-query-performance`
- Raw SKILL.md: https://api.skillmd.com/api/skills/hoangnguyen0403/database-query-performance/raw
- Safety review: pending (external: skill-scanner PASS, skillspector PASS)
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: HoangNguyen0403 (https://skillmd.com/u/hoangnguyen0403)
- Updated: 2026-09-09
- Page: https://skillmd.com/skills/hoangnguyen0403/database-query-performance

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# Database Query Performance

## **Priority: P0 (CRITICAL)**

Slow query work starts with the query shape and evidence, not with random index creation.

## Rules

- Capture the exact query, parameters, and access path.
- Run explain or equivalent plan inspection before adding indexes.
- Match indexes to filter, join, sort, and coverage needs.
- Fix N+1 at the query pattern or loading strategy, not by caching first.

## Verify

- [ ] Explain plan was reviewed.
- [ ] Proposed index maps to a named query path.
- [ ] Write cost of the index is acceptable.
- [ ] Pagination or batch loading avoids deep scans or N+1 loops.

## Anti-Patterns

- **No blind indexes**: every index needs a query owner.
- **No cache-first latency fix**: fix query shape before hiding it.
- **No ORM opacity**: inspect generated SQL when the ORM is involved.

## References

- [Framework Map](../references/framework-map.md)
- [Index Strategies](references/index-strategies.md)