# Query Rewriter

> SQL query rewriting expert. Identifies inefficient query patterns and provides optimized alternatives. Trigger when user mentions query optimization, SQL rewriting, or query performance.

- Skill: `huifer/query-rewriter` (Agent Skill, multi-file: 11 files)
- Install (CLI): `npx skillmds@latest add huifer/query-rewriter`
- Raw SKILL.md: https://api.skillmd.com/api/skills/huifer/query-rewriter/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Data & Analytics
- Author: huifer (https://skillmd.com/u/huifer)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/huifer/query-rewriter

---


# SQL Query Rewriting Expert

## Common Anti-Patterns

| Anti-Pattern | Problem | Reference |
|--------------|---------|-----------|
| SELECT * | Fetches unnecessary data | `references/select-star.md` |
| N+1 queries | Too many round trips | `references/n-plus-one.md` |
| IN with subquery | May process too many rows | `references/exists-vs-in.md` |
| OR across columns | Prevents index usage | `references/or-optimization.md` |
| OR conditions | Cannot use index | `references/union-instead-of-or.md` |
| Type mismatch | Implicit conversion | `references/implicit-conversion.md` |

## Audit Workflow

### Step 1: Identify Problem Patterns

Check for:
- SELECT * usage
- N+1 query patterns
- IN with large subqueries
- OR conditions across columns
- Type mismatches

### Step 2: Match Anti-Pattern

Identify which anti-pattern the query matches and apply the corresponding solution from the reference file.

### Step 3: Apply Rewrite

Follow the specific reference file for detailed instructions.

### Step 4: Verify Improvement

```sql
EXPLAIN ANALYZE <rewritten_query>;
```

## Benchmark Reference

| Optimization | Before | After | Improvement |
|--------------|--------|-------|-------------|
| SELECT * → specific | 100ms | 20ms | 80% |
| N+1 → batch | 500ms | 50ms | 90% |
| IN → EXISTS | 500ms | 50ms | 90% |
| OR → UNION | 200ms | 20ms | 90% |
| Type fix | 150ms | 2ms | 98.7% |

## Output Format

```
## Query Rewriting Report

### 1. Original Query
```sql
<original_query>
```

### 2. Anti-Patterns Detected
| Pattern | Problem | Reference |
|---------|---------|-----------|
| ... | ... | ... |

### 3. Recommended Rewrite
```sql
<optimized_query>
```

### 4. Expected Improvement
- Query time: Xms → Yms (Z% faster)
- Rows processed: X → Y (Z% reduction)

### 5. Implementation
Reference: references/<case-name>.md
```