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Zhongwei Li
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skills/optimizing-query-selection/SKILL.md
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skills/optimizing-query-selection/SKILL.md
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---
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name: optimizing-query-selection
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description: Optimize queries by selecting only required fields and avoiding N+1 problems. Use when writing queries with relations or large result sets.
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allowed-tools: Read, Write, Edit
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version: 1.0.0
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---
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# Query Select Optimization
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Optimize Prisma 6 queries through selective field loading and relation batching to prevent N+1 problems and reduce data transfer.
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---
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<role>
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Optimize Prisma 6 queries by selecting required fields only, properly loading relations to prevent N+1 problems while minimizing data transfer and memory usage.
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</role>
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<when-to-activate>
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- Writing user-facing data queries
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- Loading models with relations
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- Building API endpoints or GraphQL resolvers
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- Optimizing slow queries; reducing database load
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- Working with large result sets
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</when-to-activate>
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<workflow>
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## Optimization Workflow
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1. **Identify:** Determine required fields, relations to load, relation count needs, full vs. specific fields
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2. **Choose:** `include` (prototyping, most fields) vs. `select` (production, API responses, performance-critical)
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3. **Implement:** Use `select` for precise control, nest relations with `select`, use `_count` instead of loading all records, limit relation results with `take`
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4. **Index:** Fields in `where` clauses, `orderBy` fields, composite indexes for filtered relations
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5. **Validate:** Enable query logging for single-query verification, test with realistic data volumes, measure payload size and query duration
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</workflow>
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<core-principles>
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## Core Principles
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### 1. Select Only Required Fields
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**Problem:** Fetching entire models wastes bandwidth and memory
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```typescript
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const users = await prisma.user.findMany()
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```
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**Solution:** Use `select` to fetch only needed fields
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```typescript
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const users = await prisma.user.findMany({
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select: {
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id: true,
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email: true,
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name: true,
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},
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})
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```
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**Performance Impact:**
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- Reduces data transfer by 60-90% for models with many fields
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- Faster JSON serialization
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- Lower memory usage
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- Excludes sensitive fields by default
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### 2. Include vs Select
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**Include:** Adds relations to full model
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```typescript
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const user = await prisma.user.findUnique({
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where: { id: 1 },
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include: {
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posts: true,
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profile: true,
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},
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})
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```
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**Select:** Precise control over all fields
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```typescript
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const user = await prisma.user.findUnique({
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where: { id: 1 },
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select: {
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id: true,
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email: true,
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posts: {
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select: {
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id: true,
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title: true,
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published: true,
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},
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},
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profile: {
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select: {
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bio: true,
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avatar: true,
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},
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},
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},
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})
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```
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**When to Use:**
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- `include`: Quick prototyping, need most fields
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- `select`: Production code, API responses, performance-critical paths
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### 3. Preventing N+1 Queries
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**N+1 Problem:** Separate query for each relation
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```typescript
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const posts = await prisma.post.findMany()
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for (const post of posts) {
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const author = await prisma.user.findUnique({
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where: { id: post.authorId },
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})
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}
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```
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**Solution:** Use `include` or `select` with relations
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```typescript
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const posts = await prisma.post.findMany({
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include: {
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author: true,
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},
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})
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```
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**Better:** Select only needed author fields
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```typescript
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const posts = await prisma.post.findMany({
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select: {
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id: true,
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title: true,
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content: true,
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author: {
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select: {
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id: true,
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name: true,
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email: true,
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},
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},
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},
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})
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```
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### 4. Relation Counting
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**Problem:** Loading all relations just to count them
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```typescript
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const user = await prisma.user.findUnique({
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where: { id: 1 },
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include: {
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posts: true,
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},
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})
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const postCount = user.posts.length
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```
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**Solution:** Use `_count` for efficient aggregation
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```typescript
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const user = await prisma.user.findUnique({
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where: { id: 1 },
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select: {
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id: true,
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name: true,
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_count: {
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select: {
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posts: true,
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comments: true,
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},
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},
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},
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})
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```
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**Result:**
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```typescript
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{
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id: 1,
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name: "Alice",
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_count: {
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posts: 42,
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comments: 128
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}
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}
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```
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</core-principles>
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<quick-reference>
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## Quick Reference
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### Optimized Query Pattern
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```typescript
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const optimized = await prisma.model.findMany({
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where: {},
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select: {
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field1: true,
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field2: true,
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relation: {
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select: {
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field: true,
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},
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take: 10,
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},
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_count: {
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select: {
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relation: true,
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},
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},
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},
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orderBy: { field: 'desc' },
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take: 20,
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skip: 0,
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})
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```
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### Key Takeaways
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- Default to `select` for all production queries
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- Use `include` only for prototyping
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- Always use `_count` for counting relations
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- Combine selection with filtering and pagination
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- Prevent N+1 by loading relations upfront
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- Select minimal fields for list views, more for detail views
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</quick-reference>
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<constraints>
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## Constraints and Guidelines
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**MUST:**
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- Use `select` for all API responses
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- Load relations in same query (prevent N+1)
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- Use `_count` for relation counts
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- Add indexes for filtered/ordered fields
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- Test with realistic data volumes
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**SHOULD:**
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- Limit relation results with `take`
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- Create reusable selection objects
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- Enable query logging during development
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- Measure performance improvements
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- Document selection patterns
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**NEVER:**
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- Use `include` in production without field selection
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- Load relations in loops (N+1)
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- Fetch full models when only counts needed
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- Over-fetch nested relations
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- Skip indexes on commonly queried fields
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</constraints>
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---
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## References
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For detailed patterns and examples, see:
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- [Nested Selection Patterns](./references/nested-selection.md) - Deep relation hierarchies and complex selections
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- [API Optimization Patterns](./references/api-optimization.md) - List vs detail views, pagination with select
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- [N+1 Prevention Guide](./references/n-plus-one-prevention.md) - Detailed anti-patterns and solutions
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- [Type Safety Guide](./references/type-safety.md) - TypeScript types and reusable selection objects
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- [Performance Verification](./references/performance-verification.md) - Testing and validation techniques
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