Initial commit

skills/optimizing-query-selection/references/n-plus-one-prevention.md

@@ -0,0 +1,112 @@
+# N+1 Prevention Guide
+
+## Anti-Patterns
+
+### Over-fetching
+
+**Problem:**
+```typescript
+const user = await prisma.user.findUnique({
+  where: { id },
+  include: {
+    posts: {
+      include: {
+        comments: {
+          include: {
+            author: true,
+          },
+        },
+      },
+    },
+  },
+})
+```
+
+**Issue:** Fetches thousands of records, massive data transfer
+
+**Fix:** Use select with limits
+
+```typescript
+const user = await prisma.user.findUnique({
+  where: { id },
+  select: {
+    id: true,
+    name: true,
+    posts: {
+      select: {
+        id: true,
+        title: true,
+        _count: {
+          select: {
+            comments: true,
+          },
+        },
+      },
+      take: 10,
+      orderBy: {
+        createdAt: 'desc',
+      },
+    },
+  },
+})
+```
+
+### Inconsistent Selection
+
+**Problem:**
+```typescript
+const posts = await prisma.post.findMany({
+  include: {
+    author: true,
+  },
+})
+```
+
+**Issue:** Full author object when only name needed
+
+**Fix:** Select specific fields
+
+```typescript
+const posts = await prisma.post.findMany({
+  select: {
+    id: true,
+    title: true,
+    author: {
+      select: {
+        name: true,
+      },
+    },
+  },
+})
+```
+
+### Selecting Then Filtering
+
+**Problem:**
+```typescript
+const users = await prisma.user.findMany()
+const activeUsers = users.filter(u => u.status === 'active')
+```
+
+**Issue:** Fetches all users, filters in application
+
+**Fix:** Filter in database
+
+```typescript
+const activeUsers = await prisma.user.findMany({
+  where: { status: 'active' },
+  select: {
+    id: true,
+    name: true,
+    email: true,
+  },
+})
+```
+
+## Prevention Strategies
+
+1. **Always load relations upfront** - Never query in loops
+2. **Use select with relations** - Don't fetch unnecessary fields
+3. **Add take limits** - Prevent accidental bulk loads
+4. **Use _count** - Don't load relations just to count
+5. **Test with realistic data** - N+1 only shows at scale