gh-djankies-claude-configs-prisma-6/skills/optimizing-query-performance/references/query-monitoring.md

Query Monitoring Guide

Enable Query Logging

Development:

const prisma = new PrismaClient({
  log: [
    { emit: 'event', level: 'query' },
    { emit: 'stdout', level: 'error' },
    { emit: 'stdout', level: 'warn' }
  ]
})

prisma.$on('query', (e) => {
  console.log('Query: ' + e.query)
  console.log('Duration: ' + e.duration + 'ms')
})

Production (structured logging):

const prisma = new PrismaClient({
  log: [{ emit: 'event', level: 'query' }]
})

prisma.$on('query', (e) => {
  if (e.duration > 1000) {
    logger.warn('Slow query detected', {
      query: e.query,
      duration: e.duration,
      params: e.params
    })
  }
})

Analyzing Slow Queries

Identify Patterns:

1. Queries without WHERE clause on large tables (full table scans)
2. Complex JOINs without indexes on foreign keys
3. ORDER BY on unindexed fields
4. Missing LIMIT on large result sets

Use Database EXPLAIN:

await prisma.$queryRaw`EXPLAIN ANALYZE
  SELECT * FROM "User"
  WHERE status = 'active'
  ORDER BY "createdAt" DESC
  LIMIT 20
`

Look for:

• "Seq Scan" (sequential scan) - needs index
• "Index Scan" - good
• High execution time relative to query complexity

Common Query Anti-Patterns

N+1 Problem:

const users = await prisma.user.findMany()

for (const user of users) {
  const orders = await prisma.order.findMany({
    where: { userId: user.id }
  })
}

Solution - Use include:

const users = await prisma.user.findMany({
  include: {
    orders: true
  }
})

Over-fetching:

const users = await prisma.user.findMany()

Fetches ALL fields for ALL users.

Solution - Select needed fields:

const users = await prisma.user.findMany({
  select: {
    id: true,
    email: true,
    name: true
  }
})

Offset Pagination on Large Datasets:

await prisma.user.findMany({
  skip: 50000,
  take: 20
})

Database must scan and skip 50,000 rows.

Solution - Cursor pagination:

await prisma.user.findMany({
  take: 20,
  cursor: { id: lastSeenId },
  skip: 1
})

Constant time regardless of page depth.