query-optimization

Analyze and optimize database queries: EXPLAIN ANALYZE plan reading, index strategies (B-tree, GIN, GiST, partial, composite), N+1 detection in ORM code, connection pooling, and query plan regression testing.

SKILL.md

Quality

Evals

Security

Query Optimization

Name: query-optimization
Author: JaviMontano

"The fastest query is the one you never have to run — the second fastest is the one with a covering index."

TL;DR

Five-step procedure to systematically identify, analyze, and optimize slow queries: capturing slow query logs, reading EXPLAIN ANALYZE output, choosing the right index type, detecting N+1 patterns in ORM code, and preventing regressions.

Step 1 — Capture Slow Queries

Enable Slow Query Logging (PostgreSQL)

-- postgresql.conf or ALTER SYSTEM
ALTER SYSTEM SET log_min_duration_statement = '100';  -- Log queries > 100ms
ALTER SYSTEM SET log_line_prefix = '%t [%p]: [%l-1] db=%d,user=%u,app=%a,client=%h ';
SELECT pg_reload_conf();

-- pg_stat_statements — aggregate query stats
CREATE EXTENSION IF NOT EXISTS pg_stat_statements;

-- Top 10 slowest queries by total time
SELECT
  round(total_exec_time::numeric, 2) AS total_ms,
  round(mean_exec_time::numeric, 2) AS avg_ms,
  calls,
  round(stddev_exec_time::numeric, 2) AS stddev_ms,
  rows,
  left(query, 100) AS query_snippet
FROM pg_stat_statements
ORDER BY total_exec_time DESC
LIMIT 10;

Identify Problematic Queries in Application Code

// Prisma — enable query logging
const db = new PrismaClient({
  log: [
    { level: 'query', emit: 'event' },
    { level: 'warn', emit: 'stdout' },
  ],
});

// Log all queries in development
db.$on('query', (e) => {
  if (e.duration > 100) {
    console.warn(`Slow query (${e.duration}ms):`, e.query);
  }
});

Step 2 — Read EXPLAIN ANALYZE Output

-- Always use EXPLAIN (ANALYZE, BUFFERS, FORMAT TEXT)
EXPLAIN (ANALYZE, BUFFERS, FORMAT TEXT)
SELECT u.id, u.email, COUNT(o.id) as order_count
FROM users u
LEFT JOIN orders o ON o.user_id = u.id
WHERE u.created_at > '2024-01-01'
GROUP BY u.id, u.email
ORDER BY order_count DESC
LIMIT 20;

Reading the Plan — Key Signals

Signal	Meaning	Fix
`Seq Scan` on large table	No usable index	Add index on filter column
`cost=0.00..50000.00` high cost	Planner estimate is expensive	Index or rewrite
`rows=1 actual rows=100000`	Bad row estimate	ANALYZE table, update statistics
`Hash Join` vs `Nested Loop`	Hash better for large sets; Nested Loop better for small	Usually correct, verify row estimates
`Buffers: shared hit=0 read=50000`	All reads from disk	Increase `shared_buffers`, add index
`Sort Method: external merge Disk`	Sort spills to disk	Increase `work_mem`, add index for ORDER BY

Example: Identifying a Missing Index

-- Bad plan: Seq Scan on 10M row table
Seq Scan on orders  (cost=0.00..285432.00 rows=1234 width=8)
                    (actual time=0.052..2847.123 rows=1234 loops=1)
  Filter: (user_id = 42 AND status = 'pending')
  Rows Removed by Filter: 9998766

-- Fix: composite index
CREATE INDEX CONCURRENTLY idx_orders_user_status
  ON orders(user_id, status)
  WHERE status IN ('pending', 'processing');  -- Partial index reduces size

-- After index: Index Scan
Index Scan using idx_orders_user_status on orders
  (cost=0.56..12.34 rows=8 width=8)
  (actual time=0.023..0.041 rows=8 loops=1)
  Index Cond: (user_id = 42 AND status = 'pending')

Step 3 — Index Strategy Selection

Index Type Decision Framework

Use Case	Index Type	Example
Equality / range on scalar	B-tree (default)	`WHERE user_id = 42`, `WHERE created_at > ...`
Full-text search	GIN	`WHERE to_tsvector('english', body) @@ query`
JSONB field search	GIN	`WHERE metadata @> '{"type": "premium"}'`
Array contains	GIN	`WHERE tags @> ARRAY['typescript']`
Geospatial / PostGIS	GiST	`WHERE location <-> point '(0,0)' < 1`
Case-insensitive text	B-tree + `lower()` expression	`lower(email)`
Partial (subset of rows)	Partial B-tree	`WHERE deleted_at IS NULL`
Multi-column	Composite B-tree	`(user_id, created_at DESC)`

Composite Index Column Order Rule

-- Selectivity rule: most selective first, then range/sort
-- Query: WHERE user_id = ? AND status = ? ORDER BY created_at DESC

-- Correct order: equality columns first, range/sort column last
CREATE INDEX idx_orders_composite ON orders(user_id, status, created_at DESC);

-- Wrong: range column in the middle breaks index use for rest
-- CREATE INDEX bad_idx ON orders(user_id, created_at, status);

Covering Index (Index-Only Scans)

-- Include all selected columns to avoid heap fetches
CREATE INDEX idx_users_email_covering
  ON users(email)
  INCLUDE (id, name, created_at);

-- Query becomes Index Only Scan — zero heap access
SELECT id, name, created_at FROM users WHERE email = 'user@example.com';

Step 4 — N+1 Detection & Resolution

Detecting N+1 in ORM Code

// BAD — N+1: 1 query for users + N queries for each user's posts
const users = await db.user.findMany({ take: 100 });
for (const user of users) {
  const posts = await db.post.findMany({ where: { userId: user.id } });
  // 101 total queries
}

// GOOD — Eager loading with Prisma include
const users = await db.user.findMany({
  take: 100,
  include: {
    posts: {
      select: { id: true, title: true, publishedAt: true },
      where: { published: true },
      orderBy: { publishedAt: 'desc' },
      take: 5,
    },
  },
});
// 2 queries total: users + a JOIN or batched post fetch

Detecting N+1 with Query Counter

// middleware/query-counter.ts — development only
let queryCount = 0;
db.$use(async (params, next) => {
  queryCount++;
  const result = await next(params);
  return result;
});

// In tests
beforeEach(() => { queryCount = 0; });
afterEach(() => {
  if (queryCount > 10) {
    console.warn(`Potential N+1: ${queryCount} queries in this test`);
  }
});

Step 5 — Connection Pooling & Prevention

PgBouncer / Prisma Accelerate Setup

// prisma/schema.prisma
datasource db {
  provider  = "postgresql"
  url       = env("DATABASE_URL")
  // PgBouncer requires this for prepared statements
  directUrl = env("DIRECT_DATABASE_URL")
}

# .env
DATABASE_URL=postgresql://user:pass@pgbouncer-host:6432/mydb?pgbouncer=true
DIRECT_DATABASE_URL=postgresql://user:pass@postgres-host:5432/mydb

Query Plan Regression in CI

-- Store baseline plan for critical queries
-- Compare in CI to catch plan regressions
EXPLAIN (FORMAT JSON, ANALYZE)
SELECT ...
-- Parse JSON output, fail if "Seq Scan" appears on tables > 10k rows

Quality Criteria

No Seq Scan on tables > 10k rows — All filter columns have indexes.
EXPLAIN ANALYZE run before optimizing — No blind index creation; plan confirms the bottleneck.
N+1 patterns eliminated in ORM — Eager loading or DataLoader used for relationship traversal.
Composite index column order matches query — Equality columns before range/sort columns.
pg_stat_statements enabled — Slow query visibility in production.
ANALYZE run after bulk inserts — Planner statistics kept current for large data loads.
Connection pool configured — PgBouncer or application pool prevents connection storms.
Partial indexes for soft deletes — WHERE deleted_at IS NULL on most queries; index matches.

Anti-Patterns

Adding an index on every column — Index maintenance overhead slows writes; each index adds ~10-30% overhead per INSERT/UPDATE. Index only what's queried.
Creating indexes inside transactions — CREATE INDEX CONCURRENTLY cannot run inside a transaction; causes error or falls back to locking.
SELECT * in production queries — Fetches columns not needed; defeats covering indexes; increases network transfer.
ORM .findAll() without pagination — Loading entire tables into memory when the user sees 20 rows.
LIKE '%search%' on large text columns — Leading wildcard prevents B-tree use; use full-text search with GIN index and tsvector instead.
Ignoring row count estimates in EXPLAIN — A plan with rows=1 actual rows=50000 is a statistics problem that will return to different bad plans after VACUUM.
Not using EXPLAIN ANALYZE after adding an index — The planner might still choose a Seq Scan if statistics are stale or the table is small enough.

Repository: JaviMontano/mao-sovereign-architect
Path: skills/query-optimization/SKILL.md
Commit: 2699c1e

Last updated: 3 days ago
First committed: 6 months ago

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JaviMontano/jm-adk

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since Aug 28, 2026

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