Writing Faster SQL: Indexes, EXPLAIN, and Query Optimization
Most slow queries have a simple fix. Here's how to find it.
EXPLAIN ANALYZE: Your First Tool
EXPLAIN ANALYZE
SELECT u.name, 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;
Look for:
- Seq Scan on large tables — needs an index
- Hash Join vs Index Scan — hash joins are fine for small tables
- rows=10000 vs actual rows=3 — bad statistics, run ANALYZE
The Right Indexes
-- Index columns used in WHERE clauses
CREATE INDEX idx_users_created_at ON users(created_at);
-- Composite index: column order matters
-- Matches WHERE status = 'active' AND created_at > ...
CREATE INDEX idx_orders_status_created ON orders(status, created_at);
-- Partial index: only index rows you actually query
CREATE INDEX idx_active_users ON users(email) WHERE status = 'active';
-- Index for LIKE prefix searches
CREATE INDEX idx_users_name_text ON users USING gin(name gin_trgm_ops);
N+1 Queries: The Most Common Performance Bug
// Bad: 1 query for users + 1 query per user = N+1
const users = await db.users.findMany();
for (const user of users) {
user.orders = await db.orders.findMany({ where: { userId: user.id } });
}
// Good: 2 queries total with Prisma include
const users = await db.users.findMany({
include: { orders: true },
});
Pagination: Cursor vs Offset
-- Offset pagination gets slower as page number increases
SELECT * FROM posts ORDER BY created_at DESC LIMIT 20 OFFSET 10000;
-- Cursor pagination is consistently fast
SELECT * FROM posts
WHERE created_at < $cursor
ORDER BY created_at DESC
LIMIT 20;
Connection Overhead
Queries that touch the DB many times per request add up fast. Batch operations where possible:
// Bad: N database round-trips
for (const id of ids) await db.items.findUnique({ where: { id } });
// Good: 1 round-trip
await db.items.findMany({ where: { id: { in: ids } } });
SQL optimization, Prisma patterns, and DB performance monitoring are part of the production stack in the AI SaaS Starter Kit.
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