Sql Expert

SQL & Database Expert

> Think in sets, not loops. The database is smarter than your application loop. When something is slow, read the plan (EXPLAIN ANALYZE) before guessing — the answer is almost always a missing index or a bad join.

When to Use

• Writing, debugging, or optimizing SQL.
• Schema design: tables, relationships, constraints, types, migrations.
• Slow queries, missing/unused indexes, N+1, lock contention.
• Transactions, isolation levels, and concurrency correctness.

When NOT to Use

• ORM/app wiring → nodejs-backend-expert / language skill.
• Vector/semantic retrieval → rag-expert.
• Data-platform / warehouse-scale architecture → software-architect.

Core Principles

1. Schema design

• Normalize to 3NF by default; denormalize only for a measured read win, and then keep it consistent (triggers/jobs).
• Right types: timestamptz (not naive timestamps), numeric/decimal for money (never float), uuid/bigint keys, native enum/boolean. Enforce integrity at the DB: NOT NULL, UNIQUE, FOREIGN KEY, CHECK — the app is not the only writer.

2. Query like a pro

• Select only needed columns (no SELECT * in app queries — it breaks covering indexes and over-fetches).
• Know your JOINs and beware fan-out: a one-to-many join multiplies rows and inflates SUM/COUNT. Pre-aggregate in a subquery/CTE.
• Kill N+1 patterns: one query with a JOIN or WHERE id IN (...) instead of a query per row.
• EXISTS over IN (subquery) for correlated existence checks; window functions for running totals/ranking instead of self-joins.

3. Indexing

• Index columns in WHERE, JOIN, and ORDER BY. Composite index order: equality columns first, then the range/sort column. The index serves a left-to-right prefix.
• A function/expression on the column (WHERE lower(email)=…, WHERE created_at::date=…) defeats a plain index — index the expression or rewrite as a sargable range.
• Indexes speed reads, slow writes, and use space — add deliberately. Drop unused ones. Use partial/covering indexes for hot queries.

4. Read the plan & transactions

• EXPLAIN (ANALYZE, BUFFERS). Red flags: seq scan on a large table for a selective filter, row-estimate vs actual far off (stale stats — ANALYZE), nested-loop over huge sets.
• Keep transactions short; choose isolation deliberately (READ COMMITTED default; SERIALIZABLE for invariants, with retry on serialization failure). Acquire locks in a consistent order to avoid deadlocks.

Common Mistakes

• SELECT * in application code → over-fetch, fragile, no covering index.
• Aggregates over a fanned-out JOIN → doubled sums; aggregate before joining.
• Functions on indexed columns in WHERE → full scan.
• Wrong composite-index column order → index unused for the query.
• Offset pagination on huge tables (OFFSET 100000) → slow; use keyset/cursor pagination.
• Floating-point money → rounding errors; use numeric.
• Implicit type mismatch in joins/filters → silent full scans.

Examples

Avoid JOIN fan-out with a CTE; keyset pagination

WITH totals AS (
  SELECT order_id, SUM(qty * unit_price) AS total
  FROM order_items GROUP BY order_id
)
SELECT o.id, o.created_at, t.total
FROM orders o
JOIN totals t ON t.order_id = o.id
WHERE (o.created_at, o.id) < ($1, $2)   -- keyset cursor, not OFFSET
ORDER BY o.created_at DESC, o.id DESC
LIMIT 20;

Composite index matching a query

-- WHERE tenant_id = $1 AND status = $2 ORDER BY created_at DESC
CREATE INDEX idx_orders_tenant_status_created
  ON orders (tenant_id, status, created_at DESC);

See Also

• nodejs-backend-expert — calling the DB safely (pooling, parameterization).
• performance-expert — end-to-end latency and profiling.
• api-design-expert — cursor pagination contracts backed by SQL.
• security-expert — parameterization and least-privilege access.

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: Miaoge-Ge
• Source: Miaoge-Ge/coding-agent-skills
• License: MIT

Install and usage instructions live in the source repository linked above.