Language Sql

SQL

Query optimization basics

• Start with the access pattern: filters, joins, grouping, sorting, and result size.
• Index columns used for selective WHERE, join keys, and stable ORDER BY clauses.
• Prefer narrow projections over SELECT *; return only the columns callers need.
• Avoid accidental row multiplication in joins; check cardinality before adding DISTINCT.
• Watch for N+1 query loops at application boundaries.

EXPLAIN and EXPLAIN ANALYZE

• Use EXPLAIN to inspect the planned access path before changing indexes or query shape.
• Use EXPLAIN ANALYZE when you need actual timing and row counts; run it against safe data and statements.
• Compare estimated vs actual rows. Large gaps often mean stale statistics, skewed data, or missing predicates.
• Optimize the highest-cost operation first, but confirm the full query got faster.

Schema design

• Model durable facts, not current screens. Let queries influence indexes, not table names.
• Choose primary keys deliberately; use foreign keys for integrity unless there is a measured reason not to.
• Normalize to remove update anomalies, then denormalize only for proven read pressure.
• Encode invariants with constraints: NOT NULL, UNIQUE, CHECK, and referential actions.
• Plan migrations as expand-and-contract changes when existing clients need compatibility.

Transactions and isolation

• Keep transactions short; do not wait on users or remote services while holding locks.
• Pick the weakest isolation level that preserves correctness for the workflow.
• Know the anomalies you are allowing: dirty reads, non-repeatable reads, phantoms, and write skew.
• Use optimistic concurrency with version columns when conflicts are rare and retriable.

Deadlock avoidance

• Touch shared tables and rows in a consistent order across code paths.
• Lock only what you need, as late as possible, and commit as soon as the invariant is protected.
• Add retry logic for deadlock and serialization failures; they are expected under contention.
• Index foreign keys and hot predicates so updates do not scan and lock more rows than intended.

Window functions

• Use window functions for rankings, running totals, deduplication, and "top N per group" queries.
• Keep PARTITION BY and ORDER BY explicit; frame clauses matter for aggregates.

SELECT customer_id, order_id, total,
       row_number() OVER (
         PARTITION BY customer_id
         ORDER BY created_at DESC
       ) AS recency_rank
FROM orders;

CTEs vs subqueries

• Use CTEs to name meaningful intermediate results or reuse the same derived relation.
• Use subqueries when the scope is local and the surrounding query stays readable.
• Check your database's optimizer behavior; some engines inline CTEs, others may materialize them.
• Do not use CTEs as a performance hint unless the engine documents that behavior.

Pagination patterns

• Prefer keyset pagination for large or frequently changing result sets.
• Use LIMIT/OFFSET only for small, stable lists; deep offsets get slower and can skip or duplicate rows.
• Make ordering deterministic with a unique tie-breaker.

SELECT id, customer_id, created_at, total
FROM orders
WHERE (created_at, id)  .` (replace `` with e.g. `ansi`, `postgres`, `bigquery` — check `.sqlfluff` config or project README)
- **Lint:** `sqlfluff lint --dialect  .`
- **Test:** engine-specific (pgTAP for PostgreSQL, `dbt test` if using dbt)

## Avoid
- Schema changes without rollback or compatibility planning.
- Unbounded queries in production paths.
- Relying on implicit ordering without `ORDER BY`.
- Never build SQL by concatenating untrusted input — use parameterized queries or prepared statements. ORM raw-query escape hatches (e.g. Django's `extra()`, SQLAlchemy's `text()`) are equally dangerous. See `swe-workbench:principle-security`.

## Source & license

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

- **Author:** [lugassawan](https://github.com/lugassawan)
- **Source:** [lugassawan/swe-workbench](https://github.com/lugassawan/swe-workbench)
- **License:** MIT

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