SQL Idioms

SQL Idioms and Patterns

SQL rewards set-based thinking, explicit joins, and query plan awareness. Idiomatic SQL = readable, performant, migration-safe.

Scope: SQL coding idioms. For database design principles, load @.agents/rules/database-design-principles.md.

Query Patterns

1. CTEs over subqueries for readability:

   -- ✅ CTE — readable, debuggable
   WITH active_tasks AS (
       SELECT id, title, priority, user_id
       FROM tasks
       WHERE status = 'active'
   )
   SELECT u.name, COUNT(at.id) AS task_count
   FROM users u
   JOIN active_tasks at ON u.id = at.user_id
   GROUP BY u.name;
   
   -- ❌ Nested subquery — hard to read
   SELECT u.name, (SELECT COUNT(*) FROM tasks t WHERE t.user_id = u.id AND t.status = 'active')
   FROM users u;
   

2. Window functions for ranking, running totals:

   SELECT title, priority,
       ROW_NUMBER() OVER (PARTITION BY user_id ORDER BY created_at DESC) AS rn
   FROM tasks;
   

3. Explicit JOIN syntax — never implicit joins in WHERE.

4. Parameterized queries — never string concatenation. (See .agents/rules/security-mandate.md.)

Migration Safety

For migration strategy (additive-first, two-phase drops, reversibility), see @.agents/rules/database-design-principles.md § Migrations.

1. Index creation: CONCURRENTLY on PostgreSQL for zero-downtime.
2. Idempotent DDL — use IF NOT EXISTS for tables/indexes; DO $$ ... pg_constraint check ... $$ for constraints.

Index Strategy

1. Choose the right index type:

   • B-tree (default): =, <, >, BETWEEN, IN, IS NULL
   • GIN: arrays, JSONB (@>), full-text search (@@)
   • GiST: geometric data, range types, nearest-neighbor (KNN)
   • BRIN: large time-series tables (10-100x smaller than B-tree)
   • Hash: equality-only (marginally faster than B-tree for =)

2. Composite indexes — column order matters:

   -- Equality columns first, range columns last (leftmost prefix rule)
   CREATE INDEX idx ON orders (status, created_at);
   -- Works for: WHERE status = 'pending'
   -- Works for: WHERE status = 'pending' AND created_at > '2024-01-01'
   -- Does NOT work for: WHERE created_at > '2024-01-01' (alone)
   

3. Partial indexes for filtered queries:

   -- Index only active rows (5-20x smaller)
   CREATE INDEX idx_users_active_email ON users (email) WHERE deleted_at IS NULL;
   

4. Covering indexes to avoid heap fetches:

   -- INCLUDE non-searchable columns for index-only scans
   CREATE INDEX idx_orders_status ON orders (status) INCLUDE (customer_id, total);
   

5. Indexes on foreign keys — always. PostgreSQL does not auto-index FKs.

Performance

1. EXPLAIN (ANALYZE, BUFFERS) before optimizing — never guess.

   • Seq Scan on large table = missing index
   • Rows Removed by Filter = poor selectivity
   • read >> hit in Buffers = data not cached
   • Sort Method: external merge = work_mem too low

2. Avoid SELECT * — list specific columns.

3. Keyset pagination over OFFSET for large datasets:

   -- O(1) regardless of page depth
   SELECT * FROM products WHERE (created_at, id) > ($1, $2)
   ORDER BY created_at, id LIMIT 20;
   

   Use LIMIT/OFFSET only for small, bounded result sets.

Concurrency & Locking

1. Prevent deadlocks — consistent lock ordering:

   -- Acquire locks in PK order before updating
   SELECT * FROM accounts WHERE id IN (1, 2) ORDER BY id FOR UPDATE;
   

2. SKIP LOCKED for queue processing:

   -- Workers skip locked rows instead of blocking (10x throughput)
   UPDATE jobs SET status = 'processing'
   WHERE id = (
     SELECT id FROM jobs WHERE status = 'pending'
     ORDER BY created_at LIMIT 1 FOR UPDATE SKIP LOCKED
   ) RETURNING *;
   

3. Advisory locks for application-level coordination:

   SELECT pg_advisory_xact_lock(hashtext('daily_report')); -- Released on COMMIT
   

4. statement_timeout — always set per-session to prevent runaway queries.

Data Operations

1. UPSERT — atomic insert-or-update (no race conditions):

   INSERT INTO settings (user_id, key, value) VALUES ($1, $2, $3)
   ON CONFLICT (user_id, key)
   DO UPDATE SET value = EXCLUDED.value, updated_at = now();
   

2. Bulk loading — use COPY over batch INSERTs for large imports.

3. Batch inserts — multiple rows per statement, not one INSERT per row.

Diagnostics

1. pg_stat_statements — enable to identify top resource-consuming queries by total time and call frequency.

2. VACUUM/ANALYZE — run ANALYZE after large data changes. Tune autovacuum_vacuum_scale_factor for high-churn tables.

Advanced PostgreSQL

1. Full-text search: use tsvector + GIN index, not LIKE '%term%'.
2. JSONB indexing: GIN (jsonb_path_ops for @> only — 2-3x smaller), expression indexes for key lookups.

Naming

Follow conventions in @.agents/rules/database-design-principles.md § Schema (Naming).

Anti-Patterns

• ❌ Missing indexes on foreign keys
• ❌ N+1 queries (use JOIN or batch)
• ❌ String concatenation in queries (SQL injection risk)
• ❌ Storing comma-separated values in a single column
• ❌ OFFSET pagination on large datasets (use keyset)
• ❌ timestamp without timezone (use timestamptz)
• ❌ varchar(n) without reason (use text)
• ❌ Random UUID v4 as primary key on large tables (index fragmentation)
• ❌ Check-then-insert pattern (race condition — use UPSERT)

Related

• Database Design Principles @.agents/rules/database-design-principles.md
• Security Principles .agents/rules/security-principles.md
• Performance Optimization Principles @.agents/rules/performance-optimization-principles.md