Volatility & STRICT

IMMUTABLE/STABLE/VOLATILE and planner effects. Production-oriented PostgreSQL guidance.

Recipe

CREATE FUNCTION app.now_utc() RETURNS timestamptz LANGUAGE sql STABLE AS $$ SELECT now(); $$;
CREATE FUNCTION app.rand_uuid() RETURNS uuid LANGUAGE sql VOLATILE AS $$ SELECT gen_random_uuid(); $$;

When to reach for this: You implement or review SQL using this concept in PostgreSQL 18.

Working Example

EXPLAIN SELECT * FROM app.accounts WHERE created_at < app.now_utc();

What this demonstrates:

• Idiomatic PostgreSQL 18 SQL
• Patterns suitable for EXPLAIN review
• Safe defaults for OLTP workloads

Deep Dive

Mechanism

• Parser builds query tree.
• Executor runs plan nodes.
• MVCC provides snapshot isolation per statement or transaction.

Practice

• Keep examples readable.
• Test at realistic row counts.

Gotchas

• Long transactions - Block vacuum and increase bloat.. Fix: Commit quickly; set timeouts.
• Missing indexes - Seq scans under load.. Fix: Index join and filter columns.
• Implicit locks - Surprise blocking.. Fix: Use explicit FOR UPDATE when needed.
• Wrong isolation assumption - Anomalies under concurrent writes.. Fix: Pick level deliberately.
• Untested migrations - DDL lock or invalid objects.. Fix: Stage migrations with production-like data.

Alternatives

FAQs

Related

• Functions Basics - SQL vs PL/pgSQL

Stack versions: This page was written for PostgreSQL 18.4 (stable 18, maintenance 17), pgvector 0.8+, PgBouncer 1.x, Patroni 3.x, and PostGIS 3.5+.