DISTINCT ON | PostgreSQL Tutorial

DISTINCT ON in Querying Data helps you write SQL that is easier to test, review, and operate at scale.

Introduction to DISTINCT ON

Use DISTINCT ON to retrieve exactly the rows and columns your application needs.

Commonly paired with: SELECT, FROM, ORDER BY, DISTINCT.

Practical examples with DISTINCT ON in PostgreSQL

Reference pattern: start from canonical syntax and keep it explicit.

SELECT DISTINCT ON (customer_id) customer_id, order_id, placed_at
FROM orders
ORDER BY customer_id, placed_at DESC;

Production-style scenario: apply the same concept to realistic application data.

SELECT
  o.order_id,
  o.total_amount,
  o.placed_at
FROM orders o
WHERE o.placed_at >= now() - interval '30 days'
ORDER BY o.placed_at DESC
LIMIT 50;

Additional example: use a variation to validate behavior and edge cases.

SELECT customer_id, COUNT(*) AS paid_orders
FROM orders
WHERE status = 'paid'
GROUP BY customer_id
ORDER BY paid_orders DESC
LIMIT 10;

Production tips

Prefer explicit column lists and deterministic ordering when results feed APIs or batch jobs.
Validate plans with EXPLAIN before adding indexes, then re-check after schema changes.
Keep DDL, data backfills, and cleanups in transactions when possible to avoid partial state.
Use isolated environments for risky changes so query tuning and schema experiments stay safe.

Vela workflow tip

Test this pattern in an isolated branch database, share the result with your team, and promote only after query plans and row counts look correct.

Reference: PostgreSQL official documentation.