SQL optimization

Wed, 21 Oct 2026 00:03:01 +0200

SQL optimization

Two goals:

reduce roundtrips between the caller (e.g. the BE) and the DB
optimize the queries

Measure first

Don’t guess, find what is actually slow: slow query log, pg_stat_statements, EXPLAIN ANALYZE (runs the query and shows real timings, unlike plain EXPLAIN).

Reading

Ensure indexes on the columns used in WHERE, JOIN and ORDER BY.
- Indexes are not free: they slow down writes and take space. Index what your queries use, not everything.
- Composite indexes: column order matters, put equality filters before range filters.
- Covering indexes: include the selected columns so the DB doesn’t need to read the table.
Check how you wrote the query: select only the columns you need, avoid functions on indexed columns.
Avoid N+1 (classic with ORMs): fetch related rows for the whole batch at once with eager loading or WHERE id IN (...).

CREATE INDEX idx_orders_customer_created ON orders (customer_id, created_at) INCLUDE (total);

EXPLAIN ANALYZE
SELECT id, total FROM orders
WHERE customer_id = 42 AND created_at > '2026-01-01';

Reading many rows

Use pagination: never load the whole table at once.
Load in batches (e.g. 500 rows per call), process them, then write the results in batch (see below).

Prefer keyset pagination over OFFSET, it stays fast on big tables:

Database on Buro notes

SQL optimization

SQL optimization

Measure first

Reading

Reading many rows