PostgreSQL Patterns

A working reference for PostgreSQL decisions that come up while writing queries and shaping schemas: which index, which type, which lock, and which query will tell you what the database is actually doing. For a full review of an existing data layer, hand the work to the database-expert subagent.

Baseline version, current as of September 2026: PostgreSQL 18. PostgreSQL 14 is the oldest release still supported, until November 12, 2026, so treat anything older as a version to upgrade rather than a version to target.

Based on Supabase Agent Skills (credit: Supabase team), MIT License.

When to activate

• Choosing an index for a query, or deciding whether one is even needed.
• Picking a column type for identifiers, money, timestamps, or flags.
• Writing or reviewing a row-level security policy.
• Diagnosing a slow query, table bloat, or a missing foreign-key index.
• Setting connection limits, statement timeouts, or monitoring thresholds.

When not to activate

• Applying a schema change safely on a live table: use database-migrations.
• JPA and Hibernate entity mapping and fetch strategy: use springboot-patterns.
• MongoDB modelling and indexing: use mongodb-patterns.
• Application-level caching, retries, and layering: use backend-patterns.
• Profiling an endpoint end to end before blaming the database: use performance-optimization.

Pick the index that matches the predicate

Pass: equality columns first in a composite index, then the range column.

CREATE INDEX idx_orders_status_created ON orders (status, created_at);

Fail: CREATE INDEX idx ON orders (created_at, status) for WHERE status = 'pending' AND created_at > $1, which cannot use the leading column for the equality and scans far more of the index than it needs.

Two variants worth reaching for:

CREATE INDEX idx_users_email_cover ON users (email) INCLUDE (name, created_at);

CREATE INDEX idx_users_email_active ON users (email) WHERE deleted_at IS NULL;

The first avoids a heap lookup when the query selects only the included columns. The second indexes only live rows, so it stays small on a table dominated by soft-deleted ones.

Choose the type that cannot be wrong

Pass: amount_minor bigint NOT NULL for money, so no rounding exists to get wrong.

Fail: price float, which cannot represent 0.1 exactly and silently drifts across arithmetic.

varchar(n) buys no performance over text in PostgreSQL, and the length limit becomes a migration the first time the business changes its mind.

Wrap the auth call in a row-level security policy

-- PASS: the subquery is evaluated once per statement
CREATE POLICY orders_owner ON orders
  USING ((SELECT auth.uid()) = user_id);

-- FAIL: the function is re-evaluated for every row scanned
CREATE POLICY orders_owner ON orders
  USING (auth.uid() = user_id);

Index the column the policy filters on, or the policy turns every read into a sequential scan.

Use the right statement for the job

Upsert without a race between the check and the insert:

INSERT INTO settings (user_id, key, value)
VALUES (123, 'theme', 'dark')
ON CONFLICT (user_id, key) DO UPDATE SET value = EXCLUDED.value;

Paginate by cursor, which is constant cost, rather than by offset, which is linear in the rows skipped:

SELECT * FROM products WHERE id > $1 ORDER BY id LIMIT 20;

Claim a queue row without two workers taking the same one:

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 *;

Detect the common anti-patterns with a query

Unindexed foreign keys, which make every parent delete scan the child table:

SELECT conrelid::regclass, a.attname
FROM pg_constraint c
JOIN pg_attribute a ON a.attrelid = c.conrelid AND a.attnum = ANY(c.conkey)
WHERE c.contype = 'f'
  AND NOT EXISTS (
    SELECT 1 FROM pg_index i
    WHERE i.indrelid = c.conrelid AND a.attnum = ANY(i.indkey)
  );

Slow statements by mean execution time:

SELECT query, mean_exec_time, calls
FROM pg_stat_statements
WHERE mean_exec_time > 100
ORDER BY mean_exec_time DESC;

Tables carrying dead rows that autovacuum is not keeping up with:

SELECT relname, n_dead_tup, last_vacuum
FROM pg_stat_user_tables
WHERE n_dead_tup > 1000
ORDER BY n_dead_tup DESC;

Set the timeouts before you need them

ALTER SYSTEM SET max_connections = 100;
ALTER SYSTEM SET work_mem = '8MB';
ALTER SYSTEM SET idle_in_transaction_session_timeout = '30s';
ALTER SYSTEM SET statement_timeout = '30s';
CREATE EXTENSION IF NOT EXISTS pg_stat_statements;
REVOKE ALL ON SCHEMA public FROM public;
SELECT pg_reload_conf();

work_mem is per sort node, not per connection, so a single complex query can use several multiples of it. Size max_connections for the machine and put a pooler in front rather than raising it.

Route reads to replicas, except read-your-own-writes

• Writes go to the primary.
• Reads that tolerate eventual consistency go to a replica.
• A read in the same request as a preceding write goes to the primary, or the user sees their own change missing.

Alert on these before a user reports them:

Run EXPLAIN ANALYZE before the change lands

Two triggers, either one of which requires the plan in the change description:

• Any query expected to touch more than 10,000 rows.
• Any migration that touches data in an existing table, meaning UPDATE, DELETE, INSERT ... SELECT, or a

concurrent index build.

EXPLAIN (ANALYZE, BUFFERS) SELECT * FROM orders WHERE created_at > '2024-01-01';

EXPLAIN ANALYZE executes the statement it plans, so on an UPDATE, DELETE or INSERT it changes the data. Run it inside a transaction and roll back:

BEGIN;
EXPLAIN ANALYZE UPDATE orders SET status = 'active' WHERE created_at > '2024-01-01';
ROLLBACK;

This is the same gate database-migrations states for schema changes, deliberately worded identically so a change that crosses both skills is reviewed once against one rule.

Back up on a schedule you have actually tested

• Daily full backup plus continuous WAL archiving.
• Backups stored in a separate cloud account from the application.
• AES-256 encryption at rest.
• Quarterly restore drills that measure the real recovery time, not the intended one.
• RTO and RPO written in the runbook, next to the measured numbers from the last drill.

A backup nobody has restored is a hypothesis.

Erase personal data within the legal window

GDPR right to erasure runs to 30 days. Two workable shapes:

1. Soft delete then purge: set deleted_at, exclude the row everywhere, delete it for real after the window.
2. Pseudonymize in place: overwrite the identifying columns with a non-reversible value, keeping the row for referential

integrity and aggregates.

Never hard-delete before checking cascade behaviour and audit-log retention, because a DELETE that cascades into an audit trail destroys the evidence that the erasure was lawful.

Related skills

• database-migrations for applying any of these changes safely on a live table.
• backend-patterns and node-backend-patterns for connection pooling and caching above the database.
• springboot-patterns when the queries are generated by Hibernate rather than written by hand.
• performance-optimization for measuring an endpoint before assuming the database is the bottleneck.
• security-review for access control and personal-data handling beyond row-level security.
• The database-expert subagent for a full review of an existing data layer.

Checklist

• [ ] Every predicate that runs often has an index whose leading columns match it.
• [ ] Every foreign key has an index on the referencing side.
• [ ] Identifiers, money, and timestamps use the correct type, never float for money.
• [ ] Row-level security policies wrap the auth call in a subquery and filter on an indexed column.
• [ ] Pagination is cursor-based on anything unbounded.
• [ ] statement_timeout and idle_in_transaction_session_timeout are set.
• [ ] pg_stat_statements is enabled and someone reads it.
• [ ] EXPLAIN ANALYZE output is attached when either trigger above applies.
• [ ] Backups are encrypted, off-account, and restored on a schedule.
• [ ] Erasure requests have a documented path that respects audit retention.