Schema Primary Keys

title: Select Optimal Primary Key Strategy impact: HIGH impactDescription: Better index locality, reduced fragmentation tags: primary-key, identity, uuid, serial, schema

Select Optimal Primary Key Strategy

Primary key choice affects insert performance, index size, and replication efficiency.

Incorrect (problematic PK choices):

-- identity is the SQL-standard approach
create table users (
  id serial primary key  -- Works, but IDENTITY is recommended
);

-- Random UUIDs (v4) cause index fragmentation
create table orders (
  id uuid default gen_random_uuid() primary key  -- UUIDv4 = random = scattered inserts
);

Correct (optimal PK strategies):

-- Use IDENTITY for sequential IDs (SQL-standard, best for most cases)
create table users (
  id bigint generated always as identity primary key
);

-- For distributed systems needing UUIDs, use UUIDv7 (time-ordered)
-- Requires pg_uuidv7 extension: create extension pg_uuidv7;
create table orders (
  id uuid default uuid_generate_v7() primary key  -- Time-ordered, no fragmentation
);

-- Alternative: time-prefixed IDs for sortable, distributed IDs (no extension needed)
create table events (
  id text default concat(
    to_char(now() at time zone 'utc', 'YYYYMMDDHH24MISSMS'),
    gen_random_uuid()::text
  ) primary key
);

Guidelines:

Single database: bigint identity (sequential, 8 bytes, SQL-standard)
Distributed/exposed IDs: UUIDv7 (requires pg_uuidv7) or ULID (time-ordered, no fragmentation)
serial works but identity is SQL-standard and preferred for new applications
Avoid random UUIDs (v4) as primary keys on large tables (causes index fragmentation)

Reference: Identity Columns