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+---
+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):**
+
+```sql
+-- 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):**
+
+```sql
+-- 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](https://www.postgresql.org/docs/current/sql-createtable.html#SQL-CREATETABLE-PARMS-GENERATED-IDENTITY)