pg_global

pg_global pg_global#

What is pg_global in PostgreSQL?#

pg_global is a special PostgreSQL tablespace that stores cluster-wide system metadata shared by all databases in the server.

In short: pg_global stores PostgreSQL’s brain — not your data.

pg_global is the PostgreSQL system tablespace that contains global catalog tables shared across all databases in a PostgreSQL cluster.

📂 Where pg_global Lives#

On disk:

$PGDATA/global/

This directory is outside individual databases.

Unlike pg_default, it is:

  • Not per-database

  • Shared across the entire cluster

What is Stored in pg_global?#

pg_global contains cluster-wide system catalogs, including:

Authentication & Roles

  • Users (roles)

  • Password hashes

  • Role privileges

Database Metadata

  • List of all databases

  • Database ownership

  • Encoding, locale info

Tablespaces

  • Tablespace definitions

  • Physical paths

Replication & Slots

  • Replication slots

  • Subscription metadata

🔐 Security Objects

  • Access control lists

  • Shared dependencies

Key System Catalog Tables#

Catalog

Purpose

pg_database

List of all databases

pg_authid

Roles & passwords

pg_tablespace

Tablespace info

pg_shdepend

Shared dependencies

These live physically in pg_global.

🚫 What is NOT Stored in pg_global

  • ❌ User tables

  • ❌ Indexes

  • ❌ Application data

  • ❌ Schemas

  • ❌ Views

Those belong in pg_default or custom tablespaces.

⚠️ Why pg_global Is Untouchable

  • Required for PostgreSQL startup

  • Shared by all databases

  • Corruption = server won’t start

  • Cannot be dropped

  • Cannot be moved

If pg_global is damaged, all databases are affected.

pg_default vs pg_global#

Feature

pg_default

pg_global

Scope

Per database

Cluster-wide

Stores user data

✅

❌

Stores system catalogs

❌

✅

Safe to use manually

✅

🚫

Can be customized

✅

❌

Real-World Analogy#

Think of PostgreSQL as a company:

PostgreSQL

Company

pg_global

HR + Admin office

pg_default

Department offices

Databases

Departments

Tables

Files

You never edit HR records directly — same with pg_global.

Why pgAdmin Shows pg_global#

pgAdmin shows it so you:

  • Understand cluster structure

  • Can inspect (read-only)

  • Don’t accidentally misuse it

Viewing is safe. Editing is not.

Backup Importance#

Every full PostgreSQL backup must include pg_global.

Example:

pg_dumpall --globals-only > globals.sql
This backs up:

  • Roles

  • Tablespaces

  • Global privileges

Summary#

Note

pg_global is PostgreSQL’s system tablespace that stores global metadata shared across all databases, including roles, databases, tablespaces, and security information. It is essential for server operation and should never be modified manually.

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