How Can I Create PostgreSQL Roles Conditionally to Avoid Errors?

Creating PostgreSQL Roles with Conditional Execution

Problem Introduction

In PostgreSQL 9.1, creating a role using CREATE ROLE fails if the role already exists. This limitation poses a challenge when scripting database creation and role management. A desired solution is to execute the CREATE ROLE statement conditionally, only if the role does not exist.

Conditional Script Development

One approach is to utilize PL/pgSQL's DO block and the IF EXISTS condition:

DO
$do$
BEGIN
   IF EXISTS (
      SELECT FROM pg_catalog.pg_roles
      WHERE  rolname = 'my_user') THEN

      RAISE NOTICE 'Role "my_user" already exists. Skipping.';
   ELSE
      CREATE ROLE my_user LOGIN PASSWORD 'my_password';
   END IF;
END
$do$;

This script dynamically checks for the role's existence using SELECT and executes CREATE ROLE only if the role does not exist.

No Race Condition Scenario

This solution does not introduce a race condition. The IF EXISTS condition ensures that the role is created only if it does not exist at the time of checking. Any concurrent transaction creating the role between the check and the creation will not cause an issue as the role will already exist at the time of the CREATE ROLE execution.

Optimized Script (No Exception Handling)

To further optimize the script, a nested block can be used to avoid the cost of an exception handler:

DO
$do$
BEGIN
   IF EXISTS (
      SELECT FROM pg_catalog.pg_roles
      WHERE  rolname = 'my_user') THEN

      RAISE NOTICE 'Role "my_user" already exists. Skipping.';
   ELSE
      BEGIN   -- nested block
         CREATE ROLE my_user LOGIN PASSWORD 'my_password';
      EXCEPTION
         WHEN duplicate_object THEN
            RAISE NOTICE 'Role "my_user" was just created by a concurrent transaction. Skipping.';
      END;
   END IF;
END
$do$;

This script performs the check efficiently and handles any potential race condition with minimal overhead. The nested block ensures that the role is created only if it does not exist or if a concurrent transaction has just created it, in which case a notice is raised.

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