How Can I Create a PostgreSQL Role Conditionally, Handling Duplicate Role Errors?

Conditional Role Creation in PostgreSQL

The Need for Conditional Role Creation

In PostgreSQL 9.1, creating a role using a simple CREATE ROLE statement can fail if the role already exists. This can be problematic in automated scripts where it is crucial to handle such scenarios gracefully.

Achieving Conditional Creation

To avoid errors in these situations, it is necessary to add a conditional check to the script. Unfortunately, IF statements are not supported in plain SQL in PostgreSQL.

Using PL/pgSQL

The solution lies in using PL/pgSQL, which provides control flow capabilities. The following script demonstrates how to create a role conditionally using PL/pgSQL:

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
$do$;

Exception Handling with Nested Blocks

An alternative approach to handling race conditions is to use nested blocks with exception handling. By nesting the CREATE ROLE statement within an inner block, exceptions raised by potential duplicate role creations can be caught:

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 method is more efficient as it only raises an exception when the race condition occurs, minimizing overhead in most cases.

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