How to Create a User in PostgreSQL Only If It Doesn't Exist

• In PostgreSQL, roles are essentially database users that can connect to the database and interact with objects.
• Roles can have different levels of permissions, allowing you to control access to specific databases, tables, and other elements.

Creating a Role with Conditional Logic

While PostgreSQL doesn't have a built-in IF NOT EXISTS clause for creating roles, there are two common approaches to achieve this functionality:

Using a DO Block:

• This method leverages PL/pgSQL, PostgreSQL's procedural language.
• Here's the code structure:

DO $$
BEGIN
    IF NOT EXISTS (SELECT * FROM pg_user WHERE usename = 'new_role') THEN
        CREATE ROLE new_role PASSWORD 'your_password';
    END IF;
END;
$$;

• Explanation:
  • DO $$: Initiates a PL/pgSQL code block.
  • BEGIN: Starts the code execution.
  • IF NOT EXISTS clause:
  • CREATE ROLE: Creates the role 'new_role' with the specified password only if the role doesn't exist.
  • END IF: Closes the conditional statement.
  • END;: Ends the code block.

Checking for Role Existence Before CREATE:

• This approach involves a separate query to check for the role's existence followed by a CREATE ROLE statement if needed.

SELECT * FROM pg_roles WHERE rolname = 'new_role';

IF (SELECT COUNT(*) FROM pg_roles WHERE rolname = 'new_role') = 0 THEN
    CREATE ROLE new_role PASSWORD 'your_password';
END IF;

• Explanation:
  • SELECT * FROM pg_roles WHERE rolname = 'new_role': Queries the pg_roles system catalog table to see if 'new_role' exists.
  • IF (SELECT COUNT(*) FROM pg_roles WHERE rolname = 'new_role') = 0 THEN:
    • Counts the number of roles named 'new_role'.
    • The IF block executes only if the count is zero (role doesn't exist).

Choosing the Right Approach:

• Both methods achieve the desired outcome.
• The DO block approach generally offers better readability, especially when dealing with more complex logic.
• The second approach might be slightly more performant for very frequent role creation due to the separation of the check and creation statements.

Additional Considerations:

• Replace 'new_role' with your desired role name.
• Remember to use a strong password for the role ('your_password').
• Consider granting appropriate permissions to the role after creation.

DO $$
BEGIN
  IF NOT EXISTS (SELECT * FROM pg_user WHERE usename = 'new_role') THEN
    CREATE ROLE new_role WITH LOGIN PASSWORD 'your_strong_password'; -- Specify strong password here
  END IF;
END;
$$;

Explanation:

• This code combines the clarity of the DO block approach with the suggestion to explicitly use WITH LOGIN for better readability.
• It also emphasizes the importance of choosing a strong password ('your_strong_password').

Checking for Role Existence Before CREATE (Potential Performance Optimization):

SELECT pg_roles.rolname
FROM pg_roles
WHERE pg_roles.rolname = 'new_role';

IF NOT EXISTS (SELECT 1 FROM pg_roles WHERE rolname = 'new_role') THEN
  CREATE ROLE new_role WITH LOGIN PASSWORD 'your_strong_password'; -- Specify strong password here
END IF;

• This code separates the existence check from the creation statement, potentially offering a slight performance benefit in scenarios with very frequent role creation.
• It also uses a simpler SELECT 1 query within the IF condition to achieve the same result.
• Like the DO block approach, it highlights the importance of a strong password.

• If you're using a graphical user interface (GUI) tool like pgAdmin to manage your PostgreSQL database, you might be able to leverage its built-in functionality to create roles conditionally.
• The specific steps will vary depending on the pgAdmin version, but generally, you should be able to find options like "Create Login" or "Create User" where you can specify if the user should only be created if it doesn't already exist.
• Consult your pgAdmin documentation for detailed instructions.

Scripting with Shell and psql (Advanced):

• This approach involves writing a script that combines shell commands and psql to achieve conditional role creation.
• The script could first check if the role exists using psql and then execute a CREATE ROLE command only if the check fails.
• Here's a basic outline (caution: requires understanding shell scripting and error handling):

#!/bin/bash

# Role name to create
ROLE_NAME="new_role"

# Check if role exists
psql -t -U postgres -c "SELECT rolname FROM pg_roles WHERE rolname = '$ROLE_NAME'" > /dev/null 2>&1

# If the previous command exits with non-zero status (role doesn't exist)
if [ $? -ne 0 ]; then
    psql -U postgres -c "CREATE ROLE $ROLE_NAME PASSWORD 'your_password'"
    echo "Role '$ROLE_NAME' created successfully."
else
    echo "Role '$ROLE_NAME' already exists."
fi

Important considerations for this approach:

 - Requires knowledge of shell scripting and error handling.
 - Ensure proper security measures are in place when storing the password in the script (consider environment variables).
 - This method can be less portable compared to the pure SQL approaches.

Remember:

• The SQL methods (DO block or separate check and create) are generally more portable and easier to maintain compared to scripting.
• Choose the approach that best suits your environment, skillset, and desired level of control.