Types and Roles If Not Exists in PostgreSQL

March 28, 2016 | databases | programming | sql |

For certain operations such as creating types and roles in PostgreSQL you are not able to use the handy IF NOT EXISTS parameter. This makes creating reusable scripts a bit more challenging, especially if you are going to be running these scripts more than once against the same database. On solution to this problem is to Just Do It™. DO allows you to execute anonymous code blocks. For example, suppose we had a simple task table with a custom type called task_status along with a readerrole that has only select permissions. Using the pg_type and pg_roletables we can write a DO expression that will check to make sure that the elements do not exist before attempting to create them, which will prevent your script from erroring out in the event that they do exist. First we will create the custom type.

DO $$
BEGIN
    IF NOT EXISTS (SELECT 1 FROM pg_type WHERE typname = 'task_status') THEN
        CREATE TYPE task_status AS ENUM ('todo', 'doing', 'blocked', 'done');
    END IF;
END
$$;

Then we can create our table.

CREATE TABLE IF NOT EXISTS
tasks (
  id           integer PRIMARY KEY,
  title        varchar(200), 
  status       task_status NOT NULL DEFAULT 'todo',
  created_date timestamp
);

Next, we can create our new role called reader and grant read only rights on the tasks table.

DO $$
BEGIN
    IF NOT EXISTS (SELECT 1 FROM pg_roles WHERE rolname = 'reader') THEN
        CREATE ROLE reader;
    END IF;
END
$$;
GRANT SELECT ON tasks TO reader;

The entire script is shown below.

You can run this script as many times as you wish and it will just work™, which means that if you are running full migrations then you do not have to do any manual overrides or comment anything out. Note that the reason why we do not have to wrap up the GRANT in a check is because if you duplicate a grant, PostgreSQL will return a notice but not an error.

Thank you for reading! Share your thoughts with me on bluesky, mastodon, or via email.

Check out some more stuff to read down below.

Most popular posts this month

Recent Favorite Blog Posts

This is a collection of the last 8 posts that I bookmarked.

A Year Of Vibes from Armin Ronacher's Thoughts and Writings
Pluralistic: A perfect distillation of the social uselessness of finance (18 Dec 2025) from Pluralistic: Daily links from Cory Doctorow
Moving from WordPress to Substack from charity.wtf
Grow, Like a Tree Not a Cancer from Jim Nielsen’s Blog
Pluralistic: All the books I reviewed in 2025 (02 Dec 2025) from Pluralistic: Daily links from Cory Doctorow
DEP-18: A proposal for Git-based collaboration in Debian from Optimized by Otto
[RIDGELINE] No Phones in The Ten-don Shop from Craig Mod — Writer + Photographer
Open design: the opportunity design students didn’t know they were missing from Ubuntu blog

Generated by openring

Lev Lazinskiy

Types and Roles If Not Exists in PostgreSQL

Articles from blogs I follow around the net

Binance's Trust Wallet extension hacked; users lose $7 million

1Password Dependency Breaks Syntax Highlighting

How Rob Pike got spammed with an AI slop "act of kindness"