2016-03-28 Tags: databases postgresql
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 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 reader role that has only select permissions. Using the
pg_role tables 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.