DOCS

pg_tagged_unions

This is an experimental Postgres extension to add tagged unions (also known as sum types, discriminated unions, disjoint unions, variants, etc.)

Install

Copy the files in src/ to the Postgres extension directory (somewhere like /usr/share/postgresql/extension or /usr/local/share/postgresql/extension), then run:

create extension pg_tagged_unions;

Usage

This operates on existing user-defined composite types, for instance:

create type credential_password as (
  password_hash       text,
  last_updated        timestamp
);

create type credential_email as (
  email_address       text,
  verified            boolean
);

These are passed to create_tagged_union:

select create_tagged_union('credential',
  row('password', 'credential_password'),
  row('email', 'credential_email')
);

This will create a domain called credential which is an alias to another composite type (credential_union) with a check constraint which allows one non-null column which matches the given tag.

create table example_table(credential credential);
insert into example_table (credential) values (
  row('password', row('my hash', now()), null)
);
insert into example_table (credential) values (
  row('email', null, row('me@example.com', true))
);