Representing Enums in PostgreSQL

Note one other thing I have found is that postgres enums are less "portable" than plain text columns. For example, if you're using BigQuery to query postgres tables as an external connection, BQ doesn't support enums, so your external connection query has to cast any enum return values (e.g. some_enum_column::text) before returning them to BQ, which can be a pain if you want to do "SELECT * FROM ..." some table that returns enum columns.

In a way you describe how we can emulate this process. The question is why the heck wouldn't databases do this themselves? Same with adding and dropping columns.

Consider how PostgreSQL encodes null for example, by skipping them in the row as fields, and adding them in a null bitmap in front of the row. Meaning... rows are not uniformly sized, there's no math like offset = row * rowsize + field_offset; kind of addressing for reading a field in PG where recoding some of the rows breaks the entire table.

And yet we have all those huge monolithic operations that need to be done atomically. So weird.

    CREATE TABLE my_enum ( name TEXT PRIMARY KEY );

    CREATE TABLE foo ( my_enum TEXT REFERENCES my_enum (name) );

Metadata like sorting and even what date ranges the code is valid for and even security as in who's allowed to use the code in the app.

What they are absolutely not for is to replace the one standard way to define data enums that everybody use since relational algebra was created. It's such an outrageously naive idea that I'm sure most people here were attracted to the title thinking it's about algebraic types or some other similar misunderstanding.

I'm sorry, we don't have any examples in the article about adding values to enums, so I don't know to what you are referring here.

> Yes, that's true, but also very rare?

It is rare, indeed, but we just have a strong inclination towards having a clean schema, so we do prefer to clean up things that would otherwise be unused. But I understand that many people would prefer to leave them hanging there, even if they are not going to be used in any way.

Does this approach ensure data consistency? Or could you accidentally insert a number value into your table which is not represented by any of the enum values?

I am going down this path. There are a lot of downstream use cases (e.g. support, reporting, etc) that can benefit from having the canonical names & human-friendly labels in the database.

Keeping database in sync with code like this requires some discipline and manual scripting, but I think it's worth it.

If you peek at a particular table often enough and it has a crazy enough enum to maybe justify making your life a little easier, what you can do is add a table just for debug purposes and use a view that maps the int to it, but otherwise only use the direct int:enum cast in your app/orm/etc. thereby not paying any runtime cost for foreign keys or integrity checking.

  import foreign schema

  DROP SCHEMA IF EXISTS fdw_enum CASCADE;
  CREATE SCHEMA fdw_enum;
  
  -- Get names of the enums using ::regtype casting and label and sort_order from app_a and app_b.
  CREATE VIEW fdw_enum.app_a_enums AS SELECT * FROM dblink('fdw_app_a',
  $QUERY$
    SELECT
      enumtypid::regtype AS name,
      enumsortorder sort_order,
      enumlabel label
    FROM pg_enum;
  $QUERY$
  ) AS t (name text, sort_order integer, label text);
  
  CREATE VIEW fdw_enum.app_b_enums AS SELECT * FROM dblink('fdw_app_b',
  $QUERY$
    SELECT
      enumtypid::regtype AS name,
      enumsortorder sort_order,
      enumlabel label
    FROM pg_enum;
  $QUERY$
  ) AS t (name text, sort_order integer, label text);
  
  -- Ensure enums with the same names aren't defined in both app_a and app_b.
  DO
  $DO$
  DECLARE
    matching_count integer;
  BEGIN
    SELECT COUNT(*) into matching_count
    FROM fdw_enum.app_a_enums INNER JOIN fdw_enum.app_b_enums USING (name);
    ASSERT
      matching_count = 0,
      'app_a and app_b-NG have identically named enums. Implement a check that they are identically defined.';
  END
  $DO$;
  
  CREATE VIEW fdw_enum.upstream_enums AS
  SELECT * FROM fdw_enum.app_a_enums
  UNION ALL
  SELECT * FROM fdw_enum.app_b_enums;
  
  
  CREATE PROCEDURE fdw_enum.create_type_if_not_exists(name text)
  LANGUAGE plpgsql
  AS $PROC$
    BEGIN
      EXECUTE format('CREATE TYPE %s AS ENUM ()', name);
      EXCEPTION WHEN duplicate_object THEN RAISE NOTICE 'Enum type % already exists, skipping', name;
    END;
  $PROC$
  ;
  
  
  -- To make this idempotent we create the enums empty then alter them by adding values.
  -- So instead of `CREATE TYPE foo AS ENUM ('bar', 'baz');` we do
  -- `CREATE TYPE IF NOT EXISTS foo AS ENUM ();
  -- `ALTER TYPE foo ADD VALUE IF NOT EXISTS ('bar');
  -- `ALTER TYPE foo ADD VALUE IF NOT EXISTS ('baz');
  -- This ensure the procedure is re-runnable and can add new values to the enum as they are added upstream.
  -- Order is ensured by the ORDER BY in the loop query.
  CREATE PROCEDURE fdw_enum.refresh_upsteram_enums()
  LANGUAGE plpgsql
  AS $PROC$
  DECLARE
    rec record;
    ddl text;
  BEGIN
    FOR rec IN SELECT DISTINCT(name) AS name FROM fdw_enum.upstream_enums
    LOOP
      CALL fdw_enum.create_type_if_not_exists(rec.name);
    END LOOP;
  
    FOR rec in
      SELECT *
      FROM fdw_enum.upstream_enums
      ORDER BY name, sort_order
    LOOP
      ddl := FORMAT('ALTER TYPE %s ADD VALUE IF NOT EXISTS %s', rec.name, quote_literal(rec.label));
      EXECUTE ddl;
    END LOOP;
  END
  $PROC$
  ;
  
  CALL fdw_enum.refresh_upsteram_enums();

Right?