Domains and generated columns

[Fabian Pijcke <fabian.pijcke@xxxxxxxxx>]

Dear list,

I ran into an error when running the following snippet of code (I simplified as much as I could):

    CREATE DOMAIN domaintest AS VARCHAR(10) NOT NULL;

   
    CREATE TABLE tabletest (
      fieldtest domaintest GENERATED ALWAYS AS ('valuetest') STORED
    );
   

    INSERT INTO tabletest DEFAULT VALUES;

In PostgreSQL 12 and 13, I get the following error:

    domain domaintest does not allow null values.

If I move the NOT NULL constraint from the domain to the table, the error disappears.

I suspect that the NULL check is done before the GENERATED column is actually computed. I don't think this is related to the warning the documentation gives about NULL values and DOMAINs (https://www.postgresql.org/docs/13/sql-createdomain.html#id-1.9.3.62.7).

Is this the expected behaviour? Is there any way to circumvent this and use a domain with a NOT NULL constraint on a generated column? (I know the documentation recommends to avoid NOT NULL on domains, but I'd still like to do so).

Thank you,

--

Fabian Pijcke