Is "create domain" allowed with a base type of json (or jsonb)? - postgresql

Question

I have a custom type with code similar to the following (shortened for brevity):
create domain massFraction as jsonb (
value->'h' > 0 and value->'h' is not null
);
Running this provokes the following error:
ERROR: type modifier is not allowed for type "jsonb"
The documentation makes no mention of json being a disallowed base type for a domain.
https://www.postgresql.org/docs/12/sql-createdomain.html
I also thought that it was perhaps the "not null" part of the constraint, which the documentation mentions is "tricky". Additionally, I've tried similar statements without any json operators. All of these seem to be disallowed.
I am obnoxiously prone to not reading documentation, and worse still about understanding documentation when I do bother to read it... so does it not exist, or am I looking in the wrong places to see if this is permitted? Why is it not permitted?

Answer 1

You're missing a CHECK after jsonb:
create domain massFraction as jsonb check (
value->'h' > 0 and value->'h' is not null
);
This results in another error:
ERROR: 42883: operator does not exist: jsonb > integer
HINT: No operator matches the given name and argument types. You might need to add explicit type casts.
I believe what you want is
CREATE DOMAIN massFraction AS jsonb CHECK (
CASE jsonb_typeof(value->'h')
-- only cast when it is safe to do so
-- note: this will reject numeric values stored as text in the json object (eg '{"h": "1"}')
WHEN 'number' THEN (value->>'h')::integer > 0
ELSE false
END
)
The error you're getting comes from the fact that you input is parsed as
CREATE DOMAIN massFraction AS jsonb(<modifier>) -- like in varchar(10)
As an aside I would recommend against using camelCase in postgresql, as massFraction is the same as massfraction (unless quoted) and postgresql will use the lowercased form when reporting errors, hints, etc.