The following code helps me to understand how to use complex domain in PostgreSQL using pgx v5.

In this case I used an array of another domain:

CREATE DOMAIN public.http_client_error_code AS smallint
	CONSTRAINT http_client_error_code_check CHECK (VALUE = 0 OR VALUE >= 400 AND VALUE <= 499);

The above domain is a simple integer with a constraint.

CREATE DOMAIN public.http_client_error_code_list AS http_client_error_code[];

The above domain is an array of the first domain (http_client_error_code).

When trying to scan or assign it using pgx an error will be raised:

panic: failed to encode args[0]: unable to encode main.HTTPClientErrorCodeList{400} into text format for unknown type (OID 32783): cannot find encode plan

The issue is that pgx does not understand how to convert the slice into http_client_error_code_list.

By telling pgx to look for that domain (and given OID), it know how to scan and value the Go's data type.