Re: Unexpected custom type behavior using ROW(NULL)

On Sat, Mar 14, 2015 at 7:21 PM, Denver Timothy <denver@timothy.io> wrote:
> In 9.4.1, I do this:
>
> CREATE TYPE my_test_type as (part1 text, part2 text);
>
> \pset null NULL
>
> WITH test_table(test_col) AS (
>     VALUES (NULL::my_test_type), (ROW(NULL, NULL)::my_test_type)
> )
> SELECT *, (test_col).part1, (test_col).part2, test_col IS NULL AS is_null FROM test_table;
>
> And I get this result:
>
> ┌──────────┬───────┬───────┬─────────┐
> │ test_col │ part1 │ part2 │ is_null │
> ├──────────┼───────┼───────┼─────────┤
> │ NULL     │ NULL  │ NULL  │ t       │
> │ (,)      │ NULL  │ NULL  │ t       │
> └──────────┴───────┴───────┴─────────┘
>
> But I expect this result:
>
> ┌──────────┬───────┬───────┬─────────┐
> │ test_col │ part1 │ part2 │ is_null │
> ├──────────┼───────┼───────┼─────────┤
> │ NULL     │ NULL  │ NULL  │ t       │
> │ NULL     │ NULL  │ NULL  │ t       │
> └──────────┴───────┴───────┴─────────┘
>
> Is this expected behavior? I do find references in the docs to input/output of NULL values as components of anonymous
recordtypes, but it's still not clear to me if this would be expected behavior after a cast to a custom type.
 

kinda.   The SQL standard mandates that rows containing all null
values satisfy 'IS NULL = true'.  However, postgres internally has
nullibitily of container types that is distinct from their contents.
I personally find this to be a good thing for various reasons but the
facts are that postgres has some historical baggage in this area that
crashed into the standard.

To make things more confusing, look at:
postgres=# select coalesce(row(null), row(1));
 coalesce
──────────
 ()

> Is there a trick to get the result I'm expecting? So far all of the syntactical gymnastics I can think of still
producethe same result.
 

Not really.  You could make a null wrapper functions to approximate
the rules you want:

CREATE OR REPLACE FUNCTION NullWrap(anyelement) RETURNS anyelement AS
$$
  SELECT CASE WHEN $1 IS NULL THEN NULL ELSE $1 END;
$$ LANGUAGE SQL IMMUTABLE;

postgres=# select coalesce(nullwrap(row(null)), nullwrap(row(1)));
 coalesce
──────────
 (1)

If I were to seriously consider using that often, I'd probably
abbreviate it to be n() etc.

merlin