Обсуждение: domain on bit(N) type produces strange results

Creating a domain on bit(N) doesn't seem to work as expected when casting
to the domain type.

CREATE DOMAIN bit4 AS bit(4);

SELECT 7::bit(4), 7::bit4;
 bit  | bit4
------+------
 0111 | 1000
(1 row)


Reported in #postgresql by msw_alt.

Kris Jurka

Kris Jurka <books@ejurka.com> writes:
> Creating a domain on bit(N) doesn't seem to work as expected when casting
> to the domain type.

> CREATE DOMAIN bit4 AS bit(4);

> SELECT 7::bit(4), 7::bit4;
>  bit  | bit4
> ------+------
>  0111 | 1000
> (1 row)

What's going on here is that "7::bit4" is implemented as
"7::bit::bit(4)", and since 7::bit is taken to mean 7::bit(1),
the result follows.

Bit is the only typmod-using datatype for which casting to the type
with typmod -1 risks discarding information.  In a brief look I'm
not sure whether this can easily be fixed without introducing unwanted
side-effects.

            regards, tom lane

I wrote:
> Kris Jurka <books@ejurka.com> writes:
>> Creating a domain on bit(N) doesn't seem to work as expected when casting
>> to the domain type.

> What's going on here is that "7::bit4" is implemented as
> "7::bit::bit(4)", and since 7::bit is taken to mean 7::bit(1),
> the result follows.

I've fixed it to collapse this into a direct "7::bit(4)" coercion.
Thanks for the test case.

            regards, tom lane