Re: Cast INTEGER to BIT confusion

> On 15/08/2023 10:49 CEST [Quipsy] Markus Karg <karg(at)quipsy(dot)de> wrote:
>
> Hello PostgreSQL Community,
>
> I like to store just a single bit but that can be either 1 or 0, so I tried
> to do this:
>
> CREATE TABLE T (c BIT);
> INSERT INTO T VALUES (1);
> -- I MUST NOT change both lines in any way as these are part of a third-party application!
>
> Unfortunately this tells me:
>
> column "c" is of type bit but expression is of type integer
>
> So I logged in as a cluster admin and I tried this:
>
> CREATE CAST (integer AS bit) WITH INOUT AS IMPLICIT;
>
> Unfortunately that tells me:
>
> cast from type integer to type bit already exists
>
> This is confusing! Apparently PostgreSQL 15.3 comes with the needed cast
> out-of-the-box but it does not apply it? This is confusing!
>
> What is my fault?

The built-in cast is explicit (castcontext = 'e'):

=# SELECT * FROM pg_cast WHERE castsource = 'int'::regtype AND casttarget = 'bit'::regtype;
oid | castsource | casttarget | castfunc | castcontext | castmethod
-------+------------+------------+----------+-------------+------------
10186 | 23 | 1560 | 1683 | e | f
(1 row)

It's not possible to drop that cast and replace it with a custom one:

=# DROP CAST (int AS bit);
ERROR: cannot drop cast from integer to bit because it is required by the database system

You could create a custom domain if you're only interested in values 0 and 1
and don't use bit string functions. The search path must be changed so that
domain bit overrides pg_catalog.bit:

=# CREATE SCHEMA xxx;
=# CREATE DOMAIN xxx.bit AS int;
=# SET search_path = xxx, pg_catalog;
=# CREATE TABLE t (c bit);
=# INSERT INTO t VALUES (1);
INSERT 0 1

But I would do that only if the third-party code cannot be tweaked because the
custom domain could be confusing. It's also prone to errors as it relies on a
specific search path order. Also make sure that regular users cannot create
objects in schema xxx that would override objects in pg_catalog.

--
Erik