Re: Fwd: PATCH: psql boolean display

I wrote:
> Phil Sorber <phil@omniti.com> writes:
>> What my patch was intended to do was let the end user set boolean
>> output to any arbitrary values. While foo/bar is pretty useless, it
>> was meant to reinforce that it was capable of any arbitrary value. I
>> can think of a decent list of other output an end user might want,
>> such as:

>> true/false
>> yes/no
>> y/n
>> on/off
>> 1/0
>> enabled/disabled

>> Plus the different capitalized forms.

> I can readily see that people might want boolean columns displayed in
> such ways in custom applications.  I'm less convinced that there is much
> use for it in psql, though.

BTW, another point that your list brings to mind is that somebody who
wants something like this would very possibly want different displays
for different columns.  The proposed feature, being one-size-fits-all
for "boolean", couldn't handle that.

What would make a lot more sense in my mind would be to label columns
*in the database* to show how they ought to be displayed.

One conceivable method is to make a collection of domains over bool,
and drive the display off the particular domain used.  However we lack
some infrastructure that would be needed for this (in particular, I'm
pretty sure the PQftype data delivered to the client identifies the
underlying type and not the domain).

Another approach is to make a collection of enum types, in which case
you don't need any client-side support at all.  I experimented with
this method a bit, and it seems workable:

regression=# create type mybool as enum ('no', 'yes');
CREATE TYPE
regression=# create function bool(mybool) returns bool as
$$ select $1 = 'yes'::mybool $$ language sql immutable;
CREATE FUNCTION
regression=# create cast (mybool as bool) with function bool(mybool) as assignment;
CREATE CAST
regression=# create table mb(f1 mybool);
CREATE TABLE
regression=# insert into mb values('no'),('yes');
INSERT 0 2
regression=# select * from mb where f1;f1  
-----yes
(1 row)

regression=# select * from mb where f1 = 'yes';f1  
-----yes
(1 row)

I tried making the cast be implicit, but that caused problems with
ambiguous operators, so assignment seems to be the best you can do here.

A variant of this is to build casts in the other direction
(bool::mybool), declare columns in the database as regular bool,
and apply the casts in queries when you want columns displayed in a
particular way.  This might be the best solution if the desired
display is at all context-dependent.
        regards, tom lane