Re: extensible enum types

On Fri, Jun 18, 2010 at 1:59 PM, Andrew Dunstan <andrew(at)dunslane(dot)net> wrote:
>
>
> Robert Haas wrote:
>>
>> On Fri, Jun 18, 2010 at 12:59 PM, Andrew Dunstan <andrew(at)dunslane(dot)net>
>> wrote:
>>
>>>
>>> You are just bumping up the storage cost. Part of the attraction of enums
>>> is
>>> their efficiency.
>>>
>>
>> What's efficient about them?  Aren't we using 4 bytes to store a value
>> that will nearly always fit in 2, if not 1?
>>
>>
>
> This was debated when we implemented enums. As between 1,2 and 4 there is
> often not much to choose, as alignment padding makes it pretty much the
> same. But any of them are more efficient than storing a numeric value or the
> label itself.
>
> Anyway, it might well be moot.
>
> cheers
>
> andrew

Something I don't understand in all this is: why can't the type of an
enum be determined statically rather than stored in every single
value? For instance, if we have:

CREATE TYPE number AS ENUM ('one', 'two', 'three');
CREATE TYPE color AS ENUM ('red', 'green', 'blue');

PostgreSQL won't allow a comparison between two different enum types, e.g.:

> SELECT 'one'::number = 'red'::color;
ERROR: operator does not exist: number = color

However, when we say:

SELECT 'one'::number = 'one'::number

Couldn't enum_eq just use get_fn_expr_argtype to determine the type of
enum input rather than rely on it being stored in the value (either
implicitly via OID or explicitly as a word half)?

Also, I can't seem to find the original debates from when enums were
implemented. Does anyone have a link to that thread in the archives?
Thanks.

Joey Adams