Tom Lane wrote:
> Heikki Linnakangas <heikki(at)enterprisedb(dot)com> writes:
>> 1. What's the point of having comparison operators for enums? For most
>> use cases, there's no natural ordering of enum values.
> If you would like to be able to index enum columns, or even GROUP BY one,
> you need those; whether the ordering is arbitrary or not is irrelevant.
Heikki's assertion is wrong in any case. The enumeration definition
defines the ordering, and I can think of plenty of use cases where it
does matter. We do not use an arbitrary ordering. An enum type is an
*ordered* set of string labels. Without this the feature would be close
to worthless. But if a particular application doesn't need them ordered,
it need not use the comparison operators. Leaving aside the uses for
GROUP BY and indexes, I would ask what the justification would be for
leaving off comparison operators?
>> 2. The comparison routine compares oids, right? If the oids wrap around
>> when the enum values are created, the ordering isn't what the user expects.
> This is a fair point --- it'd be better if the ordering were not
> dependent on chance OID assignments. Not sure what we are willing
> to pay to have that though.
This is a non-issue. The code sorts the oids before assigning them:
/* allocate oids */
oids = (Oid *) palloc(sizeof(Oid) * n);
for(i = 0; i < n; i++)
oids[i] = GetNewOid(pg_enum);
/* wraparound is unlikely, but just to be safe...*/
qsort(oids, n, sizeof(Oid), oid_cmp);
>> 3. 4 bytes per value is wasteful if you're storing simple status codes
> I've forgotten exactly which design Tom is proposing to implement here,
> but at least one of the contenders involved storing an OID that would be
> unique across all enum types. 1 byte is certainly not enough for that
> and even 2 bytes would be pretty marginal. I'm unconvinced by arguments
> about 2 bytes being so much better than 4 anyway --- in the majority of
> real table layouts, the hoped-for savings would disappear into alignment
Globally unique is the design adopted, after much on-list discussion.
That was a way of getting it *down* to 4 bytes. The problem is that the
output routines need enough info from just the internal representation
of the type value to do their work. The original suggestions was for 8
bytes - type oid + offset in value set. Having them globally unique lets
us get down to 4.
As for efficiency, I agree with what Tom says about alignment and
padding dissolving away any perceived advantage in most cases. If we
ever get around to optimising record layout we could revisit it.
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