Re: [RFC] Unsigned integer support.

Tom,

On Fri, Jul 25, 2008 at 12:32 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Consider the idea of not having any uint4-specific arithmetic operators,
> but instead providing the following:
>
> * assignment casts from int4 and int8 to uint4
> (these throw error if out of range, of course)
> * implicit cast from uint4 to int8 (can never fail)
>
> The effect of providing the latter cast would be that any arithmetic
> involving a uint4 column would automatically be done in int8. Which
> would make it a shade slower than a native implementation, but probably
> not enough slower to be a problem --- and you'd avoid having to write
> dozens of operators and underlying support functions. Storing into the
> uint4 column would work fine with no extra notation because of the
> assignment casts.

This is an interesting idea that I will test out tonight. I did have
the following
concern looking through src/backend/utils/adt/int8.c: There is code that is
optionally compiled based on the INT64_IS_BUSTED pre-processor define.
Is this pre-processor define something I should worry about for portability
with this plan?

After I get uint types implemented, for fun I might try some benchmarks
to see if I can detect the int8 overhead on a 32-bit system.

> Moreover, you'd avoid cluttering the system with a pile of cross-type
> operators, which we have recently realized are not a good thing, because
> they increase the likelihood of "ambiguous operator" problems --- see
> http://archives.postgresql.org/pgsql-hackers/2008-06/msg00750.php

Good to know. Thanks for the link.

> For uint8 you'd have to promote to numeric to guarantee no failure
> in the implicit cast; which is going to be a rather bigger performance
> hit, but I don't really see uint8 as being a type with huge demand.

Hopefully I will not need the uint8 type. Right now for a project I am
looking at I need the uint2 and uint4 types. uint8 support can come
later if it is needed or requested.

> Now you probably *will* want cross-type comparison operators, if you
> are going to support indexing of unsigned columns, so that something
> like
> uint4col > 42
> can be indexed without any casting. But limiting yourself to the six
> basic comparison operators certainly makes it a much less bulky project.

This sounds excellent! Hopefully by using these operators I will be able to
avoid most of the casting to int8 for my use, while still providing the
complete functionality for this type.

Thanks again for your review and feedback!

- Ryan