Re: macaddr 64 bit (EUI-64) datatype support

On Wed, Nov 23, 2016 at 12:53 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:

> Haribabu Kommi <kommi.haribabu@gmail.com> writes:
> > On Wed, Nov 23, 2016 at 1:42 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> >> The precedent of int4/int8/float4/float8 is that SQL data types should
> >> be named after their length in bytes.  So I'd be inclined to call this
> >> "macaddr8" not "macaddr64".  That would suggest taking the simple
> >> approach of always storing values in the 8-byte format, rather than
> >> dealing with the complexities of having two formats internally, two
> >> display formats, etc.
>
> > Do you prefer the automatic conversion from 6 byte to 8 byte MAC address,
> > This way it takes extra space with new datatype. Is it fine to with new
> > datatype?
>
> Well, I think the space savings would be pretty illusory.  If we use a
> varlena datatype, then old-style MAC addresses would take 7 bytes and
> new-style would take 9.  That's not much of an improvement over always
> taking 8 bytes.  What's worse, if the next field has an alignment
> requirement more than char, is that it's really 8 bytes and 12 bytes (or
> 16!), making this strictly worse than a fixed-length-8-bytes approach.
>
> As against that, if we use a varlena type then we'd have some protection
> against the day when the MAC people realize they were still being
> short-sighted and go up to 10 or 12 or 16 bytes.  But even if that happens
> while Postgres is still in use, I'm not sure that we'd choose to make use
> of the varlena aspect rather than invent a third datatype to go with that
> new version of the standard.  Per the discussion in this thread, varlena
> storage in itself doesn't do very much for the client-side compatibility
> issues.  Making a new datatype with a new, well-defined I/O behavior
> ensures that applications don't get blindsided by a new behavior they're
> not ready for.
>
> In short, I'm leaning to having just a fixed-length-8-byte implementation.
> This may seem like learning nothing from our last go-round, but the
> advantages of varlena are very far in the hypothetical future, and the
> disadvantages are immediate.
>
> Also, if we define macaddr as "always 6 bytes" and macaddr8 as "always 8
> bytes", then there's a very simple way for users to widen an old-style
> address to 8 bytes or convert it back to the 6-byte format: just cast
> to the other datatype.  If the new macaddr type can store both widths
> then you need to invent at least one additional function to provide
> those behaviors.
>

Thanks for your feedback.

Here is attached updated patch with new datatype "macaddr8" with fixed
length
of 8 bytes.

If your input a 6 byte MAC address, it automatically  converts it into an 8
byte
MAC address by adding the reserved keywords and store it as an 8 byte
address.

While sending it to client it always send an 8 byte MAC address.

Currently the casting is supported from macaddr to macaddr8, but not the
other way. This is because, not all 8 byte MAC addresses can be converted
into
6 byte addresses.

Test and doc changes are also added.

comments?

Regards,
Hari Babu
Fujitsu Australia