Re: [HACKERS] cidr'

> Bruce Momjian <maillist@candle.pha.pa.us> writes:
> >> Would it make sense to use atttypmod to distinguish several different
> >> subtypes of CIDR?  "4 bytes", "4 bytes + mask", "6 bytes", "6 bytes
> >> + mask" seem like interesting possibilities.
>
> > Yes, that is the proper way to go, though atttypmod is something on
> > column, not on each data row.  It is specified when the column is
> > created.
>
> Right, that's what I had in mind.  If you *know* that every entry in
> your table only needs IPv4, you can specify that when making the table
> and save a couple of bytes per entry.
>
> The alternative solution is to make CIDR a variable-length type, but
> I think the overhead of that would be as much or more than the possible
> savings, no?
>
> I don't know whether having multiple top-level types would be better
> or worse than one type with a subtype code.

The byte size is really not an issue to me.  You can do ip6 and still
put it in eight bytes.  If you make it a variable-lengh type, you have
the length on each field, and that is four bytes right there, so you are
better doing eight bytes from the start.

    ip4    5 btyes(4 + precision)
    ip6    7 bytes(6 + precision)

If you want ip6 now, just take eight bytes and make it a fixed length.
The backend it going to round the disk storage of 5 bytes up to eight
anyway, unless the next field is int2 or char1.

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