Re: Reducing data type space usage

Ühel kenal päeval, R, 2006-09-15 kell 19:34, kirjutas Tom Lane:
> Bruce Momjian <bruce(at)momjian(dot)us> writes:
> > Oh, OK, I had high byte meaning no header, but clear is better, so
> > 00000001 is 0x01, and 00000000 is "". But I see now that bytea does
> > store nulls, so yea, we would be better using 10000001, and it is the
> > same size as 00000000.
>
> I'm liking this idea more the more I think about it, because it'd
> actually be far less painful to put into the system structure than the
> other idea of fooling with varlena headers. To review: Bruce is
> proposing a var-length type structure with the properties
>
> first byte 0xxxxxxx ---- field length 1 byte, exactly that value
> first byte 1xxxxxxx ---- xxxxxxx data bytes follow

would adding this -

first byte 0xxxxxxx ---- field length 1 byte, exactly that value
first byte 10xxxxxx ---- 0xxxxxx data bytes follow
first byte 110xxxxx -- xxxxx xxxxxxxx data bytes to follow
first byte 111xxxxx -- xxxxx xxxxxxxx xxxxxxxx xxxxxxxx bytes t.flw

be too expensive ?

it seems that for strings up to 63 bytes it would be as expensive as it is
with your proposal, but that it would scale up to 536870912 (2^29) bytes nicely.

this would be extra good for datasets that are mostly below 63 (or 127) with only
a small percentage above

> This can support *any* stored value from zero to 127 bytes long.
> We can imagine creating new datatypes "short varchar" and "short char",
> and then having the parser silently substitute these types for varchar(N)
> or char(N) whenever N <= 127 / max_encoding_length. Add some
> appropriate implicit casts to convert these to the normal varlena types
> for computation, and away you go. No breakage of any existing
> datatype-specific code, just a few additions in places like
> heap_form_tuple.
>
> regards, tom lane
>
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