Re: Alternative variable length structure

On Thu, 2005-09-08 at 09:53 -0700, Josh Berkus wrote:
> Takahiro,
>
> > PostgreSQL can treat variable-length data flexibly, but therefore
> > it consumes more spaces if we store short data. Headers of
> > variable-length types use 4 bytes regardless of the data length.
> >
> > My idea is to change the header itself to variable-length.
> > In order to reduce the size of short data, I wrote a patch to encode
> > lengths into the first several bits of structure. Also, the alignments
> > of the types were changed to 'char' from 'int'.
> >
> > I know my patch is still insufficient, for example, the types cannot
> > be TOASTed. But I guess this compression works well for short text.
>
> Hmmm. Seems like these would be different data types from the standard ones
> we deal with. I can see the value for data warehousing, for example.
>
> Wouldn't this require creating, for example, a SHORTTEXT type? Or were you
> planning this to handle VARCHAR(6) and the like? If so, how would we deal
> with users who change the length via ALTER TABLE?

I believe ALTER TABLE always rewrites the structure using a standard
cast or another expression.

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