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Re: TOAST compression

From: Alvaro Herrera <alvherre(at)commandprompt(dot)com>
To: Luke Lonergan <llonergan(at)greenplum(dot)com>
Cc: "Jim C(dot) Nasby" <jnasby(at)pervasive(dot)com>, Neil Conway <neilc(at)samurai(dot)com>,PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: TOAST compression
Date: 2006-02-26 19:18:31
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Lists: pgsql-hackers
Luke Lonergan wrote:
> Jim,
> On 2/26/06 10:37 AM, "Jim C. Nasby" <jnasby(at)pervasive(dot)com> wrote:
> > So the cutover point (on your system with very fast IO) is 4:1
> > compression (is that 20 or 25%?).
> Actually the size of the gzipp'ed binary file on disk was 65MB, compared to
> 177.5MB uncompressed, so the compression ratio is 37% (?), or 2.73:1.

I doubt our algorithm would give the same compression (though I haven't
really measured it).  The LZ implementation we use is supposed to have
lightning speed at the cost of a not-so-good compression ratio.

> No, unfortunately not.  O'Reilly's jobs data have 65K rows, so that would
> work.  How do we implement LZW compression on toasted fields?  I've never
> done it!

See src/backend/utils/adt/pg_lzcompress.c

Alvaro Herrera                      
The PostgreSQL Company - Command Prompt, Inc.

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