Re: [HACKERS] Custom compression methods

From: Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Dilip Kumar <dilipbalaut(at)gmail(dot)com>, Andres Freund <andres(at)anarazel(dot)de>, Justin Pryzby <pryzby(at)telsasoft(dot)com>, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>, Alexander Korotkov <a(dot)korotkov(at)postgrespro(dot)ru>, David Steele <david(at)pgmasters(dot)net>, Ildus Kurbangaliev <i(dot)kurbangaliev(at)gmail(dot)com>, Dmitry Dolgov <9erthalion6(at)gmail(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>
Subject: Re: [HACKERS] Custom compression methods
Date: 2021-03-19 22:38:43
Message-ID: 20210319223843.GA18811@alvherre.pgsql
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On 2021-Mar-19, Robert Haas wrote:

> On Fri, Mar 19, 2021 at 10:11 AM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> > Also added a test case for vacuum full to recompress the data.
>
> I committed the core patch (0003) with a bit more editing. Let's see
> what the buildfarm thinks.

I updated the coverage script to use --with-lz4; results are updated.
While eyeballing the results I noticed this bit in
lz4_decompress_datum_slice():

+ /* slice decompression not supported prior to 1.8.3 */
+ if (LZ4_versionNumber() < 10803)
+ return lz4_decompress_datum(value);

which I read as returning the complete decompressed datum if slice
decompression is not supported. I thought that was a bug, but looking
at the caller I realize that this isn't really a problem, since it's
detoast_attr_slice's responsibility to slice the result further -- no
bug, it's just wasteful. I suggest to add comments to this effect,
perhaps as the attached (feel free to reword, I think mine is awkward.)

--
Álvaro Herrera 39°49'30"S 73°17'W
Si no sabes adonde vas, es muy probable que acabes en otra parte.

Attachment Content-Type Size
comments.patch text/x-diff 1.2 KB

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