Re: [HACKERS] Custom compression methods

On Mon, 20 Nov 2017 00:23:23 +0100
Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com> wrote:

> On 11/15/2017 02:13 PM, Robert Haas wrote:
> > On Wed, Nov 15, 2017 at 4:09 AM, Ildus Kurbangaliev
> > <i(dot)kurbangaliev(at)postgrespro(dot)ru> wrote:
> >> So in the next version of the patch I can just unlink the options
> >> from compression methods and dropping compression method will not
> >> affect already compressed tuples. They still could be
> >> decompressed.
> >
> > I guess I don't understand how that can work. I mean, if somebody
> > removes a compression method - i.e. uninstalls the library - and you
> > don't have a way to make sure there are no tuples that can only be
> > uncompressed by that library - then you've broken the database.
> > Ideally, there should be a way to add a new compression method via
> > an extension ... and then get rid of it and all dependencies
> > thereupon.
>
> I share your confusion. Once you do DROP COMPRESSION METHOD, there
> must be no remaining data compressed with it. But that's what the
> patch is doing already - it enforces this using dependencies, as
> usual.
>
> Ildus, can you explain what you meant? How could the data still be
> decompressed after DROP COMPRESSION METHOD, and possibly after
> removing the .so library?

The removal of the .so library will broke all compressed tuples. I
don't see a way to avoid it. I meant that DROP COMPRESSION METHOD could
remove the record from 'pg_compression' table, but actually the
compressed tuple needs only a record from 'pg_compression_opt' where
its options are located. And there is dependency between an extension
and the options so you can't just remove the extension without CASCADE,
postgres will complain.

Still it's a problem if the user used for example `SELECT
<compressed_column> INTO * FROM *` because postgres will copy compressed
tuples, and there will not be any dependencies between destination and
the options.

Also thank you for review. I will look into it today.

--
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Ildus Kurbangaliev
Postgres Professional: http://www.postgrespro.com
Russian Postgres Company