Re: Proposal: custom compression methods

Hi,

On 12/13/2015 06:28 PM, Alexander Korotkov wrote:>
> Compression method of column would be stored in pg_attribute table.
> Dependencies between columns and compression methods would be tracked in
> pg_depend preventing dropping compression method which is currently in
> use. Compression method of the attribute could be altered by ALTER TABLE
> command.
>
> ALTER TABLE table_name ALTER COLUMN column_name SET COMPRESSION METHOD
> compname;

Do you plan to make this available in CREATE TABLE? For example 
Greenplum allows to specify COMPRESSTYPE/COMPRESSLEVEL per column.

What about compression levels? Do you plan to allow tweaking them? 
Tracking them would require another column in pg_attribute, probably.

> Since mixing of different compression method in the same attribute
> would be hard to manage (especially dependencies tracking), altering
> attribute compression method would require a table rewrite.

I don't think the dependency tracking would be a big issue. The easiest 
we could do is simply track which columns used the compression type in 
the past, and scan them when removing it (the compression type).

I think the main obstacle to make this possible is the lack of free 
space in varlena header / need to add the ID of the compression method 
into the value.

FWIW I'd like to allow this (mixing compression types), but I don't 
think it's worth the complexity at this point. We can add that later, if 
it turns out to be a problem in practice (which it probably won't).

regards

--
Tomas Vondra                  http://www.2ndQuadrant.com
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