Re: [HACKERS] Custom compression methods

On Fri, Apr 20, 2018 at 7:45 PM, Konstantin Knizhnik <k.knizhnik@postgrespro.ru> wrote:

On 30.03.2018 19:50, Ildus Kurbangaliev wrote:

On Mon, 26 Mar 2018 20:38:25 +0300
Ildus Kurbangaliev <i.kurbangaliev@postgrespro.ru> wrote:

Attached rebased version of the patch. Fixed conflicts in pg_class.h.

New rebased version due to conflicts in master. Also fixed few errors
and removed cmdrop method since it couldnt be tested.

 I seems to be useful (and not so difficult) to use custom compression methods also for WAL compression: replace direct calls of pglz_compress in xloginsert.c

I'm going to object this at point, and I've following arguments for that:

1) WAL compression is much more critical for durability than datatype

compression.  Imagine, compression algorithm contains a bug which

cause decompress method to issue a segfault.  In the case of datatype

compression, that would cause crash on access to some value which

causes segfault; but in the rest database will be working giving you

a chance to localize the issue and investigate that.  In the case of

WAL compression, recovery would cause a server crash.  That seems

to be much more serious disaster.  You wouldn't be able to make

your database up and running and the same happens on the standby.

Well, I do not think that somebody will try to implement its own compression algorithm...
From my point of view the main value of this patch is that it allows to replace pglz algorithm with more efficient one, for example zstd.
At some data sets zstd provides more than 10 times better compression ratio and at the same time is faster then pglz.
I do not think that risk of data corruption caused by WAL compression with some alternative compression algorithm (zlib, zstd,...) is higher than in case of using builtin Postgres compression.

2) Idea of custom compression method is that some columns may

have specific data distribution, which could be handled better with

particular compression method and particular parameters.  In the

WAL compression you're dealing with the whole WAL stream containing

all the values from database cluster.  Moreover, if custom compression

method are defined for columns, then in WAL stream you've values

already compressed in the most efficient way.  However, it might

appear that some compression method is better for WAL in general

case (there are benchmarks showing our pglz is not very good in

comparison to the alternatives).  But in this case I would prefer to just

switch our WAL to different compression method one day.  Thankfully

we don't preserve WAL compatibility between major releases.

Frankly speaking I do not believe that somebody will use custom compression in this way: implement its own compression methods for the specific data type.
May be just for json/jsonb, but also only in the case when custom compression API allows to separately store compression dictionary (which as far as I understand is not currently supported).

When I worked for SciDB (database for scientists which has to deal mostly with multidimensional arrays of data) our first intention was to implement custom compression methods for the particular data types and data distributions. For example, there are very fast, simple and efficient algorithms for encoding sequence of monotonically increased integers, ....
But after several experiments we rejected this idea and switch to using generic compression methods. Mostly because we do not want compressor to know much about page layout, data type representation,... In Postgres, from my point of view,  we have similar situation. Assume that we have column of serial type. So it is good candidate of compression, isn't it?
But this approach deals only with particular attribute values. It can not take any advantages from the fact that this particular column is monotonically increased. It can be done only with page level compression, but it is a different story.

So current approach works only for blob-like types: text, json,...  But them usually have quite complex internal structure and for them universal compression algorithms used to be more efficient than any hand-written specific implementation. Also algorithms like zstd, are able to efficiently recognize and compress many common data distributions, line monotonic sequences, duplicates, repeated series,...

3) This patch provides custom compression methods recorded in

the catalog.  During recovery you don't have access to the system

catalog, because it's not recovered yet, and can't fetch compression

method metadata from there.  The possible thing is to have GUC,

which stores shared module and function names for WAL compression.

But that seems like quite different mechanism from the one present

in this patch.

I do not think that assignment default compression method through GUC is so bad idea.

Taking into account all of above, I think we would give up with custom

WAL compression method.  Or, at least, consider it unrelated to this

patch.

Sorry for repeating the same thing, but from my point of view the main advantage of this patch is that it allows to replace pglz with more efficient compression algorithms.
I do not see much sense in specifying custom compression method for some particular columns.
It will be more useful from my point of view to include in this patch implementation of compression API not only or pglz, but also for zlib, zstd and may be for some other popular compressing libraries which proved their efficiency.

Postgres already has zlib dependency (unless explicitly excluded with --without-zlib), so zlib implementation can be included in Postgres build.
Other implementations can be left as module which customer can build himself. It is certainly less convenient, than using preexistred stuff, but much more convenient then making users to write this code themselves.


There is yet another aspect which is not covered by this patch: streaming compression.
Streaming compression is needed if we want to compress libpq traffic. It can be very efficient for COPY command and for replication. Also libpq compression can improve speed of queries returning large results (for example containing JSON columns) throw slow network.
I have  proposed such patch for libpq, which is using either zlib, either zstd streaming API. Postgres built-in compression implementation doesn't have streaming API at all, so it can not be used here. Certainly support of streaming  may significantly complicates compression API, so I am not sure that it actually needed to be included in this patch.
But I will be pleased if Ildus can consider this idea.

-- 
Konstantin Knizhnik
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company