On 19th of March 2021, Robert Haas committed patch:
Allow configurable LZ4 TOAST compression. There is now a per-column COMPRESSION option which can be set to pglz (the default, and the only option in up until now) or lz4. Or, if you like, you can set the new default_toast_compression GUC to lz4, and then that will be the default for new table columns for which no value is specified. We don't have lz4 support in the PostgreSQL code, so to use lz4 compression, PostgreSQL must be built --with-lz4. In general, TOAST compression means compression of individual column values, not the whole tuple, and those values can either be compressed inline within the tuple or compressed and then stored externally in the TOAST table, so those properties also apply to this feature. Prior to this commit, a TOAST pointer has two unused bits as part of the va_extsize field, and a compessed datum has two unused bits as part of the va_rawsize field. These bits are unused because the length of a varlena is limited to 1GB; we now use them to indicate the compression type that was used. This means we only have bit space for 2 more built-in compresison types, but we could work around that problem, if necessary, by introducing a new vartag_external value for any further types we end up wanting to add. Hopefully, it won't be too important to offer a wide selection of algorithms here, since each one we add not only takes more coding but also adds a build dependency for every packager. Nevertheless, it seems worth doing at least this much, because LZ4 gets better compression than PGLZ with less CPU usage. It's possible for LZ4-compressed datums to leak into composite type values stored on disk, just as it is for PGLZ. It's also possible for LZ4-compressed attributes to be copied into a different table via SQL commands such as CREATE TABLE AS or INSERT .. SELECT. It would be expensive to force such values to be decompressed, so PostgreSQL has never done so. For the same reasons, we also don't force recompression of already-compressed values even if the target table prefers a different compression method than was used for the source data. These architectural decisions are perhaps arguable but revisiting them is well beyond the scope of what seemed possible to do as part of this project. However, it's relatively cheap to recompress as part of VACUUM FULL or CLUSTER, so this commit adjusts those commands to do so, if the configured compression method of the table happens not to match what was used for some column value stored therein. Dilip Kumar. The original patches on which this work was based were written by Ildus Kurbangaliev, and those were patches were based on even earlier work by Nikita Glukhov, but the design has since changed very substantially, since allow a potentially large number of compression methods that could be added and dropped on a running system proved too problematic given some of the architectural issues mentioned above; the choice of which specific compression method to add first is now different; and a lot of the code has been heavily refactored. More recently, Justin Przyby helped quite a bit with testing and reviewing and this version also includes some code contributions from him. Other design input and review from Tomas Vondra, Álvaro Herrera, Andres Freund, Oleg Bartunov, Alexander Korotkov, and me. Discussion: http://postgr.es/m/20170907194236.4cefce96%40wp.localdomain Discussion: http://postgr.es/m/CAFiTN-uUpX3ck%3DK0mLEk-G_kUQY%3DSNOTeqdaNRR9FMdQrHKebw%40mail.gmail.com
As you perhaps know, when you store large text it gets internally compressed. Of course not only text, but that's what I will focus on now.
So far compression was done using PostgreSQL's implementation of LZ algorithm.
Now, thanks to this patch, we can have one more algorithm. TO use it I had to install liblz4-dev package, and then use –with-lz4 option for configure.
After compilation, I can see that LZ4 is available by doing:
=$ pg_config --configure | grep -oP '\S+lz4\S+' '--with-lz4'
To test it I made simple table:
=$ create table pgdocs ( id serial primary key, filename text not null unique, body text );
Then, I loaded there whole Pg documentation (in html format). Each file was one row. It looked like this:
$ select id, filename, length(body) from pgdocs order by random() limit 5; id │ filename │ length ─────┼────────────────────────────────────┼──────── 829 │ sql-altertsconfig.html │ 8617 2 │ admin.html │ 17620 746 │ spi-spi-cursor-open-with-args.html │ 7572 248 │ ecpg-concept.html │ 6130 726 │ source.html │ 3042 (5 rows)
Then I made a copy using LZ4 compression for body:
=$ create table pgdocs4 ( id serial primary key, filename text not null unique, body text ); =$ alter table pgdocs4 alter column body set compression lz4; =$ insert into pgdocs4 select * from pgdocs;
I checked \d+ of both tables, to make sure that compression is as I wanted:
$ \d+ pgdocs Table "public.pgdocs" Column │ Type │ Collation │ Nullable │ Default │ Storage │ Compression │ Stats target │ Description ──────────┼─────────┼───────────┼──────────┼────────────────────────────────────┼──────────┼─────────────┼──────────────┼───────────── id │ integer │ │ not null │ nextval('pgdocs_id_seq'::regclass) │ plain │ │ │ filename │ text │ │ not null │ │ extended │ pglz │ │ body │ text │ │ │ │ extended │ pglz │ │ Indexes: "pgdocs_pkey" PRIMARY KEY, btree (id) "pgdocs_filename_key" UNIQUE CONSTRAINT, btree (filename) Access method: heap $ \d+ pgdocs4 Table "public.pgdocs4" Column │ Type │ Collation │ Nullable │ Default │ Storage │ Compression │ Stats target │ Description ──────────┼─────────┼───────────┼──────────┼─────────────────────────────────────┼──────────┼─────────────┼──────────────┼───────────── id │ integer │ │ not null │ nextval('pgdocs4_id_seq'::regclass) │ plain │ │ │ filename │ text │ │ not null │ │ extended │ pglz │ │ body │ text │ │ │ │ extended │ lz4 │ │ Indexes: "pgdocs4_pkey" PRIMARY KEY, btree (id) "pgdocs4_filename_key" UNIQUE CONSTRAINT, btree (filename) Access method: heap
As you can see in Compression column, there is lz4 for body of pgdocs4 table.
First thing I checked were sizes:
$ \dt+ List of relations Schema │ Name │ Type │ Owner │ Persistence │ Access Method │ Size │ Description ────────┼──────────┼───────┼────────┼─────────────┼───────────────┼────────────┼───────────── public │ pgdocs │ table │ depesz │ permanent │ heap │ 5952 kB │ public │ pgdocs4 │ table │ depesz │ permanent │ heap │ 5936 kB │ (2 rows)
This proved to be my mistake, as pointed in comment by Kris. I forgot that copying data will not recompress it. So, I had to vacuum full the table, and afterwards:
$ \dt+ List of relations Schema │ Name │ Type │ Owner │ Persistence │ Access Method │ Size │ Description ────────┼──────────┼───────┼────────┼─────────────┼───────────────┼────────────┼───────────── public │ pgdocs │ table │ depesz │ permanent │ heap │ 5952 kB │ public │ pgdocs4 │ table │ depesz │ permanent │ heap │ 6592 kB │ (2 rows)
So – lz4 is using more disk space, by about 10%.
But what about speed?
To test it I figured I'll update the table, wrapping body with [ / ], by doing:
=$ update pgdocs set body = '[' || body || ']'; =$ update pgdocs4 set body = '[' || body || ']';
Ran it 3 times, and got nice surprise. Times for pglz update were: 279.084, 264.014, and 278.946 ms. And times for LZ4 were: 101.977, 130.052, and 112.037 ms.
This suggests that the same operation using lz4 compression using less than 50% of the time that pglz needed!
Based on Kris comment I also checked sizes afterwards: pgdocs was 25,520kB, while pgdocs4 was 25792kB – difference of a bit less than 10%.
Obviously my test was very small, but it seems to suggest that it is a very promising change, at least for the ones that store non-trivially sized blocks of data in Pg.
Thanks a lot to everyone that worked on this.