Re: why there is not VACUUM FULL CONCURRENTLY?

Alvaro Herrera <alvherre@alvh.no-ip.org> wrote:

> On 2024-Jan-30, Pavel Stehule wrote:
>
> > One of my customer today is reducing one table from 140GB to 20GB.  Now he
> > is able to run archiving. He should play with pg_repack, and it is working
> > well today, but I ask myself, what pg_repack does not be hard to do
> > internally because it should be done for REINDEX CONCURRENTLY. This is not
> > a common task, and not will be, but on the other hand, it can be nice to
> > have feature, and maybe not too hard to implement today. But I didn't try it
>
> FWIW a newer, more modern and more trustworthy alternative to pg_repack
> is pg_squeeze, which I discovered almost by random chance, and soon
> discovered I liked it much more.
>
> So thinking about your question, I think it might be possible to
> integrate a tool that works like pg_squeeze, such that it runs when
> VACUUM is invoked -- either under some new option, or just replace the
> code under FULL, not sure.  If the Cybertec people allows it, we could
> just grab the pg_squeeze code and add it to the things that VACUUM can
> run.

There are no objections from Cybertec. Nevertheless, I don't expect much code
to be just copy & pasted. If I started to implement the extension today, I'd
do some things in a different way. (Some things might actually be simpler in
the core, i.e. a few small changes in PG core are easier than the related
workarounds in the extension.)

The core idea is that: 1) a "historic snapshot" is used to get the current
contents of the table, 2) logical decoding is used to capture the changes done
while the data is being copied to new storage, 3) the exclusive lock on the
table is only taken for very short time, to swap the storage (relfilenode) of
the table.

I think it should be coded in a way that allows use by VACUUM FULL, CLUSTER,
and possibly some subcommands of ALTER TABLE. For example, some users of
pg_squeeze requested an enhancement that allows the user to change column data
type w/o service disruption (typically when it appears that integer type is
going to overflow and change bigint is needed).

Online (re)partitioning could be another use case, although I admit that
commands that change the system catalog are a bit harder to implement than
VACUUM FULL / CLUSTER.

One thing that pg_squeeze does not handle is visibility: it uses heap_insert()
to insert the tuples into the new storage, so the problems described in [1]
can appear. The in-core implementation should rather do something like tuple
rewriting (rewriteheap.c).

Is your plan to work on it soon or should I try to write a draft patch? (I
assume this is for PG >= 18.)

[1] https://www.postgresql.org/docs/current/mvcc-caveats.html

--
Antonin Houska
Web: https://www.cybertec-postgresql.com