Re: Lock-free compaction. Why not? - Mailing list pgsql-hackers

From Robert Haas
Subject Re: Lock-free compaction. Why not?
Date
Msg-id CA+TgmoZMOfX5-dTBvPkSX+bOejxYpbnCr5AHo6Tty7km-yd0Pw@mail.gmail.com
Whole thread Raw
In response to Re: Lock-free compaction. Why not?  (David Rowley <dgrowleyml@gmail.com>)
Responses Re: Lock-free compaction. Why not?
List pgsql-hackers
On Thu, Jul 18, 2024 at 7:08 AM David Rowley <dgrowleyml@gmail.com> wrote:
> On Wed, 10 Jul 2024 at 22:58, Tomas Vondra
> <tomas.vondra@enterprisedb.com> wrote:
> > IIRC long time ago VACUUM FULL actually worked in a similar way, i.e. by
> > moving rows around. I'm not sure if it did the lock-free thing as
> > proposed here (probably not), but I guess at least some of the reasons
> > why it was replaced by CLUSTER would still apply to this new thing.
>
> Yeah, that changed in 9.0.  The old version still obtained an AEL on the table.
>
> I think the primary issue with the old way was index bloat wasn't
> fixed. The release notes for 9.0 do claim the CLUSTER method "is
> substantially faster in most cases", however, I imagine there are
> plenty of cases where it wouldn't be. e.g, it's hard to imagine
> rewriting the entire 1TB table and indexes is cheaper than moving 1
> row out of place row.

The other thing I remember besides index bloat is that it was
crushingly slow. My memory is pretty fuzzy after this long, but I feel
like it was on the order of minutes to do VACUUM FULL when you could
have done CLUSTER in seconds -- and then on top of the long wait you
often ended up using more disk space at the end than you had at the
beginning due to the index bloat. I remember being surprised by the
decision to remove it entirely, but it sure was painful to use.

--
Robert Haas
EDB: http://www.enterprisedb.com



pgsql-hackers by date:

Previous
From: Tom Lane
Date:
Subject: Re: make pg_ctl more friendly
Next
From: James Coleman
Date:
Subject: Seq scan instead of index scan querying single row from primary key on large table