Re: do only critical work during single-user vacuum?

On Wed, Jan 12, 2022 at 10:57 AM Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
>
> On Tue, Jan 11, 2022 at 4:59 PM John Naylor
> <john(dot)naylor(at)enterprisedb(dot)com> wrote:
> > I've attached a PoC *untested* patch to show what it would look like
> > as a top-level statement. If the "shape" is uncontroversial, I'll put
> > work into testing it and fleshing it out.
>
> Great!

+1

>
> > For the PoC I wanted to try re-using existing keywords. I went with
> > "VACUUM LIMIT" since LIMIT is already a keyword that cannot be used as
> > a table name. It also brings "wraparound limit" to mind. We could add
> > a single-use unreserved keyword (such as VACUUM_MINIMAL or
> > VACUUM_FAST), but that doesn't seem great.
>
> This seems reasonable, but you could add a new option instead, without
> much downside. While INDEX_CLEANUP kind of looks like a keyword, it
> isn't really a keyword. (Perhaps you knew this already.)
>
> Making this a new option is a little awkward, admittedly. It's not
> clear what it means to "VACUUM (LIMIT) my_table" -- do you just throw
> an error for stuff like that? So perhaps your approach of adding
> VacuumMinimalStmt (a minimal variant of the VACUUM command) is better.

It seems to me that adding new syntax instead of a new option is less
flexible. In the future, for instance, when we support parallel heap
scan for VACUUM, we may want to add a parallel-related option to both
VACUUM statement and VACUUM LIMIT statement. VACUUM LIMIT statement
would end up becoming like VACUUM statement?

As another idea, we might be able to add a new option that takes an
optional integer value, like VACUUM (MIN_XID), VACUUM (MIN_MXID), and
VACUUM (MIN_XID 500000). We vacuum only tables whose age is older than
the given value. If the value is omitted, we vacuum only tables whose
age exceeds a threshold (say autovacuum_freeze_max_age * 0.95), which
can be used in an emergency case and output in GetNewTransactionID()
WARNINGs output. vacuumdb’s --min-xid-age and --min-mxid-age can use
this option instead of fetching the list of tables from the server.

Regards,

--
Masahiko Sawada
EDB: https://www.enterprisedb.com/