Re: [PATCH] Increase the maximum value track_activity_query_size

On Sat, Dec 21, 2019 at 1:25 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> > What is the overhead here except the memory consumption?
>
> The time to copy those strings out of shared storage, any time
> you query pg_stat_activity.

It seems like you're masterminding this, and I don't know why. It
seems unlikely that anyone will raise the value unless they have very
long queries, and if those people would rather pay the overhead of
copying more data than have their queries truncated, who are we to
argue?

If increasing the maximum imposed some noticeable cost on
installations the kept the default setting, that might well be a good
argument for not raising the maximum. But I don't think that's the
case. I also suspect that the overhead would be pretty darn small even
for people who *do* raise the default setting. It looks to me like
both reading and write operations on st_activity_raw stop when they
hit a NUL byte, so any performance costs on short queries must come
from second-order effects (e.g. the main shared memory segment is
bigger, so the OS cache is smaller) which are likely irrelevant in
practice.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company