Re: proposal: psql: psql variable BACKEND_PID

Andres Freund <andres(at)anarazel(dot)de> writes:
> On 2023-02-13 18:06:23 +0100, Peter Eisentraut wrote:
>> But what do you need the backend PID for in the first place?

> For me it's using gdb, pidstat, strace, perf, ...
> But for those %p in the PROMPTs is more useful.

Indeed, because ...

> E.g. I fire of a query, it's slower than I'd like, I want to attach perf. Of
> course I can establish a separate connection, query pg_stat_activity there,
> and then perf. But that requires manually filtering pg_stat_activity to find
> the query.

... in this case, the problem is that the session is tied up doing the
slow query. You can't run "select pg_backend_pid()", but you can't
extract a psql variable value either. If you had the foresight to
set up a PROMPT, or to collect the PID earlier, you're good. But I'm
still not seeing where a psql variable makes that easier.

I don't buy Pavel's argument that adding Yet Another built-in variable
adds ease of use. I think what it mostly adds is clutter. I realize
that "psql --help=variables | wc" is already 160+ lines, but that
doesn't mean that making it longer and longer is a net improvement.

regards, tom lane