Re: Parallel leader process info in EXPLAIN

On Thu, Nov 7, 2019 at 9:48 PM Thomas Munro <thomas.munro@gmail.com> wrote:
>
> On Thu, Nov 7, 2019 at 11:37 PM Rafia Sabih <rafia.pghackers@gmail.com> wrote:
> > ...
> > Also, I noticed that the worker details are displayed for sort node even without verbose, but for scans it is only
withverbose. Am I missing something or there is something behind? However, I am not sure if this is the introduced by
thispatch-set.
 
>
> Yeah, it's a pre-existing thing, but I agree it's an interesting
> difference.  We currently don't have a way to show a 'combined'
> version of a parallel (oblivious) sort: we always show the per-process
> version, and all this patch changes is how we label the leader's
> stats.  I suppose someone could argue that in non-VERBOSE mode we
> should show the total memory usage (sum from all processes). I suppose
> it's possible they use different sort types (one worker runs out of
> work_mem and another doesn't), and I'm not sure how how you'd
> aggregate that.

Over at [1] (incremental sort patch) I had a similar question, since
each sort node (even non-parallel) can execute multiple tuplesorts.
The approach I took was to show both average and max for both disk and
memory usage as well as all sort strategies used. It looks like this:

   ->  Incremental Sort
         Sort Key: a, b
         Presorted Key: a
         Full-sort Groups: 4 (Methods: quicksort) Memory: 26kB (avg), 26kB (max)
         ->  Index Scan using idx_t_a...

It'd be great if that had a use here too :)

James

[1]: https://www.postgresql.org/message-id/CAAaqYe_ctGqQsauuYS5StPULkES7%3Dt8vNwvEPyzXQdbjAuZ6vA%40mail.gmail.com