Re: pg_stat_statements issue with parallel maintenance (Was Re: WAL usage calculation patch)

Masahiko Sawada <masahiko.sawada@2ndquadrant.com>

From: Masahiko Sawada <masahiko.sawada@2ndquadrant.com>

To: Amit Kapila <amit.kapila16@gmail.com>

Cc: Julien Rouhaud <rjuju123@gmail.com>, Dilip Kumar <dilipbalaut@gmail.com>, Robert Haas <robertmhaas@gmail.com>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>

Date: 2020-03-29T06:19:34Z

Lists: pgsql-hackers

Commits

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Change the display of WAL usage statistics in Explain.
- 69bfaf2e1de4 13.0 landed
Cosmetic fixups for WAL usage work.
- ef08ca113fe3 13.0 landed
Allow parallel create index to accumulate buffer usage stats.
- 5c71362174eb 13.0 landed
- f179e9f01b65 11.8 landed
- 13823ad3b012 12.3 landed
Allow autovacuum to log WAL usage statistics.
- b7ce6de93b59 13.0 landed
Add the option to report WAL usage in EXPLAIN and auto_explain.
- 33e05f89c53e 13.0 landed
Allow pg_stat_statements to track WAL usage statistics.
- 6b466bf5f2be 13.0 landed
Add infrastructure to track WAL usage.
- df3b181499b4 13.0 landed
Include information on buffer usage during planning phase, in EXPLAIN output, take two.
- ce77abe63cfc 13.0 cited
Include information on buffer usage during planning phase, in EXPLAIN output.
- ed7a5095716e 13.0 cited
Allow parallel vacuum to accumulate buffer usage.
- 3a5e22138a8d 13.0 landed
Allow pg_stat_statements to track planning statistics.
- 17e03282241c 13.0 cited

On Sun, 29 Mar 2020 at 14:23, Amit Kapila <amit.kapila16@gmail.com> wrote:
>
> On Sat, Mar 28, 2020 at 8:47 PM Julien Rouhaud <rjuju123@gmail.com> wrote:
> >
> > On Sat, Mar 28, 2020 at 02:38:27PM +0100, Julien Rouhaud wrote:
> > > On Sat, Mar 28, 2020 at 04:14:04PM +0530, Amit Kapila wrote:
> > > >
> > > > I see some basic problems with the patch.  The way it tries to compute
> > > > WAL usage for parallel stuff doesn't seem right to me.  Can you share
> > > > or point me to any test done where we have computed WAL for parallel
> > > > operations like Parallel Vacuum or Parallel Create Index?
> > >
> > > Ah, that's indeed a good point and AFAICT WAL records from parallel utility
> > > workers won't be accounted for.  That being said, I think that an argument
> > > could be made that proper infrastructure should have been added in the original
> > > parallel utility patches, as pg_stat_statement is already broken wrt. buffer
> > > usage in parallel utility, unless I'm missing something.
> >
> > Just to be sure I did a quick test with pg_stat_statements behavior using
> > parallel/non-parallel CREATE INDEX and VACUUM, and unsurprisingly buffer usage
> > doesn't reflect parallel workers' activity.
> >
>
> Sawada-San would like to investigate this? If not, I will look into
> this next week.

Sure, I'll investigate this issue today.

Regards,

-- 
Masahiko Sawada            http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services