Date: Fri, 9 Jul 2021 10:43:55 +0200
From: hubert depesz lubaczewski <depesz@depesz.com>
Sender: depesz@depesz.com
To: PostgreSQL mailing lists <pgsql-bugs@lists.postgresql.org>
Subject: I/O timigns don't include time for temp buffers
Message-ID: <20210709084355.GA6251@depesz.com>
Reply-To: depesz@depesz.com
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
User-Agent: Mutt/1.10.1 (2018-07-13)
Archived-At: 
 <https://www.postgresql.org/message-id/20210709084355.GA6251%40depesz.com>
Precedence: bulk

Hi,
I tested it on newest 15devel from git, but I believe it goes back.

It looks that I/O timings in explain show that that was necessary to
read/write *shared* and *local* buffers, but temp are excluded for some
reason.

For example:
$ explain (analyze, buffers) select count(*) from plans.part_0;
                                                                    QUERY PLAN                                                                    
──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
 Aggregate  (cost=464.74..464.75 rows=1 width=8) (actual time=14.580..14.582 rows=1 loops=1)
   Buffers: shared hit=13 read=16 dirtied=7
   I/O Timings: read=6.325
   ->  Index Only Scan using part_0_added_by_idx on part_0  (cost=0.29..413.82 rows=20369 width=0) (actual time=0.031..11.428 rows=20359 loops=1)
         Heap Fetches: 50
         Buffers: shared hit=13 read=16 dirtied=7
         I/O Timings: read=6.325
 Planning:
   Buffers: shared hit=166
 Planning Time: 0.414 ms
 Execution Time: 14.618 ms
(11 rows)

Shows that we have normal I/O Timings info, 6.3ms for reading 16 pages.

But when I don't touch any shared/local pages:
$ explain (analyze, buffers) select count(*) from generate_series(1,10000000);
                                                                 QUERY PLAN                                                                 
────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
 Aggregate  (cost=125000.00..125000.01 rows=1 width=8) (actual time=1536.899..1536.900 rows=1 loops=1)
   Buffers: temp read=17090 written=17090
   ->  Function Scan on generate_series  (cost=0.00..100000.00 rows=10000000 width=0) (actual time=611.066..1210.260 rows=10000000 loops=1)
         Buffers: temp read=17090 written=17090
 Planning Time: 0.071 ms
 JIT:
   Functions: 4
   Options: Inlining false, Optimization false, Expressions true, Deforming true
   Timing: Generation 0.202 ms, Inlining 0.000 ms, Optimization 0.089 ms, Emission 1.528 ms, Total 1.819 ms
 Execution Time: 1565.635 ms
(10 rows)

The I/O timings info is fully missing.

With more info you can see it in https://explain.depesz.com/s/xWRP#source

Line 11 (Hash) has info:
Buffers: shared hit=229 read=19,627 written=3, temp written=1,100
I/O Timings: read=31.173 write=0.049

and it's only subnode (Nested loop) shows:

Buffers: shared hit=229 read=19,627 written=3
I/O Timings: read=31.173 write=0.049

This shows that the Hash wrote 1100 temp buffers, but I/O timings didn't increase at all.

So far it looks that only shared/local buffers are taken into account
for I/O timings, but, as far as I can read it,
https://www.postgresql.org/docs/current/sql-explain.html doesn't mention
this.

Best regards,

depesz