I/O timigns don't include time for temp buffers
От | hubert depesz lubaczewski |
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Тема | I/O timigns don't include time for temp buffers |
Дата | |
Msg-id | 20210709084355.GA6251@depesz.com обсуждение исходный текст |
Ответы |
Re: I/O timigns don't include time for temp buffers
(Masahiko Sawada <sawada.mshk@gmail.com>)
|
Список | pgsql-bugs |
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..11.428rows=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=10000000loops=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
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