| From: | "tsunakawa(dot)takay(at)fujitsu(dot)com" <tsunakawa(dot)takay(at)fujitsu(dot)com> |
|---|---|
| To: | "houzj(dot)fnst(at)cn(dot)fujitsu(dot)com" <houzj(dot)fnst(at)cn(dot)fujitsu(dot)com> |
| Cc: | Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>, "tanghy(dot)fnst(at)cn(dot)fujitsu(dot)com" <tanghy(dot)fnst(at)cn(dot)fujitsu(dot)com>, Antonin Houska <ah(at)cybertec(dot)at>, Dilip Kumar <dilipbalaut(at)gmail(dot)com>, vignesh C <vignesh21(at)gmail(dot)com>, Bharath Rupireddy <bharath(dot)rupireddyforpostgres(at)gmail(dot)com>, Andres Freund <andres(at)anarazel(dot)de>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>, Amit Langote <amitlangote09(at)gmail(dot)com> |
| Subject: | RE: Parallel INSERT (INTO ... SELECT ...) |
| Date: | 2021-02-10 05:00:35 |
| Message-ID: | TYAPR01MB2990212C1E8FD175957E2881FE8D9@TYAPR01MB2990.jpnprd01.prod.outlook.com |
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From: Hou, Zhijie/侯 志杰 <houzj(dot)fnst(at)cn(dot)fujitsu(dot)com>
> table and index's size after parallel insert
> ----------------------
> postgres=# select pg_size_pretty(pg_indexes_size('testscan_index'));
> pg_size_pretty
> ----------------
> 4048 kB
> (1 row)
>
> postgres=# select pg_size_pretty(pg_relation_size('testscan_index'));
> pg_size_pretty
> ----------------
> 4768 kB
> (1 row)
Which of the above shows the table size? What does pg_indexes_size() against an index (testscan_index) return?
> IMO, due to the difference of inserts with parallel execution,
> the btree insert's cost is more than serial.
>
> At the same time, the parallel does not have a huge performance gain with
> bitmapscan,
> So the extra cost of btree index will result in performance degradation.
How did you know that the parallelism didn't have a huge performance gain with bitmap scan?
[serial]
-> Bitmap Heap Scan on public.x (cost=3272.20..3652841.26 rows=79918 width=8) (actual time=8.096..41.005 rows=129999 loops=1)
[parallel]
-> Parallel Bitmap Heap Scan on public.x (cost=3272.20..1260119.35 rows=19980 width=8) (actual time=5.832..14.787 rows=26000 loops=5)
Regards
Takayuki Tsunakawa
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