From: | Movead Li <movead(dot)li(at)highgo(dot)ca> |
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To: | "Kyotaro Horiguchi" <horikyota(dot)ntt(at)gmail(dot)com> |
Cc: | "bruce" <bruce(at)momjian(dot)us>, "ahsan(dot)hadi" <ahsan(dot)hadi(at)highgo(dot)ca>, "robertmhaas" <robertmhaas(at)gmail(dot)com>, "thomas(dot)munro" <thomas(dot)munro(at)gmail(dot)com>, "pgsql-hackers" <pgsql-hackers(at)postgresql(dot)org>, "sfrost" <sfrost(at)snowman(dot)net> |
Subject: | Re: Append with naive multiplexing of FDWs |
Date: | 2020-01-29 06:41:07 |
Message-ID: | 16ff008362d.edad0e7a133214.1991835179355654367@highgo.ca |
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Lists: | pgsql-hackers |
Hello Kyotaro,
>"Parallel scan" at the moment means multiple workers fetch unique
>blocks from *one* table in an arbitrated manner. In this sense
>"parallel FDW scan" means multiple local workers fetch unique bundles
>of tuples from *one* foreign table, which means it is running on a
>single session. That doesn't offer an advantage.
It maybe not "parallel FDW scan", it can be "parallel shards scan"
the local workers will pick every foreign partition to scan. I have ever
draw a picture about that you can see it in the link below.
https://www.highgo.ca/2019/08/22/parallel-foreign-scan-of-postgresql/
I think the "parallel shards scan" make sence in this way.
>If parallel query processing worked in worker-per-table mode,
>especially on partitioned tables, maybe the current FDW would work
>without much of modification. But I believe asynchronous append on
>foreign tables on a single process is far resource-effective and
>moderately faster than parallel append.
As the test result, current patch can not gain more performance when
it returns a huge number of tuples. By "parallel shards scan" method,
it can work well, because the 'parallel' can take full use of CPUs while
'asynchronous' can't.
Highgo Software (Canada/China/Pakistan)
URL : http://www.highgo.ca/
EMAIL: mailto:movead(dot)li(at)highgo(dot)ca
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