Re: Append with naive multiplexing of FDWs

Thanks!

At Wed, 29 Jan 2020 14:41:07 +0800, Movead Li <movead.li@highgo.ca> wrote in
> >"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.

It is "asynchronous append on async-capable'd postgres-fdw scans". It
could be called as such in the sense that it is intended to be used
with sharding.

> >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. 

Did you looked at my benchmarking result upthread?  Even it gives
significant gain even when gathering large number of tuples from
multiple servers or even from a single server.  It is because of its
asynchronous nature.

regards.

--
Kyotaro Horiguchi
NTT Open Source Software Center