Re: ParallelFinish-hook of FDW/CSP (Re: [HACKERS] Steps inside ExecEndGather)

On Sat, Feb 25, 2017 at 10:00 AM, Kohei KaiGai <kaigai@kaigai.gr.jp> wrote:
> I tried to add a description how custom-scan callbacks performs under the
> executor, and when invoked roughly.
> However, it is fundamentally not easy for most of people because it assumes
> FDW/CSP author understand the overall behavior of optimizer / executor, not
> only APIs specifications.

I think the statements about Shutdown only being useful in parallel
mode aren't quite striking the right tone.  In theory a node can hold
any kind of resources and find it worthwhile to release them at
shutdown time.  I think we'll eventually find it useful to run
shutdown callbacks in other cases as well - e.g. when a Limit has been
filled.  It's probably true that the undeniable need for this today is
in the parallel query case, but I'd like to have this text read in a
way that doesn't assert that this is the only possible use case,
because I don't think it is.

I rewrote the documentation along those lines a bit and committed this.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
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