I think you are right after reading code in compute_parallel_vacuum_workers() :)

________________________________
发件人: Tom Lane <[email protected]>
发送时间: 2020年12月1日 2:54
收件人: Yulin PEI <[email protected]>
抄送: Masahiko Sawada <[email protected]>; [email protected] 
<[email protected]>
主题: Re: 回复: [PATCH] BUG FIX: Core dump could happen when VACUUM FULL in 
standalone mode

Yulin PEI <[email protected]> writes:
> Yes, I agree because (IsNormalProcessingMode() ) means that current process 
> is not in bootstrap mode and postmaster process will not build index.
> So my new modified patch is attached.

This is a good catch, but the proposed fix still seems pretty random
and unlike how it's done elsewhere.  It seems to me that since
index_build() is relying on plan_create_index_workers() to assess
parallel safety, that's where to check IsUnderPostmaster.  Moreover,
the existing code in compute_parallel_vacuum_workers (which gets
this right) associates the IsUnderPostmaster check with the initial
check on max_parallel_maintenance_workers.  So I think that the
right fix is to adopt the compute_parallel_vacuum_workers coding
in plan_create_index_workers, and thereby create a model for future
uses of max_parallel_maintenance_workers to follow.

                        regards, tom lane

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