On Wed, Aug 16, 2017 at 11:03 AM, Tom Lane <t...@sss.pgh.pa.us> wrote:
> Robert Haas <robertmh...@gmail.com> writes:
>> On Tue, Aug 15, 2017 at 6:40 PM, Tom Lane <t...@sss.pgh.pa.us> wrote:
>>> (In fact, a quick look shows a counterexample: if we pick a MinMaxAgg
>>> path, that's parallel unsafe, but the original query might've been
>>> completely safe.)
>> I'm quite confused here.  What's parallel-unsafe about a MinMaxAgg?
>> There might be some reason why it's parallel-restricted, but it
>> shouldn't be parallel-unsafe.
> Well, it has subplans, so formally I think it's restricted not unsafe
> --- but the parallel_safe marking on constructed paths/plans is only
> safe/not-safe, not a three-way.

True, but when parallel_safe it not set, that means it's not
parallel-safe, so either parallel-restricted or parallel-unsafe.  But
if parallelModeOK is true, then it had better be parallel-restricted,
not parallel-unsafe.  Which in turn means that it had better be
perfectly safe to run the plan under
EnterParallelMode()/ExitParallelMode().  If it's not, then <original
query> UNION ALL <something that makes a Gather> would blow up.

So I still don't see what's wrong here, other than that the comment is
evidently not half clear enough.  I don't think this is the first time
we've been over all this.

Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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