Re: Why does create_gather_merge_plan need make_sort?

On 11/22/20 10:31 PM, Tom Lane wrote:
> Tomas Vondra <tomas.vondra@enterprisedb.com> writes:
>> On 11/20/20 11:24 PM, James Coleman wrote:
>>> While looking at another issue I noticed that create_gather_merge_plan
>>> calls make_sort if the subplan isn't sufficiently sorted. In all of
>>> the cases I've seen where a gather merge path (not plan) is created
>>> the input path is expected to be properly sorted, so I was wondering
>>> if anyone happened to know what case is being handled by the make_sort
>>> call. Removing it doesn't seem to break any tests.
> 
>> Yeah, I think you're right this is dead code, essentially. We're only
>> ever calling create_gather_merge_path() with pathkeys matching the
>> subpath. And it's like that on REL_12_STABLE too, i.e. before the
>> incremental sort was introduced.
> 
> It's probably there by analogy to the other callers of
> prepare_sort_from_pathkeys, which all do at least a conditional
> make_sort().  I'd be inclined to leave it there; it's cheap insurance
> against somebody weakening the existing behavior.
> 

But how do we know it's safe to actually do the sort there, e.g. in
light of the volatility/parallel-safety issues discussed in other threads?

I agree the check may be useful, but maybe we should just do elog(ERROR)
instead.



regards

-- 
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company