Re: [HACKERS] Partition-wise aggregation/grouping

On Sat, Dec 2, 2017 at 4:08 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Fri, Dec 1, 2017 at 7:41 AM, Ashutosh Bapat
> <ashutosh(dot)bapat(at)enterprisedb(dot)com> wrote:
>> This code creates plans where there are multiple Gather nodes under an Append
>> node.
>
> We should avoid that. Starting and stopping workers is inefficient,
> and precludes things like turning the Append into a Parallel Append.

Ah, I didn't think about it. Thanks for bringing it up.

>
>> AFAIU, the workers assigned to one gather node can be reused until that
>> Gather node finishes. Having multiple Gather nodes under an Append mean that
>> every worker will be idle from the time that worker finishes the work till the
>> last worker finishes the work.
>
> No, workers will exit as soon as they finish. They don't hang around idle.

Sorry, I think I used wrong word "idle". I meant that if a worker
finishes and exists, the query can't use it that worker slot until the
next Gather node starts. But as you pointed out, starting and stopping
a worker is costlier than the cost of not using the slot. So we should
avoid such plans.

>
>> index b422050..1941468 100644
>> --- a/src/tools/pgindent/typedefs.list
>> +++ b/src/tools/pgindent/typedefs.list
>> @@ -2345,6 +2345,7 @@ UnlistenStmt
>> UnresolvedTup
>> UnresolvedTupData
>> UpdateStmt
>> +UpperPathExtraData
>> UpperRelationKind
>> UpperUniquePath
>> UserAuth
>>
>> Do we commit this file as part of the feature?
>
> Andres and I regularly commit such changes; Tom rejects them.
>

We will leave it to the committer to decide what to do with this hunk.

--
Best Wishes,
Ashutosh Bapat
EnterpriseDB Corporation
The Postgres Database Company