Re: [HACKERS] Removing [Merge]Append nodes which contain a single subpath

On 26 October 2017 at 23:42, Antonin Houska <ah@cybertec.at> wrote:
> David Rowley <david.rowley@2ndquadrant.com> wrote:
>
>> Method 1:
>>
>> In set_append_rel_size() detect when just a single subpath would be
>> added to the Append path.
>
> I spent some time reviewing the partition-wise join patch during the last CF
> and have an impression that set_append_rel_size() is called too early to find
> out how many child paths will eventually exist if Append represents a join of
> a partitioned table. I think the partition matching takes place at later stage
> and that determines the actual number of partitions, and therefore the number
> of Append subpaths.

I understand that we must wait until set_append_rel_size() is being
called on the RELOPT_BASEREL since partitions may themselves be
partitioned tables and we'll only know what's left after we've
recursively checked all partitions of the baserel.

I've not studied the partition-wise join code yet, but if we've
eliminated all but one partition in set_append_rel_size() then I
imagine we'd just need to ensure partition-wise join is not attempted
since we'd be joining directly to the only partition anyway.


-- David Rowley                   http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training & Services


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