Re: [HACKERS] multi-level partitions and partition-wise joins

Another question: do we want to commit the code for creating
unflattened hierarchy separately or along with partition-wise join?

On Fri, Dec 23, 2016 at 9:58 AM, Ashutosh Bapat
<ashutosh.bapat@enterprisedb.com> wrote:
> On Thu, Dec 22, 2016 at 10:52 PM, Robert Haas <robertmhaas@gmail.com> wrote:
>> On Wed, Dec 21, 2016 at 11:31 PM, Ashutosh Bapat
>> <ashutosh.bapat@enterprisedb.com> wrote:
>>> Given the scenario described above, it looks like we have to retain
>>> partition hierarchy in the form of inheritance hierarchy in order to
>>> implement partition-wise joins for multi-leveled partition tables. Is
>>> that the right thing to do? PFA a patch retained by Amit Langote to
>>> translate partition hierarchy into inheritance hierarchy. Is this
>>> something on the right direction?
>>
>> I am not sure whether Amit's patch is the right way to go.  I don't
>> fully understand it, and I remember complaining about some aspects of
>> it before, such as this unexplained and fairly random-looking
>> exception:
>>
>> +    /*
>> +     * Do not flatten the inheritance hierarchy if partitioned table, unless
>> +     * this is the result relation.
>> +     */
>>
>> However, I think the overall idea of doing flattening later in the
>> process for partitioned tables is probably correct.  It's not that we
>> shouldn't do flattening at all -- the final Plan shouldn't involve
>> nested Append nodes -- but maybe we should delay it. Perhaps the Path
>> tree retains the structure and the final Plan flattens it.
>
> While creating append paths we flatten any append paths added to the children.
>
>> We might
>> consider doing that way for both inheritance trees and partitioning,
>> just so we don't have two different code paths to validate.
>>
>
> AFAIU the reason why we chose to flatten the inheritance hierarchy is
> multiple inheritance. Since the same child can inherit from two
> parents, in an unflattened version its paths would be included twice.
> It would be clumsy to keep the inheritance unflattened but not include
> a relation more than once in the final plan tree.
>
> However, for partitioned tables, we are guaranteed that there's only a
> single parent and thus every child relation will be considered only
> once. We will need separate code to handle (possible) multiple
> inheritance and strictly single inheritance imposed by partitioning.
>
> --
> Best Wishes,
> Ashutosh Bapat
> EnterpriseDB Corporation
> The Postgres Database Company



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