Re: partition tree inspection functions

On 2018/10/09 19:05, Michael Paquier wrote:
> On Tue, Oct 09, 2018 at 06:41:59PM +0900, Amit Langote wrote:
>> Partitioned indexes, just like partitioned tables, don't have their own
>> storage, so pg_relation_size() cannot be used to obtain their size.  We
>> decided that the correct way to get the partitioned table's size is *not*
>> to modify pg_relation_size itself to internally find all partitions and
>> sum their sizes, but to provide a function that returns partitions and
>> tell users to write SQL queries around it to calculate the total size.
>> I'm saying that the new function should be able to be used with
>> partitioned indexes too.
> 
> I have to admit that I find the concept non-intuitive.  A partition tree
> is a set of partitions based on a root called a partitioned table.  A
> partitioned index is not a partition, it is a specific object which is
> associated to a partitioned table without any physical on-disk presence.
> An index is not a partition as well, it is an object associated to one
> relation.

I see it as follows: a partitioned index *does* have partitions and the
partitions in that case are index objects, whereas, a partitioned table's
partitions are table objects.  IOW, I see the word "partition" as
describing a relationship, not any specific object or kind of objects.

> I am not saying that there is no merit in that.  I honestly don't know,
> but being able to list all the partitioned tables and their partition
> tables looks enough to cover all the grounds discussed, and there is no
> need to work out more details for a new clone of find_all_inheritors and
> get_partition_ancestors.

Sorry if I'm misunderstanding something, but why would we need a new
clone?  If we don't change pg_partition_tree() to only accept tables
(regular/partitioned/foreign tables) as input, then the same code can work
for indexes as well.  As long as we store partition relationship in
pg_inherits, same code should work for both.

Thanks,
Amit