Autovacuum on partitioned table

Hello,

Greg reported in [1] before, autovacuum ignores partitioned tables.
That is, even if individual partitions’ statistics are updated, its parent's
statistics are not updated.  This is TODO for declarative partitioning.
As Amit mentioned in [2], a way to make parent's statistics from
partitions' statistics without scanning the partitions would be nice,
but it will need a lot of modifications.  So I tried to fix that using the
current analyze method.

The summary of the attached patch is as follows:
* If the relation is a partitioned table, check its children if they need
  vacuum or analyze.  Children need to do that are added to
  a table list for autovacuuum.  At least one child is added to the list,
  the partitioned table is also added to the list.  Then, autovacuum
  runs on all the tables in the list.
* If the partitioned table has foreign partitions, ignore them.

When the parent has children don't need vacuum/analyze or foreign
partitions, parent's stats are updated scanning the current data of all
children, so old stats and new are mixed within the partition tree.
Is that suitable?  Any thoughts?

[1] https://www.postgresql.org/message-id/CAM-w4HMQKC8hw7nB9TW3OV%2BhkB5OUcPtvr_U_EiSOjByoa-e4Q%40mail.gmail.com
[2] https://www.postgresql.org/message-id/CA%2BHiwqEeZQ-H2OVbHZ%3Dn2RNNPF84Hygi1HC-MDwC-VnBjpA1%3DQ%40mail.gmail.com

-- 
Best regards,
Yuzuko Hosoya
NTT Open Source Software Center