RE: Parallel Inserts in CREATE TABLE AS

Hi

> Attaching v11 patch set. Please review it further.

Currently with the patch, we can allow parallel CTAS when topnode is Gather.
When top-node is Append and Gather is the sub-node of Append, I think we can still enable 
Parallel CTAS by pushing Parallel CTAS down to the sub-node Gather, such as:

Append
------>Gather
--------->Create table
------------->Seqscan
------>Gather
--------->create table
------------->Seqscan

And the use case seems common to me, such as:
select * from A where xxx union all select * from B where xxx;

I attatch a WIP patch which just show the possibility of this feature.
The patch is based on the latest v11-patch.

What do you think?

Best regards,
houzj