make default TABLESPACE belong to target table.

Dear pgsql community,

I've been using postgres for a long time. Recently I'm doing table
sharding over a bunch of pgsql instances. I'm using multiple tablespaces
one per disk to utilize all the IO bandwidth. Things went on pretty
well, however there is a troublesome problem I have when adding
auxiliary structures to sharded tables, such as Indexes. These objects
have their storage default to the database's tablespace, and it's
difficult to shard them by hand.

I'd like to implement this small feature --- making table's auxiliary
structures store their data to the target table's tablespace by default.
I've done a thorough search over the mailing list and there is nothing
relevant. Well I may miss some corners :-)

What do you think?

Regards,
Amos