It seems ALTER TABLE does not support table space. I think this is
neccessary because:
1) if a data disk if full, users need to move table to another table spaces.
2) if performance hits, users could move the table which is likely a bottle neck to another table space.
I know that pg_dump+pg_restore will do the trick, but I think
tablespace can handle this better.
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Tatsuo Ishii