Re: BUG #19686: Rolling back SET TABLESPACE

Hi Shihao,

> I personally prefer v6, but that should be a committer decision.

Agreed it's a committer call. I'm re-attaching v7 as a plain .patch: it
went as .patch.txt before, which the cfbot skips, so the entry was still
testing the first version. No code change -- it applies unchanged on
today's master and REL_19 through REL_14, make check and the standby,
crash and wal_level=minimal checks are green on each, and the regression
test fails without the fix.

For whoever makes the v6-vs-v7 call, the two differ only in the shortcut
that skips copying the indexes:

- v6 keeps the shortcut for both protocols. Its cost is that a
SET TABLESPACE that is the first statement of a pipeline commits on
its own, so a later failure in that pipeline no longer rolls the
move back.

- v7 drops that. The shortcut then applies only to a single-statement
simple-protocol query, so through the extended protocol (most
drivers) the ALTER always copies the indexes: more WAL and one lock
per index, and it can fail if the destination tablespace has room
for the heap but not for the indexes.

I attached v7, as the one that changes no behavior, but either is ready
and I'm happy to switch to v6 if that's the preference.

One user-visible change in both: when the indexes are copied, the
destination tablespace has to have room for them too, not just the heap.

Regards,
Manu