PATCH: Warn users about tablespace abuse data loss risk

Hi all

I've just seen another case of data loss due to misuse of /
misunderstanding of tablespaces:

http://dba.stackexchange.com/questions/58704/how-do-i-access-a-old-saved-tablespace-after-reinstalling-postgres

and it's prompted me to write some docs amendments to make it more
obvious that *you shouldn't do that*.

Not that it'll stop people, but it'll at least mean they can't say we
didn't warn them.

This is actually quite important, because many users are used to MySQL's
MyISAM, where each table contains its own metadata and is readable by
simply copying the table into a different MySQL install's data
directory. It doesn't even have to be the same version! Users are
clearly surprised that PostgreSQL tablespaces don't have the same
properties.

Thoughts?

--
Craig Ringer http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services