tableam: abstracting relation sizing code

Hi,

It looks to me as though any table AM that uses the relation forks
supported by PostgreSQL in a more or less normal manner is likely to
require an implementation of the relation_size callback that is
identical to the one for heap, and an implementation of the
estimate_rel_size method that is extremely similar to the one for
heap. The latter is especially troubling as the amount of code
duplication is non-trivial, and it's full of special hacks.

Here is a patch that tries to improve the situation. I don't know
whether there is some better approach; this seemed like the obvious
thing to do.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company