extensible external toast tuple support

Hi,

In
http://archives.postgresql.org/message-id/20130216164231.GA15069%40awork2.anarazel.de
I presented the need for 'indirect' toast tuples which point into memory
instead of a toast table. In the comments to that proposal, off-list and
in-person talks the wish to make that a more general concept has
been voiced.

The previous patch used varattrib_1b_e.va_len_1be to discern between
different types of external tuples. That obviously only works if the
data sizes of all possibly stored datum types are distinct which isn't
nice. So what the newer patch now does is to rename that field into
'va_tag' and decide based on that what kind of Datum we have. To get the
actual length of that datum there now is a VARTAG_SIZE() macro which
maps the tags back to size.
To keep on-disk compatibility the size of an external toast tuple
containing a varatt_external is used as its tag value.

This should allow for fairly easy development of a new compression
scheme for out-of-line toast tuples. It will *not* work for compressed
inline tuples (i.e. VARATT_4B_C). I am not convinced that that is a
problem or that if it is, that it cannot be solved separately.

FWIW, in some quick microbenchmarks I couldn't find any performance
difference due to the slightly more complex size computation which I do
*not* find surprising.

Opinions?

Greetings,

Andres Freund

--
 Andres Freund                       http://www.2ndQuadrant.com/
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