Prototype: In-place upgrade

Attached patch is prototype of in-place upgrade as was presented on PGCon this year.

Main idea is to learn postgres to handle different version of page and tuple
structures.

1) page - Patch contains new page API and all code access page through this API.
Functions check page version and return correct data to caller. It is mostly
complete now. Only ItemId flags need finish.

2) tuple - HeapTuple structure has been extended with t_ver attribute which
contains page layout version and direct access to HeapTupleHeader is forbidden.
It is possible now only through HeapTuple* functions (see htup.c).
(HeapTupleHeader access still stays in a several functions like heap_form_tuple).

This patch version still does not allow to read old database, but it shows how
it should work. Main disadvantage of this approach is performance penalty.

Please, let me know your opinion about this approach.

Future work:
1) learn WAL to process different tuple structure version
2) tuple conversion to new version and put it into executor (ExecStoreTuple)
3) multiversion MaxItemSize constant

thanks for your comments Zdenek

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Zdenek Kotala Sun Microsystems
Prague, Czech Republic http://sun.com/postgresql