Hannu Krosing <hannu@tm.ee> writes:
> I don't think the main issues are with file _formats_ but rather with
> system file structures - AFAIK it is a fundamental design decision
> (arguably a design flaw ;( ) that we use system tables straight from
> page cache via C structure pointers,
The system tables are not the problem. pg_upgrade has shown how we
can have cross-version upgrades no matter how much the system catalogs
change (a good thing too, because we cannot freeze the system catalog
layout without bringing development to a standstill). A schema-only
dump and restore is cheap enough that there's no real reason to look
for any other solution.
Changes in the on-disk representation of user tables would be harder to
deal with, but they are also much rarer (AFAIR we've only done that
twice: WAL required additions to page and tuple headers, and then there
were Manfred's space-saving changes in 7.3). And as of 7.3 there is a
version field in page headers, which would in theory allow for a
page-at-a-time update process to work.
There isn't any fundamental reason why we cannot have a pg_upgrade
utility; claiming that there is something wrong with how we handle
catalog changes misses the point. The point is that *someone would
have to do the work*. Unless someone wants to step up and volunteer,
there's little value in discussing it.
regards, tom lane