Re: What does Page Layout version mean? (Was: Re: Reducing NUMERIC size for 8.3)

Heikki Linnakangas <heikki(at)enterprisedb(dot)com> writes:
> * Page format changes that grow data size are problematic, because there
> can be pages that can't be expanded to the new format because there's
> not enough space. However, it would be possible to write a pre-upgrade
> program to look for the problematic pages, and do a dummy UPDATE on some
> tuples to move them away from such pages, to make room. The pre-upgrade
> program could be run while the old database is still up, so it doesn't
> cause downtime.

That sounds good, but there are corner cases where it wouldn't work ---
consider a page containing a single maximum-length tuple.

In general I don't see us accepting changes that would increase data
size, but there's at least one troubling exception on the horizon:
per-column or per-value locale support. Another problem is that a strict
rule of "no data size increase ever" might forbid acceptance of changes
that achieve average space savings at the cost of increasing the size of
some lesser-used cases.

In short, this point seems to need more thought.

regards, tom lane