Re: Proposal: Multiversion page api (inplace upgrade)

On Jun 11, 2008, at 10:42 AM, Heikki Linnakangas wrote:

>> Another issue is that it might not be possible to update a page for
>> lack of space. Are we prepared to assume that there will never be a
>> transformation we need to apply that makes the data bigger?
>
> We do need some solution to that. One idea is to run a pre-upgrade
> script in the old version that scans the database and moves tuples
> that would no longer fit on their pages in the new version. This
> could be run before the upgrade, while the old database is still
> running, so it would be acceptable for that to take some time.

That means old versions have to have some knowledge of new versions.
There's also a big race condition unless the old version starts
taking size requirements into account every time a page is dirtied.

> No doubt people would prefer something better than that. Another
> idea would be to have some over-sized buffers that can be used as
> the target of conversion, until some tuples are moved off to
> another page. Perhaps the over-sized buffer wouldn't need to be in
> shared memory, if they're read-only until some tuples are moved.
>
> This is pretty hand-wavy, I know. The point is, I don't think these
> problems are insurmountable.

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