On 6 July 2017 at 15:29, Jim Finnerty <jfinnert@amazon.com> wrote:
>
> Feel free to knock down this 'straw man' and propose something better!
I think the pattern in this design that we don't want is that it
imposes extra complexity on every user of every page even when the
page doesn't have the problem and even when the problem isn't anywhere
in the database. Even years from now when this problem is long gone
you'll have code paths for dealing with this special page format that
are rarely executed and never tested that will have to be maintained
blind.
Ideally a solution to this problem that imposes a cost only on the
weird pages and only temporarily and leave the database in a
"consistent" state that doesn't require any special processing when
reading the data would be better.
The "natural" solution is what was discussed for incompatible page
format changes in the past where there's an point release of one
Postgres version that tries to ensure there's enough space on the page
for the next version and keeps track of whether there are any
problematic pages. Then you would be blocked from upgrading until you
had ensured all pages had space (presumably by running some special
"vacuum upgrade" or something like that).
Incidentally it's somewhat intriguing to think about what would happen
if we *always* did such a tombstone for deletes. Or perhaps only when
it's a full_page_write. Since the whole page is going into the log and
that tuple will never be modified again you could imagine just
replacing the tuple with the LSN of the deletion and letting anyone
who really needs it fetch it from the xlog. That would be a completely
different model from the way Postgres works though. More like a
log-structured storage system.
--
greg