Heikki Linnakangas <heikki.linnakangas@enterprisedb.com> writes:
> Or, we could store only the delta between current record and the
> previous one. Assuming we know what the current record is, that wouldn't
> lose any precision. That way xl_prev only needs to be as big as the
> biggest possible WAL record we can have.
The trouble with either approach is that it discards forensic
intelligence in the name of bit squeezing. The high bits of xl_prev are
the only direct evidence *within* a WAL record of where it thinks it is
in the WAL sequence, and the back-comparison against where we thought
the previous record was is correspondingly the only really strong
protection against a torn page problem within a WAL page, should the
sector boundary happen to fall exactly at a WAL record boundary.
I fear that a delta would be completely unacceptable for that check,
because it's entirely possible that a lot of different WAL records would
be the same size (consider bulk load into a fixed-width table for an
example). Simon's scheme merely removes some of the protection, not all
of it ;-).
But I don't really like removing any of it. If we need more bits in WAL
headers, then so be it --- they'd likely still be smaller than they were
a couple releases ago.
regards, tom lane
