On 07/08/2019 13:52, Dilip Kumar wrote:
I have one more problem related to compression of the command id
field. Basically, the problem is that we don't set the command id in
the WAL and we will always store FirstCommandId in the undo[1]. So
suppose there were 2 operations under a different CID then during DO
time both the undo record will store the CID field in their respective
undo records but during REDO time, all the commands will store the
same CID(FirstCommandId) so as per the compression logic the
subsequent record for the same transaction will not store the CID
field. I am not sure what is the best way to handle this but I have
few ideas.
1) Don't compress the CID field ever.
2) Write CID in WAL, but just for compressing the CID field in undo
(which may not necessarily go to disk) we don't want to add extra 4
bytes in the WAL.
Most transactions have only a few commands, so you could optimize for
that. If you use some kind of a variable-byte encoding for it, it could
be a single byte or even just a few bits, for the common cases.
For the first version, I'd suggest keeping it simple, though, and
optimize later.
- Heikki
