On 30 July 2015 at 14:26, Alexander Korotkov <a.korot...@postgrespro.ru> wrote:
> As I mentioned in CSN thread, it would be nice to replace XID with CSN > when setting hint bits for tuple. In this case when hint bits are set we > don't need any additional lookups to check visibility. > > http://www.postgresql.org/message-id/CAPpHfdv7BMwGv=ofug3s-jgvfkqhi79pr_zk1wsk-13oz+c...@mail.gmail.com > Introducing 32-bit CSN doesn't seem reasonable for me, because it would > double our troubles with wraparound. > Your idea to replace XIDs with CSNs instead of hinting them was a good one. It removes the extra-lookup we thought we needed to check visibility with CSN snapshots. I agree 32-bit CSNs would not be a good idea though, a 64-bit CSN is needed. If we break a CSN down into an Epoch and a 32-bit value then it becomes more easily possible. The Epoch for XID and CSN can be the same - whichever wraps first we just increment the Epoch. By doing this we can reuse the page-level epoch for both XID and CSN. Now hinting a tuple is just replacing a 32-bit XID with a 32-bit CSN. We would probably need an extra flag bit for the case where the CSN is one epoch later than the XID. -- Simon Riggs http://www.2ndQuadrant.com/ <http://www.2ndquadrant.com/> PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
