On Fri, Jul 24, 2015 at 1:00 PM, Simon Riggs <si...@2ndquadrant.com> wrote: > It depends on the exact design we use to get that. Certainly we do not want > them if they cause a significant performance regression.
Yeah. I think the performance worries expressed so far are: - Currently, if you see an XID that is between the XMIN and XMAX of the snapshot, you hit CLOG only on first access. After that, the tuple is hinted. With this approach, the hint bit doesn't avoid needing to hit CLOG anymore, because it's not enough to know whether or not the tuple committed; you have to know the CSN at which it committed, which means you have to look that up in CLOG (or whatever SLRU stores this data). Heikki mentioned adding some caching to ameliorate this problem, but it sounds like he was worried that the impact might still be significant. - Mixing synchronous_commit=on and synchronous_commit=off won't work as well, because if the LSN ordering of commit records matches the order in which transactions become visible, then an async-commit transaction can't become visible before a later sync-commit transaction. I expect we might just decide we can live with this, but it's worth discussing in case people feel otherwise. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
