On Thu, Apr 23, 2015 at 06:24:00PM +0300, Heikki Linnakangas wrote: > >I've often wondered if there was some way we could consolidate XMIN/XMAX > >from multiple tuples at the page level; that could be a big win for OLAP > >environments where most of your tuples belong to a pretty small range of > >XIDs. In many workloads you could have 80%+ of the tuples in a table > >having a single inserting XID. > > It would be doable for xmin - IIRC someone even posted a patch for > that years ago - but xmax (and ctid) is difficult. When a tuple is > inserted, Xmax is basically just a reservation for the value that > will be put there later. You have no idea what that value is, and > you can't influence it, and when it's time to delete/update the row, > you *must* have the space for that xmax. So we can't > opportunistically use the space for anything else, or compress them > or anything like that.
Also SELECT FOR UPDATE uses the per-row xmax too. -- Bruce Momjian <br...@momjian.us> http://momjian.us EnterpriseDB http://enterprisedb.com + Everyone has their own god. + -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
