Jeff Janes <jeff.janes@xxxxxxxxx> writes: > I apologize if it is bad form to respond to a message that is two > months old, but I did not see this question answered elsewhere and > thought it would be helpful to have it answered. This my rough > understanding. Oracle never "takes" a snapshot, it computes one the > fly, if and when it is needed. It maintains a structure of recently > committed transactions, with the XID for when they committed. If a > process runs into a tuple that is neither from the future nor from the > deep past, it consults this structure to see if that transaction has > committed, and if so whether it did so before or after the current > query was started. The structure is partionable so it does not have > one global lock to serialize on, and the lock is short as it only gets > the info it needs, not the entire set of global open transactions. Are you sure it's partitionable? I've been told that Oracle's transaction log is a serious scalability bottleneck. (But I think I first heard that in 2001, so maybe they've improved it in recent releases.) We know that Postgres' WAL log is a bottleneck --- check for recent discussions involving XLogInsert. But the WAL log is only touched by read-write transactions, whereas in Oracle even read-only transactions often have to go to the transaction log. regards, tom lane -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
