Tom, > > Count() on Oracle and MySQL is almost instantaneous, even for very > > large tables. So why can't Postgres do what they do? > > AFAIK the above claim is false for Oracle. They have the same > transactional issues we do. Nope. Oracle's MVCC is implemented through rollback segments, rather than non-overwriting the way ours is. So Oracle can just do a count(*) on the index, then check the rollback segment for any concurrent update/delete/insert activity and adjust the count. This sucks if there's a *lot* of concurrent activity, but in the usual case it's pretty fast. I've been thinking that when we apply the Dead Space Map we might be able to get a similar effect in PostgreSQL. That is, just do a count over the index, and visit only the heap pages flagged in the DSM. Again, for a heavily updated table this wouldn't have any benefit, but for most cases it would be much faster. -- --Josh Josh Berkus PostgreSQL @ Sun San Francisco -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
