Josh Berkus <josh@xxxxxxxxxxxx> writes: > 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. Well, scanning an index to get a count might be significantly faster than scanning the main table, but it's hardly "instantaneous". It's still going to take time proportional to the table size. Unless they keep a central counter of the number of index entries; which would have all the same serialization penalties we've talked about before... 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
