Re: count * performance issue

Josh Berkus <josh@agliodbs.com> 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