Re: count * performance issue

Mark Mielke wrote:
> Josh Berkus wrote:
>>>> 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 read the "almost instantaneous" against "the above claim is false" and
> "Nope.", and I am not sure from the above whether you are saying that
> Oracle keeps an up-to-date count for the index (which might make it
> instantaneous?), or whether you are saying it still has to scan the
> index - which can take time if the index is large (therefore not
> instantaneous).
>
> Cheers,
> mark
>
> --
> Mark Mielke <mark@mielke.cc>
>

Oracle scans the index pages, if the b-tree index is on non-nullable
columns, or if the bitmap index is on low-ish cardinality data.
Otherwise, it table scans.  MyISAM in MySQL would be an example where a
counter is kept.