Re: count * performance issue

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



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

[Postgresql General]     [Postgresql PHP]     [PHP Users]     [PHP Home]     [PHP on Windows]     [Kernel Newbies]     [PHP Classes]     [PHP Books]     [PHP Databases]     [Yosemite]

  Powered by Linux