Hi Chris,

That's a great question that is going to be hard to answer on a
general basis as it is implementation-specific (database server,
database engine, memory, and hard drive speed).

To do a gross oversimplification, if you have a single database server
and you get a random mix of traffic from all of the sites, then having
everything in one database and using your 'org_id' should be faster
since you will have potentially fewer files open and the database has
fewer indexes to cache.  But each database server and database engine
handles table-to-file mapping and caching differently, so your mileage
may vary.

It would probably be very worthwhile to write some test applications
to simulate queries to the database and get some numbers for your
particular setup.  With a little tuning of the database cache and
memory settings for a particular usage scenario, you can substantially
increase the performance.

To give you an idea of performance increases, I have some databases
with over 900,000,000 records and just rebuilding the indexes took
around 3 days with MySQL and MyISAM.  I checked the memory usage and
the darned thing wasn't using even half of the memory it could.  It
turned out that I needed to tweak the index (key) buffer settings
which reduced the rebuild time to less than 8 hours.  Some of the
newer database engines like InnoDB do a much better job with file and
memory management so you don't have to tweak as much.  I've heard that
other databases such as PostGRES are much better, but I don't have any
first-hand experience to comment to that.

Hope that helps somewhat.

-Eric
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