What are the problems with a join between two tables in two different databases?

Question

I am interested in your thoughts about the the pitfalls of joining two or more tables from different databases. I\'ll try to give an example.

Suppose table Tab

Answer 1

The answer to your questions is...it depends.

I have noticed that there is no serious degradation in performance when you keep the queries nice and simple (fewer join etc).

The more complex the queries, the more chance that the optimizer will produce a suboptimal execution plan.

The optimizer ultimately gets to decide how to execute the query. The more complex the query, the more opportunity for the optimizer to get the order of operations "wrong".

I recently experimented with this problem...

I ran a query with roughly 8 joins on a single database. I then put up a copy of that database on the same server with a different name, and then I modified the query so that it would join to a couple tables in the second copy of the database.

As a single database query, it ran in under 3 seconds; expected given the volume of data.

The cross database joined query run in just under 3 minutes.

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