[wp-hackers] MySQL: Union vs global table

[Dino Termini]

Hi list, 

I am working on a plugin that stores information in a table. In MU environments, each blog will have its own instance of that table (just like wp itself does for posts etc). Now, I am also being asked to provide a "network view" of that information. I evidently have two choices: UNION ALL on wp_*_mytable, or use a global table with blog_id. We may be talking about millions of rows per blog. So the latter approach would affect performance when analyzing the info for a given site (more frequent), I assume. 

What would you do in this case?

Thanks
Dino