In Blog with many Media and Posts the Posts Table is very Big (my, by example, is 173223 entrys in 2 Years). Sharing between Sites in an Network ist Impossible so we need to Upload every Image seperatly to all sites.
With an own Table for media for all Site together (similar Users) this is not longer needed because the Database Table is the same for all Sites.
To realize this we need to create 2 extra tables. The first is an copy of the post table with some extra collums for identify the Site it have the entry created and if other Site is used this Media in an post. T
The second one is an copy of post_meta with an extra field to identify the Site it have the entry created and if other Site is used this Media in an post.
Feel free to comments this :)
ps: sry for my Very Bad English