Craig Farrell (2/17/2011)
Isn't the RID and the "uniquifier" pretty much equivalent from an overhead standpoint, being a 4 byte identifier?
No. To start with, the RID's not 4 bytes.
The RID is the row identifier. An 8 byte combination of file, page and slot. Every row has one, it is unique always. The uniquifier is a 4-byte sequential value that only appears on the rows that have duplicate clustered index key values (the first row SQL encounters won't have a uniquifier, any rows subsequent with the same clustering key will gain one, sequential value, starting at (I believe) 1. It's ony unique in combination with the clustering key, not by itself.
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