RID Lookup Or Key Lookup?

Aaron Bertrand asks which is faster, RID lookups or key lookups?

I’ve seen multiple people state that a heap can be better than a clustered index for certain scenarios. I cannot disagree with that. One of the interesting reasons I’ve seen stated, though, is that a RID Lookup is faster than a Key Lookup. I’m a big fan of clustered indexes and not a huge fan of heaps, so I felt this needed some testing.

So, let’s test it!

I thought it would be good to create a database with two tables, identical except that one had a clustered primary key, and the other had a non-clustered primary key. I would time loading some rows into the table, updating a bunch of rows in a loop, and selecting from an index (forcing either a Key or RID Lookup).

It looks like RID lookups are slightly faster than key lookups.  But check out the comments:  this is a best-case scenario.

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