Which is better: Bookmark/Key Lookup or Index Scan

Question

I know that an index seek is better than an index scan, but which is preferable in SQL Server explain plans: Index seek or Key Lookup (Book

Answer 1

Index seek, every time.

Lookups are expensive, so this is covering indexes and especially the INCLUDE clause was added to make them better.

Saying that if you expect exactly one row, for example, a seek followed a lookup can be better than trying to cover a query. We rely on this to avoid yet another index in certain situations.

Edit: Simple talk article: Using Covering Indexes to Improve Query Performance

Edit, Aug 2012

Lookups happen per row which is why they scale badly. Eventually, the optimiser will choose a clustered index scan instead of a seek+lookup because it's more efficient than many lookups.