I understand what she says or so I think.
A lookup occurs when a nonclustered index was used to locate rows for a query, but the nonclustered index did not contain all of the columns required for the query. To fetch the remaining columns, a lookup is done to the clustered index.
A lookup is equivalent to a single row clustered index seek. Lookups are always done one row at a time. For this reason, they are very expensive operations, especially when lots of rows are involved
Precisely why I asked the question -- so what does it mean for a Clustered Index. Am I missing something in her article.