SQL Server Execution Plans - Red Gate Software

Chapter 2: Graphical Execution Plans for Basic Queries
Assuming that the number of rows in a table is relatively small, Table Scans are generally
not a problem. On the other hand, if the table is large and many rows are returned, then
you might want to investigate ways to rewrite the query to return fewer rows, or add an
appropriate index to speed performance.
RID Lookup
RID Lookup is the heap equivalent of the Key Lookup operation. As was mentioned
before, non-clustered indexes don't always have all the data needed to satisfy a query.
When they do not, an additional operation is required to get that data. When there is a
clustered index on the table, it uses a Key Lookup operator as described above. When
there is no clustered index, the table is a heap and must look up data using an internal
identifier known as the Row ID or RID.
If we specifically filter the results of our previous DatabaseLog query using the
primary key column, we see a different plan that uses a combination of an Index Seek
and a RID Lookup.
SELECT *
FROM [dbo].[DatabaseLog]
WHERE DatabaseLogID = 1
Listing 2.6
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