"Estimated vs Actual, and Why Actual Matters"

1. What does the optimizer use to estimate the number of rows for a query execution plan?

Statistics about the data Actual data values User-defined parameters XML query plans

2. What is indicated by a significant difference between the estimated number of rows and the actual number of rows?

The query is optimized There is a data entry error The optimizer planned incorrectly The database has been corrupted

3. Which of the following is a common cause for estimates going wrong in query execution?

Using a temporary table Mismatched data types Stale statistics Adding indexes

4. What is the recommended first step to take if you encounter a bad estimate in a query execution plan?

Use query hints to optimize Update the statistics Rewrite the query Change the database settings