Before you select Save query and run report or Run report, you can preview the report to make sure it meets your needs. Click Preview. The preview function enables you to simulate the output of the query you’ve built before it runs. Preview can save you time and effort, especially if the query is complex and may take a long time to execute.
This sampling provides you with an idea of what the query will look like without actually running the query. Since the goal of preview is to highlight the arrangement of data, the sampling will vary slightly from the actual query output (which is shown as a tree table).
The preview mimics the output of the report. In other words, you see a tree table and up to 200 rows of data. The layout is an exact match but the metrics display dummy data.