You can preview a transformation at any time by clicking Preview, to see the effect it will have on your project data set. Preview mode is also a useful debugging tool, as it detects any runtime errors or corner case exceptions your transformation contains.
When you click Preview, the Transformation Editor updates the transformation script.
When you preview a transformation, Transform finds and displays runtime errors that weren't detected by the static parser. It is therefore recommended that you preview your transformations and fix any errors they contain before saving them to your transformation script. You can revert the changes made in the preview by clicking Cancel.
Preview only updates your project data set in Studio's internal files backing the Transform; it does not affect any data sets in a Dgraph database, and the results are not visible to other users of your project in Studio. Additionally, your project data set is still associated with its source Hive data, so any changes made to the source are still reflected within your project.