Pentaho

I dont remember the exact moment but the truncate goes off on either the activation or validation of the step during the transformation execution. It means it doesnt matter if at some point it fails to insert, the table gets truncated before the error (unless the error is related to accessing the table itself)! 

To give you an idea you can have a table truncated, and a parcial dataset inserted on it before the error!

All steps are initialized when a transformation start. Then also the truncation take place. So when in your first step there is an error and no data comes to the table output step. The table is already truncated

I did some testing and I monitored the database server and I saw that Pentaho does a transaction rollback if there is a failure. So it first truncates the table, but reverts it when an insert fails.

Yes, but keep in mind that TRUNCATE isn't a DML (data manipulation language) order, so there's no rollback for a TRUNCATE, the rollback only affects the inserts after the TRUNCATE.

Regards

You are correct that the truncation will be rollback when an error occur when "Make the tranformation database transactional" in the Miscellaneous tab in the Transformation properties. The database must also allow that