Query Scripting in Pentaho using groovy

Question

I am using pentaho as reporting engine in my project, I have a situation where I have to dynamically generate a query based on parameters and from there the report out come will be processed, so I found query scripting is the best way to do that, but I can't find any guide/tutorial on query scripting using groovy. I am using pentaho report designer 5.x and postgres database. Any help on this topic would be greatly appreciated.