The Internal Merged option is used if you wish to merge the data from more than one table already in existence in the PostgreSQL database.
Click on the green + 
to display the Merge Constructor dialog.
Select Fields From
Select your table from the drop down list and then select the fields from the table that you wish to include in your data source.
Now click on the green + again to select the table that you wish to link with the first table that you selected. Once you have selected the second table the Link Field and Parent Link Field entries will become enabled.
Link Field
This is the field in the currently selected table that you wish to use to link to the Parent or first table that you selected.
Parent Link Field
This is the field in the parent or first table that you wish to use to match against the link field.
Click on OK and the dialog will be updated to show the hierarchy.
If you wish to amend any of the details simply select the entry in the right hand panel and click on the Edit button 
and you can edit the details. To remove an entry select it and click on the red cross 
.
Click OK to add the new Data Source.