XPath in a Desicion does not go to that length. But let Mendix do the work for you. If you do a retrieve-activity followed by a count-activity into for instance “numofrecords”, then this will result in only one database call. Then you can follow with a Decision which is just checking $numofrecords > 0.
Something like this:
I guess, 'isNew()' can be used in split which will return a boolean value whether the object already exists in DB or not.
Hope this helps your requirement! Thanks.