XML projection is one of the main adopted optimization techniques for reducing memory consumption in XQuery in-memory engines. The main idea behind this technique is quite simple: given a query Q over an XML document D, instead of evaluating Q on D, the query Q is evaluated on a smaller document D' obtained from D by pruning out, at loading-time, parts of D that are unrelevant for Q. The actual queried document D' is a projection of the original one, and is often much smaller than D due to the fact that queries tend to be quite selective in general.While projection techniques have been extensively investigated for XML querying, we are not aware of applications to XML updating.This Thesis investigates application of a projection based optimization mechanism for XQuery Update Facility expressions in the presence of a schema. The current work includes study of the method and a formal development of Merge algorithm as well as experiments testifying its effectiveness.