Ensuring the correctness of XML queries is crucial for accurate data retrieval. A subexpression in a query that will never contribute data to the query answer should be regarded as an error. This principle has been recently accepted into mainstream XML query processing. The goal is to prevent errors and improve query performance by identifying and eliminating unnecessary subexpressions. This research activity aims to investigate the principles and techniques for ensuring the correctness of XML queries, with a focus on path correctness and query optimization.