Can One Escape Red Chains? Regular Path Queries Determinacy is Undecidable

For a given set of queries (which are expressions in some query language) Q = {Q1, Q2, . . . Qk} and for another query Q0 we say that Q determines Q0 if – informally speaking – for every database D, the information contained in the views Q(D) is sufficient to compute Q0(D). Query Determinacy Problem is the problem of deciding, for given Q and Q0, whether Q determines Q0. Many versions of this problem, for different query languages, were studied in database theory. In this paper we solve a problem stated in [CGLV02] and show that Query Determinacy Problem is undecidable for the Regular Path Queries – the paradigmatic query language of graph databases.