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We study the problem of type checking and type inference for queries over semistructured data. Introducing a novel traces technique, we show that the problem is difficult in general (NP-complete), but can be solved in PTIME for many practical cases, including, in particular, queries over XML data. Besides being interesting by itself, we show that type inference and the related traces technique have several important applications, facilitating query formulation, optimization, and verification.doi:10.1145/303976.303998 dblp:conf/pods/MiloS99 fatcat:x323vhzoyrg4lb6jg4oamupcxu