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We establish a framework for characterizing structures representing trees for which conjunctive queries can be evaluated efficiently. Then we completely chart the tractability frontier of the problem and establish a dichotomy theorem for our axis relations, that is, we find all subset-maximal sets of axes for which query evaluation is in polynomial time and show that for all other cases, query evaluation is NP-complete. All polynomial-time results are obtained immediately using the proof techniques from our framework. Finally, we study the expressiveness of conjunctive queries over trees and show that for each conjunctive query, there is an equivalent acyclic positive query (i.e., a set of acyclic conjunctive queries), but that in general this query is not of polynomial size.<\/jats:p>","DOI":"10.1145\/1131342.1131345","type":"journal-article","created":{"date-parts":[[2006,7,25]],"date-time":"2006-07-25T14:14:26Z","timestamp":1153836866000},"page":"238-272","update-policy":"https:\/\/doi.org\/10.1145\/crossmark-policy","source":"Crossref","is-referenced-by-count":46,"title":["Conjunctive queries over trees"],"prefix":"10.1145","volume":"53","author":[{"given":"Georg","family":"Gottlob","sequence":"first","affiliation":[{"name":"Oxford University, Oxford, United Kingdom"}]},{"given":"Christoph","family":"Koch","sequence":"additional","affiliation":[{"name":"Universit\u00e4t des Saarlandes, Saarbr\u00fccken, Germany"}]},{"given":"Klaus U.","family":"Schulz","sequence":"additional","affiliation":[{"name":"Ludwig-Maximilians-Universit\u00e4t M\u00fcnchen, Munich, Germany"}]}],"member":"320","published-online":{"date-parts":[[2006,3]]},"reference":[{"key":"e_1_2_1_1_1","doi-asserted-by":"publisher","unstructured":"Abiteboul S. 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