@inproceedings{STOC-2001-GroheSS,
	author        = "Martin Grohe and Thomas Schwentick and Luc Segoufin",
	booktitle     = "{Proceedings of the 33rd Annual ACM Symposium on Theory of Computing}",
	doi           = "10.1145/380752.380867",
	isbn          = "1-58113-349-9",
	pages         = "657--666",
	publisher     = "{ACM}",
	title         = "{When is the evaluation of conjunctive queries tractable?}",
	year          = 2001,
}