We describe a method of inferring join plans for a set of relation instances, in the absence of any metadata, such as attribute domains, attribute names, or constraints (e.g., keys or foreign keys). Our method enumerates the possible join plans in order of likelihood, based on the compatibility of a pair of columns and their suitability as join attributes (i.e. their appropriateness as keys). We outline two variants of the approach. The first variant is accurate but potentially time-consuming, especially for large relations that do not fit in memory. The second variant is an approximation of the former and hence less accurate, but is considerably more efficient, allowing the method to be used online, even for large relations. We provide experimental results showing how both forms scale in terms of performance as the number of candidate join attributes and the size of the relations increase. We also characterize the accuracy of the approximate variant with respect to the exact variant. Copyright ©2009 ACM.