In this paper, we study the following problem. Given a database and a set of queries, we want to find a set of views that can compute the answers to the queries, such that the amount of space, in bytes, required to store the viewset is minimum on the given database. (We also handle problem instances where the input has a set of database instances, as described by an oracle that returns the sizes of view relations for given view definitions.) This problem is important for applications such as distributed databases, data warehousing, and data integration. We explore the decidability and complexity of the problem for workloads of conjunctive queries. We show that results differ significantly depending on whether the workload queries have self-joins. Further, for queries without self-joins we describe a very compact search space of views, which contains all views in at least one optimal viewset. We present techniques for finding a minimum-size viewset for a single query without self-joins by using the shape of the query and its constraints, and validate the approach by extensive experiments.

Huanglongbing (HLB) is the most devastating disease of citrus in Florida. HLB is caused by the phloem-inhabiting bacterium ‘Candidatus Liberibacter asiaticus’ (Las), which is transmitted by psyllid vector Diaphorina citri. The current management strategies of HLB are to control psyllids and eradicate infected plants. However, these management practices have not been able to stop the spreading of HLB (Duan et al. 2009). Alternative approaches are needed to control HLB. In previous studies we isolated multiple bacterial strains from roots of healthy citrus plants in HLB infected groves in Florida, which have the potential to enhance plant growth and suppress diseases (Trivedi et al. 2011). Recently, early infection of roots by Las leading to root decline was suggested to be important in HLB disease development (Johnson et al. 2012). We hypothesize that introduction of beneficial bacteria to roots of citrus plants could decrease root damage by promoting root growth and reducing Las infection and thus improve HLB management. To test this hypothesis, six beneficial bacteria were assessed for their plant growth-promotion ability; and three were found to be able to promote growth of both grapefruit and Arabidopsis (Col-0 ecotype) in greenhouse experiments, with increases in root length and weight, compared to mock treated plants. Thus, the three isolates were selected to evaluate the effect on Las infection in greenhouse assays. Details of the beneficial bacterial isolates, plant growth-promotion activity, and effect on Las infection will be discussed.