PSALM: Cardinality Estimation in the Presence of Fine-Grained Access Controls

In database systems that support fine-grained access controls, each user has access rights that determine which tuples are accessible and which are inaccessible. Queries are answered as if the inaccessible tuples are not present in the database. Thus, users with different access rights may get different answers to a given query. To process queries efficiently in the presence of fine-grained access controls, the database system needs accurate estimates of the number of tuples that are both accessible according to the access rights of the submitting user and relevant according to the selection predicates in the query. In this paper, we present PSALM, a sampling-based cardinality estimation technique for use in the presence of fine-grained access controls. Our technique exploits the fact that access rights are relatively static and are common to all queries that are evaluated on behalf of a particular user. We show that PSALM provides more accurate estimates than techniques that do not exploit knowledge of access rights.