It is well known that using conventional concurrency control techniques for obtaining serializable answers to longrunning queries leads to an unacceptable drop in systems performance. as a result, most current DBMSs execute such queries under a reduced degree of consistency, thus providing non-serializable answers. in this paper, we present a new and highly concurrent approach for processing large decision support queries in relational databases. in this new approach, called compensation-based query processing, concurrent updates to any data participating in a query are communicated to the query's on-line query processor, which then compensates for these updates so that the final answer reflects changes caused by the updates. very high concurrency is achieved by locking data only briefly, while still delivering transaction-consistent answer to queries.