19 Vacuuming Christian

Base tables supporting transaction time are ever-growing because all logical updates, including deletions, transform into insertions at the physical level. This contrasts snapshot tables where logical and physical deletion coincide. Logical deletion in TSQL2 was covered in 16. As there is a need for physical deletion capabilities in snapshot tables (and valid-time tables), there is also a need for such capabilities in temporal tables supporting transaction time (i.e., transaction-time tables and bitemporal tables). This chapter is divided into two parts. In the first, the general notion of vacuuming and associated concepts are introduced and motivated. It is shown that straightforward physical deletion may adversely affect the usefulness of query results and that special attention thus should be devoted to the design of appropriate vacuuming facilities. We then give reasons why disciplined vacuuming is a highly desirable capability. Subsequently, the important concepts of vacuuming are presented by means of a larger example and a survey. In the second part, the specific design of vacuuming capabilities for TSQL2 is presented. The goal has been to design minimal facilities that are easily implemented and yet provide adequate functionality.