A Critique Of ANSI SQL Isolation Levels

Abstract

ANSI SQL-92 [MS, ANSI] defines Isolation <i>Levels</i> in terms of <i>phenomena:</i> Dirty Reads, Non-Repeatable Reads, and Phantoms. This paper shows that these phenomena and the ANSI SQL definitions fail to properly characterize several popular isolation levels, including the standard locking implementations of the levels covered. Ambiguity in the statement of the phenomena is investigated and a more formal statement is arrived at; in addition new phenomena that better characterize isolation types are introduced. Finally, an important multiversion isolation type, called Snapshot Isolation, is defined.