Isolation (database Systems) - Default Isolation Level

The default isolation level of different DBMS's varies quite widely. Most databases that feature transactions allow the user to set any isolation level. Some DBMS's also require additional syntax when performing a SELECT statement to acquire locks (e.g. SELECT ... FOR UPDATE to acquire exclusive write locks on accessed rows).

However, the definitions above have been criticised as being ambiguous, and as not accurately reflecting the isolation provided by many databases:

This paper shows a number of weaknesses in the anomaly approach to defining isolation levels. The three ANSI phenomena are ambiguous. Even their broadest interpretations do not exclude anomalous behavior. This leads to some counter-intuitive results. In particular, lock-based isolation levels have different characteristics than their ANSI equivalents. This is disconcerting because commercial database systems typically use locking. Additionally, the ANSI phenomena do not distinguish among several isolation levels popular in commercial systems.

There are also other criticisms concerning ANSI SQL's isolation definition, in that it encourages implementors to do "bad things":

... it relies in subtle ways on an assumption that a locking schema is used for concurrency control, as opposed to an optimistic or multi-version concurrency scheme. This implies that the proposed semantics are ill-defined.