Editing Isolation (database systems) (section)

==Default isolation level==
The ''default isolation level'' of different [[Database management system|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 criticized 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, and even in their loosest interpretations do not exclude some 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 implementations. Additionally, the ANSI phenomena do not distinguish between a number of types of isolation level behavior that are popular in commercial systems.<ref name="sql-isolation">
{{cite web
| url = http://www.cs.umb.edu/~poneil/iso.pdf
| title = A Critique of ANSI SQL Isolation Levels
| access-date = 29 July 2012 }}
</ref>

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''.<ref>{{cite web
| access-date = 2010-03-09
| publisher = DataStax
| location = www.DataStax.com
| title = Customer testimonials (SimpleGeo, CLOUDSTOCK 2010)
| author = salesforce
| date = 2010-12-06
| url = https://www.youtube.com/v/7J61pPG9j90?version=3
| quote = (see above at about 13:30 minutes of the webcast!)}}</ref>