Phenomena 

If several transactions access the same database object, such as a table, at the same time, this can result in inconsistencies in the results. The following phenomena can occur:

·        Dirty Read

Transaction T1 changes a row. Transaction T2 reads this row before T1 ends the transaction with COMMIT. T1 then performs a ROLLBACK.

In this case, T2 has read a row that never existed.

·        Non Repeatable Read

Transaction T1 reads a row. Transaction T2 then changes or deletes this row and closes the transaction with COMMIT. If T1 then reads the row again, it either gets the modified row or a message indicating that the row no longer exists.

·        Phantom

Transaction T1 performs an SQL statement S with a search condition that returns a result set M of rows. Transaction T2 then creates at least one additional row that meets this search condition, for example, by adding or changing data. If T1 subsequently executes S again, then the new result set is different to M.

You can configure which phenomena are possible with the isolation level.

Relationship Between the Isolation Level and Phenomena

Phenomenon	Isolation Level
	0	1 or 10	2 or 20	3 or 30
Dirty Read	Yes	No	No	No
Non Repeatable Read	Yes	Yes	No	No
Phantom	Yes	Yes	Yes	No

See also:

Locking for Database Objects