Two basic isolation levels are used frequently within Oracle: READ COMMITTED
and SERIALIZABLE. (A third level, READ ONLY, is described later in this section.)
Both of these isolation levels create serializable database operations. The difference
between the two levels is in the duration for which they enforce serializable
operations:
READ COMMITTED
Enforces serialization at the statement level. This means that every statement will
get a consistent view of the data as it existed at the start of that statement. However,
since a transaction can contain more than one statement, it??™s possible that
nonrepeatable reads and phantom reads can occur within the context of the
complete transaction. The READ COMMITTED isolation level is the default
isolation level for Oracle.
SERIALIZABLE
Enforces serialization at the transaction level. This means that every statement
within a transaction will get the same consistent view of the data as it existed at
the start of the transaction.
Because of their differing spans of control, these two isolation levels also react differently
when they encounter a transaction that blocks their operation with an exclusive
lock on a requested row.
Pages:
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427