Database Isolation Levels and Serialization for Transactions
Before this chapter delves into a detailed explanation of how
locks work within Oracle, it would be beneficial to quickly
review a few basics on how transactions operate within the
relational database model.
In terms of how database isolation levels function, the
ANSI/ISO SQL standard (SQL92) breaks these down into four levels
of transaction isolation. Each isolation level contains various
degrees of impact on transaction processing. The isolation levels
are explained in terms of the following three phenomena that must
be prevented between concurrently executing transactions:
- Dirty reads - Dirty reads occur when a transaction reads
data that has been written by another transaction that has not yet
- Fuzzy or non-repeatable reads - Fuzzy reads occur when a
database transaction re-reads data it has already read and then
finds that another committed transaction has modified or deleted
the same data.
- Phantom reads - Phantom reads occur when a transaction
in Oracle re-executes a query that returns the rows that satisfy a
particular search and discovers that another committed transaction
has already inserted additional rows that meet the condition.
The database standard for SQL92 expounds these four levels of
isolation as shown in the following table.
Read Phenomena per Isolation Level
The Oracle database uses the read committed and serializable
isolation levels in addition to a read-only mode. By default,
Oracle uses the Read committed isolation level for database
transactions. Next to be examined are how locks operate within
Since Oracle is a database system that permits hundreds, if not
thousands, of multiple users to access data, it requires a method
to lock data in order to resolve problems associated with data
concurrency, consistency, and integrity. Here is where locks enter
Database locks are
mechanisms that prevent errors from occurring that could cause
dire consequences during user interaction between transactions
accessing the same particular resource within Oracle. As such,
these resources contain two basic types of data locks for user
objects such as tables and data and system objects which are the
hidden underlying data structures within the database engine such
as the Oracle data dictionary and memory.
For all purposes of explanation, the Oracle database will
automatically obtain the necessary locks when SQL statements are
executed by users. This process is hidden from users so that they
are able to perform tasks without manual intervention. By
default, Oracle will use the lowest possible level of restriction
to provide for the highest rate of data concurrency while also
providing perfect data integrity. In addition, users have the
ability to lock data manually within Oracle based on the
nature of the application design.
As was mentioned earlier, locks function to protect data
integrity and consistency within the Oracle database environment.
Locks operate both implicitly when SQL statements are executed by
users as well as explicitly when user application design dictates
that locking be performed in such a manner. Explicit locking
prevents the resource from being shared by other users so that a
particular application user can modify data without interference.
Share locks allow resources
to be shared based on the nature of the operation performed.
Multiple transactions can obtain a shared lock on the same
resource within Oracle.