Beginning Oracle Database 11g Administration From Novice to Professional

CHAPTER 11 FIXING PROBLEMS 243
Consider a query that begins at 9 a.m. and completes at 10 a.m. Suppose that the
query is counting the number of records in a very large table and suppose that the table
is very active—that is, records are constantly being inserted into the table. Suppose that
Oracle’s answer to the query is ten billion records. Does the answer indicate the number
of records at 9 a.m., at 10 a.m., or at some intermediate time in between those times?
If we were working with other database technologies such as IBM’s DB2 or Informix,
Microsoft’s SQL Server, or Sybase, the answer would be 10 a.m. because the query
would not complete until it had locked all the records in the table; it would wait for all
insert, update, and delete operations to complete and it would block others from starting.
In other words, readers block writers, and writers block readers. With Oracle, on
the other hand, readers do not block writers, and writers do not block readers. Instead,
Oracle determines what the answer to the query would have been at the instant the
query started, that is, at 9 a.m.; this manner of operation is called read consistency. 3 Read
consistency extends to a single query by default or can extend to an entire transaction.
For example, the entire contents of a database can be exported by using read- consistent
mode to ensure that there are no conflicts or inconsistencies in the data.
To ensure read consistency, Oracle must construct a snapshot of the database at the
time the query or transaction started. If the required data has changed since the query
started, Oracle must obtain a prior version of the data from the rollback segments. However,
the rollback segments are a shared resource, and the information they contain may
be overwritten if other transactions need the space. Therefore, Oracle may not be able to
construct the snapshot—that is, the snapshot may be too old to be reconstructed.
Prior to Oracle 9i, the number and size of rollback segments was left to the database
administrator. Beginning with Oracle 9i, this task should be entrusted to Oracle by changing
the value of the setting to . If the error occurs,
increase the value of the setting and ensure, through trial and error, that
the tablespace specified by the setting is big enough, adding data files or
increasing the size of data files as necessary. The default value of the setting
is 900 seconds (15 minutes); a large value such as 14,400 (4 hours) may be more appropriate,
depending on the circumstances, but must be supported by an undo tablespace that
is large enough. Note that the error can never be completely avoided
because it is always possible for queries to take so long that the available resources are
exhausted. Also, transactions that modify the data typically take precedence over transactions
that read the data—that is, the modified transactions will overwrite the information
in the rollback segments if space is short, irrespective of the value of the
setting. This can be prevented by imposing a retention guarantee on the undo tablespace
by using the command. More care than usual must be taken to ensure
that the undo tablespace is adequately sized if a retention guarantee is in effect.
3. SQL Server offers read consistency beginning with SQL Server 2005. However, it is not the default
manner of operation.