Oracle Database 11 g - Online Public Access Catalog

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128 CHAPTER 3 ■ DATABASE ADMINISTRATIONNew Data Partitioning SchemesIn Oracle Database 11g, there are several new partitioning techniques to facilitate storing largeamounts of data as well as increasing the performance of queries based on the partitionedtables. We list the four new partitioning schemes here. You’ll find a more detailed discussion ofthe new partitioning schemes in Chapter 12, which discusses new data warehousing featuresin Oracle Database 11g.• Reference partitioning enables you to logically equipartition tables with parent-childrelationships by inheriting the partitioning key from the parent table.• Interval partitioning lets you automate the creation of range partitions by creating partitionson demand.• System partitioning enables application controlled partitioning.• New composite partitioning schemes let you perform logical range partitioning alongtwo dimensions.• Virtual column partitioning lets you use partitioning key columns defined on a table’svirtual columns (discussed earlier in this chapter), thus overcoming limitations wherethe partitioning needs aren’t being served by the currently defined table columns.Allowing DDL Locks to Wait for DML LocksIn previous releases, by default, any DDL locks wouldn’t wait for a DML lock but would failright away if they couldn’t obtain the DDL lock. In Oracle Database 11g, you can use the newinitialization parameter ddl_lock_timeout to specify the duration for which a DDL statementwill wait for a DML lock.The default value of the ddl_lock_timeout parameter is zero, meaning that DDL statementswon’t wait for a DML lock. The maximum value of 1,000,000 seconds means you canpotentially set the wait duration for as long as 11.5 days. You can use the alter session statementto specify the duration of wait for a DML lock, as shown here:SQL> alter session set ddl_lock_timeout = 30;Session altered.SQL>Explicit Locking of TablesIf you want to add a column to a table, Oracle requires that an exclusive lock be requestedwhen you issue the DDL command to add the column. If Oracle fails to obtain an immediateDDL lock on the table in question, the DDL command will fail right away. Oracle Database 11glets you allow a DDL command to wait for a specified length of time before it fails because of itsinability to acquire the necessary DML lock on the table.Oracle now allows new syntax with the lock table command that lets you specify themaximum time a statement should wait to obtain a DML lock on a table. This feature is handyfor operations such as adding a column to a table that users are frequently updating. The newsyntax of the lock table command is as follows:
CHAPTER 3 ■ DATABASE ADMINISTRATION 129LOCK TABLE ... IN lockmode MODE [NOWAIT | WAIT integer]Here’s what the mode parameters nowait and wait mean:• If you want the database to return control to you immediately upon finding that a necessarytable is already locked by other users, specify the nowait option.• You can use the wait parameter to specify the number of seconds the lock table statementcan wait in order to acquire a DML lock. You can set the value of this parameter toany integer value you want—there’s no limit.• If you don’t specify either wait or nowait, the database will wait until the locked table isavailable and then lock it before returning control to you.Invisible IndexesSuppose you have a situation where you have to create a huge index for some special queriesbut don’t want the optimizer to change its query plans based on this index. Furthermore, thesituation may call for dropping and re-creating the index often, which leads to problems becausethe index building takes considerable time. In Oracle Database 11g, you can create an index as aninvisible index, which really means the index won’t be visible to the cost optimizer when it’sfiguring out the execution plans.You can use the invisible index only by specifying the index hint in your SQL statements.In sum, this means you can make selective use of an index, choosing to employ the index onlywhen you want it, not when the optimizer thinks it’s necessary. As far as the optimizer is concerned,the index simply doesn’t exist, unless you reveal its existence!The following situations make the invisible index a handy feature:• You want to test the usefulness of a new index but don’t want to change the optimizer’sexecution plans before you are sure about the usefulness of the new index. You can nowmake the index invisible and have the optimizer generate execution plans without thatindex. If the performance is the same or better, you may decide to drop that index forgood. If, on the other hand, the performance is better with the index, you want to makethat index visible, that is, change it back to a normal index or change its status to visible,as we show later.• You want to use an index for a temporary purpose, such as during an application upgrade,but don’t want the index to be considered by the optimizer subsequently.Although an invisible index is unknown to the cost optimizer unless you cause the index tobe considered by using the index hint, the index continues to be maintained just as any othernormal index. This means when you add new data to the table or modify existing data, theinvisible index will be updated accordingly by the database.Creating an Invisible IndexBy default, all indexes are visible to the optimizer. The new visible|invisible clause of thecreate index statement lets you designate the visibility of an index. If you don’t use this clause,Oracle creates the index as a normal index. You can create an invisible index by simply appendingthe keyword invisible at the end of the normal create index statement, as shown here:
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The EXPERT’s VOIce ® IN ORACLEAl
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Contents at a GlanceAbout the Autho
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■CONTENTSixSQL*Plus New Features
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■CONTENTSxiEnterprise Manager Int
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■CONTENTSxiiiCombined Capture and
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■CONTENTSxvImprovements in Redo T
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About the Authors■SAM ALAPATI is
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xxii■ACKNOWLEDGMENTSNothing I do
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xxiv■INTRODUCTIONPerformance Mana
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226 CHAPTER 5 ■ DATABASE SECURITY
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CHAPTER 7■ ■ ■Data PumpDBAs a
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CHAPTER 7 ■ DATA PUMP 315if test
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CHAPTER 7 ■ DATA PUMP 317DBA11g1
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CHAPTER 7 ■ DATA PUMP 319Encrypti
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CHAPTER 7 ■ DATA PUMP 323Remappin
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CHAPTER 7 ■ DATA PUMP 325SELECT d
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CHAPTER 7 ■ DATA PUMP 331SQL> sel
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CHAPTER 7 ■ DATA PUMP 333DBMS_OUT
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CHAPTER 9■ ■ ■Storage Managem
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CHAPTER 9 ■ STORAGE MANAGEMENT 36
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416 CHAPTER 10 ■ DATA GUARD• Lo
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446 CHAPTER 11 ■ APPLICATION DEVE
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CHAPTER 12■ ■ ■Data Warehousi
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566 ■INDEXadvise failure command,
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568 ■INDEXDBMS_ADDM package, 216-
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