Thursday, 26 September 2013

New features in Oracle Database 12c

New features in Oracle Database 12c 

During this Oracle Database 12c new features article, I shall be extensively exploring some of the very important new additions and enhancements introduced in the area of Database Administration, RMAN- Recovery Manger, High Availability and Performance Tuning.
Some of the new features in Oracle 12c
Online migration of an active data file
Online table partition or sub-partition migration
Invisible column
Multiple indexes on the same column
DDL logging
Temporary undo in- and- outs

New backup user privilege
How to execute SQL statement in RMAN
Table level recovery in RMAN
Restricting PGA size
1. Online rename and relocation of an active data file
Unlike in the previous releases, a data file migration or renaming in Oracle database 12c R1 no longer requires a number of steps i.e. putting the table space in READ ONLY mode, followed by data file offline action. In 12c R1, a data file can be renamed or moved online simply using the ALTER DATABASE MOVE DATAFILE SQL statement. While the data file is being transferred, the end user can perform queries, DML and DDL tasks. Additionally, data files can be migrated between storages e.g. from non-ASM to ASM and vice versa.

Rename a data file:

SQL> ALTER DATABASE MOVE DATAFILE '/u00/data/users01.dbf' TO '/u00/data/users_01.dbf';
Migrate a data file from non-ASM to ASM:

SQL> ALTER DATABASE MOVE DATAFILE '/u00/data/users_01.dbf' TO '+DG_DATA';
Migrate a data file from one ASM disk group to another:

Overwrite the data file with the same name, if it exists at the new location:

SQL> ALTER DATABASE MOVE DATAFILE '/u00/data/users_01.dbf' TO '/u00/data new/users_01.dbf' REUSE;
Copy the file to a new location whilst retaining the old copy in the old location:

SQL> ALTER DATABASE MOVE DATAFILE '/u00/data/users_01.dbf' TO '/u00/data_new/users_01.dbf' KEEP;
You can monitor the progress while a data file being moved by querying the vssession_longops dynamic view. Additionally, you can also refer the alert.log of the database where Oracle writes the details about action being taken place.

2. Online migration of table partition or sub-partition
Migration of a table partition or sub-partition to a different tablespace no longer requires a complex procedure in Oracle 12c R1. In a similar way to how a heap (non-partition) table online migration was achieved in the previous releases, a table partition or sub-partition can be moved to a different tablespace online or offline. When an ONLINE clause is specified, all DML operations can be performed without any interruption on the partition|sub-partition which is involved in the procedure. In contrast, no DML operations are allowed if the partition|sub-partition is moved offline.

Here are some working examples:

SQL> ALTER TABLE table_name  MOVE PARTITION|SUBPARTITION partition_name  TO tablespace tablespace_name;
SQL> ALTER TABLE table_name  MOVE PARTITION|SUBPARTITION partition_name  TO tablespace tablespace_name UPDATE INDEXES ONLINE;
The first example is used to move a table partition|sub-partition to a new tablespace offline. The second example moves a table partition/sub-partitioning online maintaining any local/global indexes on the table. Additionally, no DML operation will get interrupted when ONLINE clause is mentioned.

Important notes:

The UPDATE INDEXES clause will avoid any local/global indexes going unusable on the table.
Table online migration restriction applies here too.
There will be locking mechanism involved to complete the procedure, also it might leads to performance degradation and can generate huge redo, depending upon the size of the partition, sub-partition.
3. Invisible columns
In Oracle 11g R1, Oracle introduced a couple of good enhancements in the form of invisible indexes and virtual columns. Taking the legacy forward, invisible column concepts has been introduced in Oracle 12c R1. I still remember, in the previous releases, to hide important data –columns from being displayed in the generic queries– we used to create a view hiding the required information or apply some sort of security conditions.

In 12c R1, you can now have an invisible column in a table. When a column is defined as invisible, the column won’t appear in generic queries, unless the column is explicitly referred to in the SQL statement or condition, or DESCRIBED in the table definition. It is pretty easy to add or modify a column to be invisible and vice versa:

SQL> CREATE TABLE emp (eno number(6), ename name varchar2(40), sal number(9) INVISIBLE);
SQL> ALTER TABLE emp MODIFY (sal visible);
You must explicitly refer to the invisible column name with the INSERT statement to insert the database into invisible columns. A virtual column or partition column can be defined as invisible too. However, temporary tables, external tables and cluster tables won’t support invisible columns.

4. Multiple indexes on the same column
Pre Oracle 12c, you can’t create multiple indexes either on the same column or set of columns in any form. For example, if you have an index on column {a} or columns {a,b}, you can’t create another index on the same column or set of columns in the same order. In 12c, you can have multiple indexes on the same column or set of columns as long as the index type is different. However, only one type of index is usable/visible at a given time. In order to test the invisible indexes, you need to set the optimizer_use_use_invisible_indexes=true.

Here’s an the example:

5. DDL logging
There was no direction option available to log the DDL action in the previous releases. In 12cR1, you can now log the DDL action into xml and log files. This will be very useful to know when the drop or create command was executed and by who. The ENABLE_DDL_LOGGING initiation parameter must be configured in order to turn on this feature. The parameter can be set at the database or session levels. When this parameter is enabled, all DDL commands are logged in an xml and a log file under the $ORACLE_BASE/diag/rdbms/DBNAME/log|ddl location. An xml file contains information, such as DDL command, IP address, timestamp etc. This helps to identify when a user or table dropped or when a DDL statement is triggered.

To enable DDL logging

The following DDL statements are likely to be recorded in the xml/log file:

6. Temporary Undo
Each Oracle database contains a set of system related tablespaces, such as, SYSTEM, SYSAUX, UNDO & TEMP, and each are used for different purposes within the Oracle database. Pre Oracle 12c R1, undo records generated by the temporary tables used to be stored in undo tablespace, much similar to a general/persistent table undo records. However, with the temporary undo feature in 12c R1, the temporary undo records can now be stored in a temporary table instead of stored in undo tablespace. The prime benefits of temporary undo includes: reduction in undo tablespace and less redo data generation as the information won’t be logged in redo logs. You have the flexibility to enable the temporary undo option either at session level or database level.

Enabling temporary undo
To be able to use the new feature, the following needs to be set:

Compatibility parameter must be set to 12.0.0 or higher
Enable TEMP_UNDO_ENABLED initialization parameter
Since the temporary undo records now stored in a temp tablespace, you need to create the temporary tablespace with sufficient space
For session level, you can use: ALTER SESSION SET TEMP_UNDO_ENABLE=TRUE;
Query temporary undo information
The dictionary views listed below are used to view/query the information/statistics about the temporary undo data:

To disable the feature, you simply need to set the following:

7. Backup specific user privilege
In 11g R2, SYSASM privilege was introduced to perform ASM specific operations. Similarly, backup and recovery tasks specific privilege SYSBACKUP has been introduced in 12c to execute backup and recovery commands in Recovery Manager (RMAN). Therefore, you can create a local user in the database and grant the SYSBACKUP privilege to perform any backup and recovery related tasks in RMAN without being granting the SYSDBA privilege.

$ ./rman target "username/password as SYSBACKUP"
8. How to execute SQL statement in RMAN
In 12c, you can now execute any SQL and PL/SQL commands in RMAN without the need of a SQL prefix: you can execute any SQL and PLS/SQL commands directly from RMAN. How you can execute SQL statements in RMAN:

RMAN> SELECT username,machine FROM v$session;
                RMAN> ALTER TABLESPACE users ADD DATAFILE SIZE 121m;
9. Table or partition recovery in RMAN
Oracle database backups are mainly categorized into two types: logical and physical. Each backup type has its own pros and cons. In previous editions, it was not feasible to restore a table or partition using existing physical backups. In order to restore a particular object, you must have logical backup. With 12c R1, you can recover a particular table or partition to a point-in-time or SCN from RMAN backups in the event of a table drop or truncate.

When a table or partition recovery is initiated via RMAN, the following action is performed:

Required backup sets are identified to recover the table/partition
An auxiliary database will be configured to a point-in-time temporarily in the process of recovering the table/partition
Required table/partitions will be then exported to a dumpfile using the data pumps
Optionally, you can import the table/partitions in the source database
Rename option while recovery
An example of a table point-in-time recovery via RMAN (ensure you already have a full database backup from earlier):
                RMAN> connect target "username/password as SYSBACKUP";
                RMAN> RECOVER TABLE username.tablename UNTIL TIME 'TIMESTAMP…'
                                AUXILIARY DESTINATION '/u01/tablerecovery'
                                DATAPUMP DESTINATION '/u01/dpump'
                                DUMP FILE 'tablename.dmp'
                                NOTABLEIMPORT    -- this option avoids importing the table automatically.
REMAP TABLE 'username.tablename': 'username.new_table_name';    -- can rename table with this option.
Important notes:
Ensure sufficient free space available under /u01 filesystem for auxiliary database and also to keep the data pump file
A full database backup must be exists, or at least the SYSTEM related tablespaces
The following limitations/restrictions are applied on table/partition recovery in RMAN:

SYS user table/partition can’t be recovered
Tables/partitions stored under SYSAUX and SYSTEM tablespaces can’t be recovered
Recovery of a table is not possible when REMAP option used to recovery a table that contains NOT NULL constraints
10. Restricting PGA size
Pre Oracle 12c R1, there was no option to limit and control the PGA size. Although, you set a certain size to PGA_AGGREGATE_TARGET initialization parameter, Oracle could increase/reduce the size of the PGA dynamically based on the workload and requirements. In 12c, you can set a hard limit on PGA by enabling the automatic PGA management, which requires PGA_AGGREGATE_LIMIT parameter settings. Therefore, you can now set the hard limit on PGA by setting the new parameter to avoid excessive PGA usage.

SQL> ALTER SYSTEM SET PGA_AGGREGATE_LIMIT=0; --disables the hard limit
Important notes:
When the current PGA limits exceeds, Oracle will automatically terminates/abort the session/process that holds the most untenable PGA memory.

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