Best New Features of Oracle Database 12c

Editor's Notes

• #36: The invisible columns feature which allows you to mark a specific column or set of columns as invisible. This means that If you will execute a SELECT * from the table, the invisible columns will not be displayed in the output of the select statement; however, if you will explicitly specify these columns as part of the SELECT statement they will be displayed in the output of the select statement.The Invisible columns is a very simple and straight-forward feature which can be useful if you would like to add a new column to a table but at the first stage you don't want that the application will be affected by the new column. SELECT * and INSERT statements will continue to function normal (unless an integrity constraint on the invisible column was violated).
• #37: You can either mark the columns as invisible as part of the CREATE TABLE command, or as part of the ALTER TABLE command. If you will execute an INSERT statement and you don't specify the columns, the INSERT will work; however, you can insert value to the invisible columns if you will explicitly specify the invisible columns as part of the INSERT statement.
• #38: In the moment that you decide that you want to make the column visible, you can manage to do so in a single command (ALTER TABLE table_name MODIFY column_name VISIBLE).
• #40: New Type of column which automatically generates an auto increment value upon every row insertion. It has the following options: ALWAYS (default option) - In this option, Oracle forces the usage of the sequence generator. If an insert or update statements reference the identity column, Oracle raises an error. BY DEFAULT - This option allows to explicitly assign a value for the column as part of an INSERT or UPDATE statements. BY DEFAULT ON NULL - In the previous option (BY DEFAULT), If you will attempt to reference the identity column, and assign it a NULL value, you will encounter an error. Using the BY DEFAULT ON NULL option tells Oracle to use the identity generator when there is an attempt to insert a NULL to the table.
• #41: In the first demonstration, I will create a sample table named "PRODUCTS" with an identity column for the primary key, that uses the ALWAYS option. In this option, Oracle forces the usage of the sequence generator. If an insert or update statements reference the identity column, Oracle raises an error.
• #42: The "BY DEFAULT" clause is used for the identity column, which allows us to explicitly assign a value for the identity column (as long as it is not NULL), as follows:
• #43: - In the previous option (BY DEFAULT), If you will attempt to reference the identity column, and assign it a NULL value, you will encounter an error. Using the BY DEFAULT ON NULL option tells Oracle to use the identity generator when there is an attempt to insert a NULL to the table.
• #44: When creating an identity column, Oracle "behind the scenes" creates a sequence. This means that all the options that available for sequences (e.g. START WITH , INCREMENT BY, MAX_VALUE, etc.) are available for identify columns as well, as you can see in the following example Click It is possible to query the [CDB|DBA|ALL|USER]_tab_identity_cols dictionary view to obtain all of the table identity columns, as follows
• #45: Another enhacement in the improved defaults which was introduced in Oracle 12c, it the ability to use a sequence as a default value for a column. It can be either CURRVAL or NEXTVAL. The following is a demonstration of this feature Demo As you can see in this demo, Oracle will use the sequence as a default value if the column is not referenced, or if it is referenced and the value "DEFAULT" is specified. When an explicit NULL value was inserted, the default sequence value is not being used, instead, a NULL was inserted to the table.
• #46: In order to use a default sequence number even when an explicit NULL is being inserted, you can use the ON NULL clause. Let us see a demonstration of this feature
• #48: Before Oracle 11g  Prior to Oracle 11g, adding a new column with a default value to an existing table, requires updating all the rows in the table which generates lots of redo and undo, and can impact the entire database performance. Obviously, it is a very long and high consuming operation, especially if the table is big. Oracle 11g  In Oracle 11g a new feature named "Metadata-only value" was introduced. This means that upon every new column addition with a default value, Oracle will not update every row in a table, but rather make a small metadata-only change in the data dictionary. As a result, every time that a user queries the table, Oracle will obtain the default value from the data dictionary. This feature allows us to add a new column with a default value within milliseconds instead of waiting for a long time and without generating lots of redo and undo which may impact the entire database.
• #49: In the following example, a table named "TEST" is being created and populated with 100,000 rows.Click Now, we will test how much time it takes to add a new column with a default value Click As you can see in the above demonstration, the column addition operation completed instantly.However, this feature works only if the column was added with the NOT NULL constraint.Let us see what will happen if we will try to add a nullable column with a default value (i.e. a column that doesn't have the NOT NULL constraint) Click Now, the operation took more than 1 minute! This table is only populated with 100K rows. Imagine how much time it would take if a table contains hundreds of millions of rows ... Click As you can see, in Oracle 12c adding a column with a default value is an instant operation, regardless of the the nullable attribute.
• #51: Before Oracle 12c in order to support strings longer than 4000 bytes we mainly used LOB. If you decide to enable this feature you will need to take into account that internally Oracle is storing these extended types in out-of-line LOB segments but you can't manage these lob segments using DBMS_LOB as Oracle is managing them internally. From my point of view I think this feature can be useful although its biggest disadvantage is that it requires downtime to enable it which can be problematic for many Oracle customers.
• #54: Limiting the number of rows returned from ordered sets of data, which is also known as Top-N queries is a common use-case. Examples of Top-N queries could be: Query the top 3 employees with the highest salaries Query the first 2 employees who have joined the company Click Prior to Oracle 12c we had to use the ROWNUM pseudo column in order to implement Top-N reporting. Using the ROWNUM pseudo column had worked well, however it had major disadvantages: It's not a straight-forward syntax, especially if you need to implement query paging such as query the second block of the top 3 employees with the highest salaries. It allows us specifying the Top-N rows but it doesn't allows specifying the Top-N percent (for example, query the top 25% percent of employees with the highest salaries.
• #59: Moving data files is a common use-case, for example, when DBA wants to move data file to a faster disk or larger disk. Or DBA wants to migrate the data files from traditional file system to ASM. Before Oracle 12c, this task was a painful because it required a downtime. Common Solution Take tablespace offline Move data file to a new location using OS command Change the location of data file using DB command “ALTER DATABASE” command A common workaround to improve the downtime was to take the tablespace to a READ ONLY state which make the process less painful, but a shortdown was still required.
• #62: Oracle 11g Introduced a nice feature named "Invisible Indexes" which allows us to mark an index as invisible. This index will still be maintained by Oracle during DML operations like a "regular" index but it is being ignored by the optimizer, i.e. execution plans will not use the invisible indexes. Click Oracle 12c leverages the "Invisible Indexes" feature by allowing us create multiple indexes on the same column or set of columns, as long as only one index is visible and all the indexes are different, i.e. it is impossible to create 2 B-Tree indexes on the same column, even if one of them is invisible.
• #63: This can be useful when you want to test the impact of different indexes easily without dropping and creating a new index. However, it is important to bear in mind that additional indexes means additional overhead during DML operations. This additional overhead is necessary in order to maintain the indexes, therefore use this feature with cautious
• #65: RMAN Table point-in-time recovery can be very useful in the following cases : The table was truncated - so you can't use the 10g Flashback Table feature as the undo is not available The table was dropped with the purge clause -so you can't use the 10g Flashback Drop feature Table was logically corrupted and no undo is available - so you can't use the 10g Flashback Table feature Table was logically corrupted and a DDL operation modified the structure of the table - so you can't use the 10g Flashback Table feature in order to flashback the table prior to the DDL This feature requires minimum work and effort from the DBA Due to the fact that as part of the recovery process RMAN needs to restore an auxiliary instance, and also export and import the relevant table/s, this whole process has an overhead (mainly in terms of I/O load), therefore If you can use other options for recovering the table (such as Flashback Table/Flashback Drop) you should use them and avoid using this feature. Also, table-recovery using this feature takes much longer than using the Flashback drop/Flashback table When you can't use Flashback drop/Flashback table to recover the table, you may find this feature very useful. This feature is available with the Enterprise Edition only
• #67: Problem: Oracle 11gR2 required that a user account had been granted the SYSDBA privilege to perform any RMAN backup, restoration, or recovery operation. The problem with this approach is that the user who is responsible to backup the database has an access to the data and in some companies it is important to have a separation of duty. Solution: Oracle 12cR1 now provides the SYSBACKUP privilege that limits a user account to handle just these tasks. SYSBACKUP administrative privilege allows performing backup and recovery operations. This includes performing STARTUP and SHUTDOWN operations.​