Every Oracle Database must have a method of maintaining information that is used to undo changes to the database. Such information consists of records of the actions of transactions, primarily before they are committed. Collectively these records are called undo data.

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To set up an environment for automatic undo management using an undo tablespace.

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• Set the UNDO_MANAGEMENT initialization parameter to AUTO, which is the default.

• UNDO_MANAGEMENT Initialization Parameter
  The UNDO_MANAGEMENT initialization parameter determines whether an instance starts in automatic undo management mode, which stores undo in an undo tablespace. Set this parameter to AUTO to enable automatic undo management mode. AUTO is the default if the parameter is omitted or is null.

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• UNDO_TABLESPACE Initialization Parameter
  The UNDO_TABLESPACE initialization parameter enables you to override that default undo tablespace for an instance.

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You can specify the amount of time that blocking DDL statements wait for locks.

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A data definition language (DDL) statement is either nonblocking or blocking, and both types of DDL statements require exclusive locks on internal structures. If these locks are unavailable when a DDL statement runs, then nonblocking and blocking DDL statements behave differently:

• Nonblocking DDL waits until every concurrent DML transaction that references the object affected by the DDL either commits or rolls back.
• Blocking DDL fails, though it might have succeeded if it had been executed subseconds later when the locks become available.

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To enable blocking DDL statements to wait for locks, specify a DDL lock timeout—the number of seconds a DDL command waits for its required locks before failing.

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• To specify a DDL lock timeout, set the DDL_LOCK_TIMEOUT parameter.

The permissible range of values for DDL_LOCK_TIMEOUT is 0 to 1,000,000. The default is 0. You can set DDL_LOCK_TIMEOUT at the system level, or at the session level with an ALTER SESSION statement.

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The PROCESSES initialization parameter determines the maximum number of operating system processes that can be connected to Oracle Database concurrently.

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• Set the PROCESSES initialization parameter.

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The value of this parameter must be a minimum of one for each background process plus one for each user process. The number of background processes will vary according the database features that you are using. For example, if you are using Advanced Queuing or the file mapping feature, then you will have additional background processes. If you are using Automatic Storage Management, then add three additional processes for the database instance.

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If you plan on running 50 user processes, a good estimate would be to set the PROCESSES initialization parameter to 70.

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In a bitmap index, the database stores a bitmap for each index key. In a conventional B-tree index, one index entry points to a single row. In a bitmap index, each index key stores pointers to multiple rows.

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Bitmap indexes are primarily designed for data warehousing or environments in which queries reference many columns in an ad hoc fashion. Situations that may call for a bitmap index include:

• The indexed columns have low cardinality, that is, the number of distinct values is small compared to the number of table rows.
• The indexed table is either read-only or not subject to significant modification by DML statements.
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For a data warehouse example, the sh.customers table has a cust_gender column with only two possible values: M and F. Suppose that queries for the number of customers of a particular gender are common. In this case, the customers.cust_gender column would be a candidate for a bitmap index.

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Each bit in the bitmap corresponds to a possible rowid. If the bit is set, then the row with the corresponding rowid contains the key value. A mapping function converts the bit position to an actual rowid, so the bitmap index provides the same functionality as a B-tree index although it uses a different internal representation.

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If the indexed column in a single row is updated, then the database locks the index key entry (for example, M or F) and not the individual bit mapped to the updated row. Because a key points to many rows, DML on indexed data typically locks all of these rows. For this reason, bitmap indexes are not appropriate for many OLTP applications.

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You can create tablespaces of nonstandard block sizes.

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To create tablespaces of nonstandard block sizes:

• Specify the BLOCKSIZE clause in a CREATE TABLESPACE statement.

These nonstandard block sizes can have any of the following power-of-two values: 2K, 4K, 8K, 16K or 32K. Platform-specific restrictions regarding the maximum block size apply, so some of these sizes may not be allowed on some platforms.

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To use nonstandard block sizes, you must configure subcaches within the buffer cache area of the SGA memory for all of the nonstandard block sizes that you intend to use. The initialization parameters used for configuring these subcaches are described in “Using Automatic Shared Memory Management”.

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The ability to specify multiple block sizes for your database is especially useful if you are transporting tablespaces between databases. You can, for example, transport a tablespace that uses a 4K block size from an OLTP environment to a data warehouse environment that uses a standard block size of 8K.

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The most commonly used block size should be picked as the standard block size. In many cases, this is the only block size that you must specify.

• Set the DB_BLOCK_SIZE initialization parameter.

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Typically, DB_BLOCK_SIZE is set to either 4K or 8K. If you do not set a value for this parameter, then the default data block size is operating system specific, which is generally adequate.

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You cannot change the block size after database creation except by re-creating the database. If the database block size is different from the operating system block size, then ensure that the database block size is a multiple of the operating system block size. For example, if your operating system block size is 2K (2048 bytes), the following setting for the DB_BLOCK_SIZE initialization parameter is valid: DB_BLOCK_SIZE=4096

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A larger data block size provides greater efficiency in disk and memory I/O (access and storage of data). Therefore, consider specifying a block size larger than your operating system block size if the following conditions exist:

• Oracle Database is on a large computer system with a large amount of memory and fast disk drives. For example, databases controlled by mainframe computers with vast hardware resources typically use a data block size of 4K or greater.
• The operating system that runs Oracle Database uses a small operating system block size. For example, if the operating system block size is 1K and the default data block size matches this, the database may be performing an excessive amount of disk I/O during normal operation. For best performance in this case, a database block should consist of multiple operating system blocks.

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Advanced index compression works at the block level to provide the best compression for each block. The database uses the following technique:

• During index creation, as a leaf block becomes full, the database automatically compresses the block to the optimal level.
• When reorganizing an index block as a result of DML, if the database can create sufficient space for the incoming index entry, then a block split does not occur. During DML without advanced index compression, however, an index block split always occurs when the block becomes full.

Advanced Index Compression HIGH

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In releases previous to Oracle Database 12c Release 2 (12.2), the only form of advanced index compression was low compression (COMPRESS ADVANCED LOW). Now you can also specify high compression (COMPRESS ADVANCED HIGH), which is the default. Advanced index compression with the HIGH option offers the following advantages:

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• Gives higher compression ratios in most cases, while also improving performance for queries that access the index
• Employs more complex compression algorithms than advanced low
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• Stores data in a compression unit, which is a special on-disk format

Note

When you apply HIGH compression, all blocks have compression. When you apply LOW compression, the database may leave some blocks uncompressed. You can use statistics to determine how many blocks were left uncompressed.

Example 3-5 Creating an Index with Advanced High Compression

This example enables advanced index compression for an index on the hr.employees table:

CopyCREATE INDEX hr.emp_mndp_ix
  ON hr.employees(manager_id, department_id)
  COMPRESS ADVANCED;

The following query shows the type of compression:

CopySELECT COMPRESSION FROM DBA_INDEXES WHERE INDEX_NAME ='EMP_MNDP_IX';

COMPRESSION
-------------
ADVANCED HIGH

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The DB_BLOCK_SIZE initialization parameter specifies the standard block size for the database.

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• Set the DB_BLOCK_SIZE initialization parameter.

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This block size is used for the SYSTEM tablespace and by default in other tablespaces. Oracle Database can support up to four additional nonstandard block sizes.

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• DB_BLOCK_SIZE Initialization Parameter
  The most commonly used block size should be picked as the standard block size. In many cases, this is the only block size that you must specify.
• Nonstandard Block Sizes
  You can create tablespaces of nonstandard block sizes.

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Starting with Oracle Database 12c Release 1 (12.1.0.2), advanced index compression improves on traditional prefix compression for supported indexes on heap-organized tables.

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Benefits of Advanced Index Compression

Prefix compression has limitations for types of indexes supported, compression ratio, and ease of use. Unlike prefix compression, which uses fixed duplicate key elimination for every block, advanced index compression uses adaptive duplicate key elimination on a per-block basis. The main advantages of advanced index compression are:

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• The database automatically chooses the best compression for each block, using a number of internal algorithms such as intra-column level prefixes, duplicate key elimination, and rowid compression. Unlike in prefix compression, advanced index compression does not require the user to know data characteristics.
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• Advanced compression works on both non-unique and unique indexes. Prefix compression works well on some non-unique indexes, but the ratios are lower on indexes whose leading columns do not have many repeats.
• The compressed index is usable in the same way as an uncompressed index. The index supports the same access paths: unique key lookups, range scans, and fast full scans.
• Indexes can inherit advanced compression from a parent table or containing tablespace.

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Every database has a control file, which contains entries that describe the structure of the database (such as its name, the timestamp of its creation, and the names and locations of its data files and redo files). The CONTROL_FILES initialization parameter specifies one or more names of control files, separated by commas.

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• Set the CONTROL_FILES initialization parameter.

When you execute the CREATE DATABASE statement, the control files listed in the CONTROL_FILES parameter are created.

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If you do not include CONTROL_FILES in the initialization parameter file, then Oracle Database creates a control file in the same directory as the initialization parameter file, using a default operating system–dependent file name. If you have enabled Oracle Managed Files, the database creates Oracle managed control files.

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If you want the database to create new operating system files when creating database control files, the file names listed in the CONTROL_FILES parameter must not match any file names that currently exist on your system. If you want the database to reuse or overwrite existing files when creating database control files, ensure that the file names listed in the CONTROL_FILES parameter match the file names that are to be reused, and include a CONTROLFILE REUSE clause in the CREATE DATABASE statement.

Oracle strongly recommends you use at least two control files stored on separate physical disk drives for each database.

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