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Partitioning Concept in Oracle

Partitioning enhances the performance, manageability, and availability of a wide variety of applications and helps reduce the total cost of ownership for storing large amounts of data. Partitioning allows tables, indexes, and index-organized tables to be subdivided into smaller pieces, enabling these database objects to be managed and accessed at a finer level of granularity. Moreover, since it is entirely transparent, partitioning can be applied to almost any application without the need for potentially expensive and time consuming application changes.

Basics of Partitioning
Partitioning allows a table, index, or index-organized table to be subdivided into smaller pieces, where each piece of such a database object is called a partition. Each partition has its own name, and may optionally have its own storage characteristics.

From the perspective of a database administrator, a partitioned object has multiple pieces that can be managed either collectively or individually. This gives the administrator considerable flexibility in managing partitioned objects. However, from the perspective of the application, a partitioned table is identical to a non-partitioned table; no modifications are necessary when accessing a partitioned table using SQL queries and DML statements.

Each partition of a table or index must have the same logical attributes, such as column names, datatypes, and constraints, but each partition can have separate physical attributes such as compression enabled or disabled, physical storage settings, and tablespaces.

Partitioning is useful for many different types of applications, particularly applications that manage large volumes of data. OLTP systems often benefit from improvements in manageability and availability, while data warehousing systems benefit from performance and manageability.

Partitioning offers these Advantages:
  • It enables data management operations such as data loads, index creation and rebuilding, and backup/recovery at the partition level, rather than on the entire table. This results in significantly reduced times for these operations.
  • It improves query performance. In many cases, the results of a query can be achieved by accessing a subset of partitions, rather than the entire table. For some queries, this technique (called partition pruning) can provide order-of-magnitude gains in performance.
  • It significantly reduces the impact of scheduled downtime for maintenance operations.
    Partition independence for partition maintenance operations lets you perform concurrent maintenance operations on different partitions of the same table or index. You can also run concurrent SELECT and DML operations against partitions that are unaffected by maintenance operations.
  • It increases the availability of mission-critical databases if critical tables and indexes are divided into partitions to reduce the maintenance windows, recovery times, and impact of failures.
  • Parallel execution provides specific advantages to optimize resource utilization, and minimize execution time. Parallel execution against partitioned objects is key for scalability in a clustered environment. Parallel execution is supported for queries as well as for DML and DDL.
Partitioning enables faster data access within an Oracle database. Whether a database has 10 GB or 10 TB of data, partitioning can improve data access by orders of magnitude. Partitioning can be implemented without requiring any modifications to your applications. For example, you could convert a nonpartitioned table to a partitioned table without needing to modify any of the SELECT statements or DML statements which access that table. You do not need to rewrite your application code to take advantage of partitioning.
When to Partition a Table
Here are some suggestions for when to partition a table:
  • Tables greater than 2 GB should always be considered as candidates for partitioning.
  • Tables containing historical data, in which new data is added into the newest partition. A typical example is a historical table where only the current month's data is updatable and the other 11 months are read only.
  • When the contents of a table need to be distributed across different types of storage devices.
When to Partition an Index
Here are some suggestions for when to consider partitioning an index:
  • Avoid rebuilding the entire index when data is removed.
  • Perform maintenance on parts of the data without invalidating the entire index.
  • Reduce the impact of index skew caused by an index on a column with a monotonically increasing value.

Partitioning and LOB Data

Unstructured data (such as images and documents) which is stored in a LOB column in the database can also be partitioned. When a table is partitioned, all the columns will reside in the tablespace for that partition, with the exception of LOB columns, which can be stored in their own tablespace.
This technique is very useful when a table is comprised of large LOBs because they can be stored separately from the main data. This can be beneficial if the main data is being frequently updated but the LOB data isn't. For example, an employee record may contain a photo which is unlikely to change frequently. However, the employee personnel details (such as address, department, manager, and so on) could change. This approach also means that cheaper storage can be used for storing the LOB data and more expensive, faster storage used for the employee record.

Partitioning Strategies

Oracle Partitioning offers three fundamental data distribution methods as basic partitioning strategies that control how data is placed into individual partitions:
  • Range
  • Hash
  • List
Using these data distribution methods, a table can either be partitioned as a single list or as a composite partitioned table:
  • Single-Level Partitioning
  • Composite Partitioning
Each partitioning strategy has different advantages and design considerations. Thus, each strategy is more appropriate for a particular situation.

Single-Level Partitioning

A table is defined by specifying one of the following data distribution methodologies, using one or more columns as the partitioning key:

  • Range Partitioning
  • Hash Partitioning
  • List Partitioning

List Partitioning
List partitioning enables you to explicitly control how rows map to partitions by specifying a list of discrete values for the partitioning key in the description for each partition. The advantage of list partitioning is that you can group and organize unordered and unrelated sets of data in a natural way. For a table with a region column as the partitioning key, the North America partition might contain values Canada, USA, and Mexico.

The DEFAULT partition enables you to avoid specifying all possible values for a list-partitioned table by using a default partition, so that all rows that do not map to any other partition do not generate an error.

Range Partitioning
Range partitioning maps data to partitions based on ranges of values of the partitioning key that you establish for each partition. It is the most common type of partitioning and is often used with dates. For a table with a date column as the partitioning key, the January-2005 partition would contain rows with partitioning key values from 01-Jan-2005 to 31-Jan-2005.

Each partition has a VALUES LESS THAN clause, which specifies a non-inclusive upper bound for the partitions. Any values of the partitioning key equal to or higher than this literal are added to the next higher partition. All partitions, except the first, have an implicit lower bound specified by the VALUES LESS THAN clause of the previous partition.

A MAXVALUE literal can be defined for the highest partition. MAXVALUE represents a virtual infinite value that sorts higher than any other possible value for the partitioning key, including the NULL value.

Hash Partitioning
Hash partitioning maps data to partitions based on a hashing algorithm that Oracle applies to the partitioning key that you identify. The hashing algorithm evenly distributes rows among partitions, giving partitions approximately the same size.

Hash partitioning is the ideal method for distributing data evenly across devices. Hash partitioning is also an easy-to-use alternative to range partitioning, especially when the data to be partitioned is not historical or has no obvious partitioning key.

Composite Partitioning

Composite partitioning is a combination of the basic data distribution methods; a table is partitioned by one data distribution method and then each partition is further subdivided into subpartitions using a second data distribution method. All subpartitions for a given partition together represent a logical subset of the data.

Composite partitioning supports historical operations, such as adding new range partitions, but also provides higher degrees of potential partition pruning and finer granularity of data placement through subpartitioning. 

Various Type of Composite Partitioning:
Composite Range-Range Partitioning
Composite Range-Hash Partitioning
Composite Range-List Partitioning
Composite List-Range Partitioning
Composite List-Hash Partitioning

Composite List-List Partitioning

Composite Range-Range Partitioning
Composite range-range partitioning enables logical range partitioning along two dimensions; for example, partition by order_date and range subpartition by shipping_date.

Composite Range-Hash Partitioning
Composite range-hash partitioning partitions data using the range method, and within each partition, subpartitions it using the hash method. Composite range-hash partitioning provides the improved manageability of range partitioning and the data placement, striping, and parallelism advantages of hash partitioning.

Composite Range-List Partitioning
Composite range-list partitioning partitions data using the range method, and within each partition, subpartitions it using the list method. Composite range-list partitioning provides the manageability of range partitioning and the explicit control of list partitioning for the subpartitions.

Composite List-Range Partitioning
Composite list-range partitioning enables logical range subpartitioning within a given list partitioning strategy; for example, list partition by country_id and range subpartition by order_date.

Composite List-Hash Partitioning
Composite list-hash partitioning enables hash subpartitioning of a list-partitioned object; for example, to enable partition-wise joins.

Composite List-List Partitioning
Composite list-list partitioning enables logical list partitioning along two dimensions; for example, list partition by country_id and list subpartition by sales_channel.

Virtual Column-Based Partitioning

In previous releases of the Oracle Database, a table could only be partitioned if the partitioning key physically existed in the table. In Oracle Database 11g, virtual columns remove that restriction and allow the partitioning key to be defined by an expression, using one or more existing columns of a table. The expression is stored as metadata only.
Oracle Partitioning has been enhanced to allow a partitioning strategy to be defined on virtual columns. For example, a 10 digit account ID can include account branch information as the leading 3 digits. With the extension of virtual column based Partitioning, an ACCOUNTS table containing an ACCOUNT_ID column can be extended with a virtual (derived) column ACCOUNT_BRANCH that is derived from the first three digits of the ACCOUNT_ID column, which becomes the partitioning key for this table.

Overview of Partitioned Indexes

Just like partitioned tables, partitioned indexes improve manageability, availability, performance, and scalability. They can either be partitioned independently (global indexes) or automatically linked to a table's partitioning method (local indexes). In general, you should use global indexes for OLTP applications and local indexes for data warehousing or DSS applications. Also, whenever possible, you should try to use local indexes because they are easier to manage. When deciding what kind of partitioned index to use, you should consider the following guidelines in order:
  1. If the table partitioning column is a subset of the index keys, use a local index. If this is the case, you are finished. If this is not the case, continue to guideline 2.
  2. If the index is unique and does not include the partitioning key columns, then use a global index. If this is the case, then you are finished. Otherwise, continue to guideline 3.
  3. If your priority is manageability, use a local index. If this is the case, you are finished. If this is not the case, continue to guideline 4.
  4. If the application is an OLTP one and users need quick response times, use a global index. If the application is a DSS one and users are more interested in throughput, use a local index.
Local Partitioned Indexes

Local partitioned indexes are easier to manage than other types of partitioned indexes. They also offer greater availability and are common in DSS environments. The reason for this is equipartitioning: each partition of a local index is associated with exactly one partition of the table. This enables Oracle to automatically keep the index partitions in sync with the table partitions, and makes each table-index pair independent. Any actions that make one partition's data invalid or unavailable only affect a single partition.

Local partitioned indexes support more availability when there are partition or subpartition maintenance operations on the table. A type of index called a local nonprefixed index is very useful for historical databases. In this type of index, the partitioning is not on the left prefix of the index columns.
You cannot explicitly add a partition to a local index. Instead, new partitions are added to local indexes only when you add a partition to the underlying table. Likewise, you cannot explicitly drop a partition from a local index. Instead, local index partitions are dropped only when you drop a partition from the underlying table.

A local index can be unique. However, in order for a local index to be unique, the partitioning key of the table must be part of the index's key columns.

Global Partitioned Indexes

Oracle offers two types of global partitioned indexes: range partitioned and hash partitioned.

Global Range Partitioned Indexes
Global range partitioned indexes are flexible in that the degree of partitioning and the partitioning key are independent from the table's partitioning method.

The highest partition of a global index must have a partition bound, all of whose values are MAXVALUE. This ensures that all rows in the underlying table can be represented in the index. Global prefixed indexes can be unique or nonunique.
You cannot add a partition to a global index because the highest partition always has a partition bound of MAXVALUE. If you wish to add a new highest partition, use the ALTER INDEX SPLIT PARTITION statement. If a global index partition is empty, you can explicitly drop it by issuing the ALTER INDEX DROP PARTITIONstatement. If a global index partition contains data, dropping the partition causes the next highest partition to be marked unusable. You cannot drop the highest partition in a global index.

Global hash partitioned indexes improve performance by spreading out contention when the index is monotonically growing. In other words, most of the index insertions occur only on the right edge of an index.

Miscellaneous Information about Creating Indexes on Partitioned Tables

You can create bitmap indexes on partitioned tables, with the restriction that the bitmap indexes must be local to the partitioned table. They cannot be global indexes.
Global indexes can be unique. Local indexes can only be unique if the partitioning key is a part of the index key.

Partitioned Indexes on Composite Partitions

Here are a few points to remember when using partitioned indexes on composite partitions:
  • Subpartitioned indexes are always local and stored with the table subpartition by default.
  • Tablespaces can be specified at either index or index subpartition levels.


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