Due to the selectivity of queries and high concurrency of OLTP applications, the choice of the right index strategy is indisputably an important decision for the use of partitioning in an OLTP environment. With less contention, the application can support a larger user population.

The following basic rules explain the main benefits and trade-offs for the various possible index structures:

• A nonpartitioned index, while larger than individual partitioned index segments, always leads to a single index probe (or scan) if an index access path is chosen; there is only one segment for a table. The data access time and number of blocks being accessed are identical for both a partitioned and a nonpartitioned table.

  A nonpartitioned index does not provide partition autonomy and requires an index maintenance operation for every partition maintenance operation that affects rowids (for example, drop, truncate, move, merge, coalesce, or split operations).

• With partitioned indexes, there are always multiple segments. Whenever Oracle Database cannot prune down to a single index segment, the database has to access multiple segments. This potentially leads to higher I/O requirements (n index segment probes compared with one probe for a nonpartitioned index) and can have an impact (measurable or not) on the run-time performance. This is true for all partitioned indexes.

  Partitioned indexes can either be local partitioned indexes or global partitioned indexes. Local partitioned indexes always inherit the partitioning key from the table and are fully aligned with the table partitions. Consequently, any kind of partition maintenance operation requires little to no index maintenance work. For example, dropping or truncating a partition does not incur any measurable overhead for index maintenance; the local index partitions are either dropped or truncated.

  Partitioned indexes that are not aligned with the table are called global partitioned indexes. Unlike local indexes, there is no relation between a table and an index partition. Global partitioned indexes give the flexibility to choose a partitioning key that is optimal for an efficient partition index access. Partition maintenance operations normally affect more (if not all) partitions of a global partitioned index, depending on the operation and partitioning key of the index.

• Under some circumstances, having multiple segments for an index can be beneficial for performance. It is very common in OLTP environments to use sequences to create artificial keys. Consequently, you create key values that are monotonically increasing, which results in many insertion processes competing for the same index blocks. Introducing a global partitioned index (for example, using global hash partitioning on the key column) can alleviate this situation. If you have, for example, four hash partitions for such an index, then you now have four index segments into which you are inserting data, reducing the concurrency on these segments by a factor of four for the insertion processes.

Enforcing uniqueness is important database functionality for OLTP environments. Uniqueness can be enforced with nonpartitioned and partitioned indexes. However, because partitioned indexes provide partition autonomy, the following requirements must be met to implement unique indexes:

• A nonpartitioned index can enforce uniqueness for any given column or combination of columns. The behavior of a nonpartitioned index is no different for a partitioned table compared to a nonpartitioned table.

• Each partition of a partitioned index is considered an autonomous segment. To enforce the autonomy of these segments, you always have to include the partitioning key columns as a subset of the unique key definition.

  • Global partitioned indexes must always be prefixed with at least the first leading column of the index column (the partitioning column of the partitioned global index).

  • Unique local indexes must have the partitioning key of the table as a subset of the unique key definition.

Example 7-1 shows the creation of a unique index on the order_id column of the orders_oltp table. The order_id in the OLTP application is filled using a sequence number. The unique index uses hash partitioning to reduce contention for the monotonically increasing order_id values. The unique key is then used to create the primary key constraint.

CREATE UNIQUE INDEX orders_pk
  ON orders_oltp(order_id)
  GLOBAL PARTITION BY HASH (order_id)
  ( PARTITION p1 TABLESPACE tbs1
   , PARTITION p2 TABLESPACE tbs2
   , PARTITION p3 TABLESPACE tbs3
   , PARTITION p4 TABLESPACE tbs4
  ) NOLOGGING;

ALTER TABLE orders_oltp ADD CONSTRAINT orders_pk
  PRIMARY KEY (order_id)
  USING INDEX;

When your workload fits the use of index-organized tables, then you must consider how to use partitioning on your index-organized table and on any secondary indexes.

You must decide whether to partition secondary indexes on index-organized tables based on the same considerations as indexes on regular heap tables. You can partition an index-organized table, but the partitioning key must be a subset of the primary key. A common reason to partition an index-organized table is to reduce contention; this is typically achieved using hash partitioning.

Another reason to partition an index-organized table is to be able to physically separate data sets based on a primary key column. For example, an application-hosting company can physically separate application instances for different customers by list partitioning on the company identifier. Queries in such a scenario can often take advantage of index partition pruning, shortening the time for the index scan. ILM scenarios with index-organized tables and partitioning are less common because they require a date column to be part of the primary key.

For more information about how to create partitioned index-organized tables, refer to Partition Administration.

When a partition maintenance operation takes place, Oracle Database locks the affected table partitions for any DML operation, except in the case of an ONLINE MOVE.

Data in the affected partitions, except a DROP or TRUNCATE operation, is still fully accessible for any SELECT operation. Because local indexes are logically coupled with the table (data) partitions, only the local index partitions of the affected table partitions have to be maintained as part of a partition maintenance operation, enabling optimal processing for the index maintenance.

For example, when you move an older partition from a high-end storage tier to a low-cost storage tier using the ALTER TABLE MOVE ONLINE functionality, the data and the index are always available for SELECT and DML operations; the necessary index maintenance is to update the existing index partition to reflect the new physical location of the data. If you drop an older partition after you have archived it, then its local index partitions get dropped as well, and for global indexes the orphaned entries for the removed partitions get marked, enabling a split-second partition maintenance operation that affects only the data dictionary.

Whenever a global index is defined on a partitioned or nonpartitioned table, there is no correlation between a distinct table partition and the index. Consequently, any partition maintenance operation affects all global indexes or index partitions.

The partitions of tables containing local indexes are locked to prevent DML operations against the affect table partitions, except for an ONLINE MOVE operation. However, unlike the index maintenance for local indexes, any global index is still fully available for DML operations and does not affect the online availability of the OLTP system.

Conceptually and technically, the index maintenance for global indexes for a partition maintenance operation is comparable to the index maintenance that would become necessary for a semantically identical DML operation, except for DROP and TRUNCATE where the global index maintenance gets delayed to a later point in time. For more information about managing global indexes, refer to Management of Global Partitioned Indexes.

For example, dropping an old partition is semantically equivalent to deleting all the records of the old partition using the SQL DELETE statement. In the DML case, all index entries of the deleted data set have to be removed from any global index as a standard index maintenance operation, which does not affect the availability of an index for SELECT and DML operations.

The DROP PARTITION also does not affect the index availability, but enables you to decouple the necessary index maintenance from the initial data removal without affecting the availability of the global indexes. In this scenario, a drop operation represents the optimal approach: data is removed without the overhead of a conventional DELETE operation and the global indexes are maintained in a nonintrusive manner.

The two most common partition maintenance operations are the removal of data and the relocation of data onto lower-cost storage tier devices.

• Removing (Purging) Old Data

• Moving or Merging Older Partitions to a Low-Cost Storage Tier Device