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Don Burleson Blog 







Oracle Global Index vs. Local Index

Oracle Database Tips by Donald BurlesonMay 14, 2015

Oracle Global Index vs. Local Index

Question:  What is the difference between a oracle global index and a local index?

Answer:  When using Oracle partitioning, you can specify the "global" or "local" parameter in the create index syntax:

  • Global Index:  A global index is a one-to-many relationship, allowing one index partition to map to many table partitions.  The docs says that a "global index  can be partitioned by the range or hash method, and it can be defined on any type of partitioned, or non-partitioned, table".

  • Local Index: A local index is a one-to-one mapping between a index partition and a table partition.  In general, local indexes allow for a cleaner "divide and conquer" approach for generating fast SQL execution plans with partition pruning.

For complete details, see my tips for Oracle partitioning.

Global and Local Index partitioning with Oracle

The first partitioned index method is called a LOCAL partition. A local partitioned index creates a one-for-one match between the indexes and the partitions in the table. Of course, the key value for the table partition and the value for the local index must be identical. The second method is called GLOBAL and allows the index to have any number of partitions.

The partitioning of the indexes is transparent to all SQL queries. The great benefit is that the Oracle query engine will scan only the index partition that is required to service the query, thus speeding up the query significantly. In addition, the Oracle parallel query engine will sense that the index is partitioned and will fire simultaneous queries to scan the indexes.

Local partitioned indexes

Local partitioned indexes allow the DBA to take individual partitions of a table and indexes offline for maintenance (or reorganization) without affecting the other partitions and indexes in the table.

In a local partitioned index, the key values and number of index partitions will match the number of partitions in the base table.

on all_fact (order_date)
(PARTITION name_idx1),
(PARTITION name_idx2),
(PARTITION name_idx3);

Oracle will automatically use equal partitioning of the index based upon the number of partitions in the indexed table. For example, in the above definition, if we created four indexes on all_fact, the CREATE INDEX would fail since the partitions do not match. This equal partition also makes index maintenance easier, since a single partition can be taken offline and the index rebuilt without affecting the other partitions in the table.

Global partitioned indexes

A global partitioned index is used for all other indexes except for the one that is used as the table partition key. Global indexes partition OLTP (online transaction processing) applications where fewer index probes are required than with local partitioned indexes. In the global index partition scheme, the index is harder to maintain since the index may span partitions in the base table.

For example, when a table partition is dropped as part of a reorganization, the entire global index will be affected. When defining a global partitioned index, the DBA has complete freedom to specify as many partitions for the index as desired.

Now that we understand the concept, let's examine the Oracle CREATE INDEX syntax for a globally partitioned index:

on all_fact (item_nbr)

Here, we see that the item index has been defined with five partitions, each containing a subset of the index range values. Note that it is irrelevant that the base table is in three partitions. In fact, it is acceptable to create a global partitioned index on a table that does not have any partitioning.

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