Refresh Methods

Cubes are updated in a batch window at regular intervals: weekly, daily, or periodically throughout the day. Some administrators refresh their data by adding periods to the time dimension of a measure, and may also roll off an equal number of the oldest time periods. Each update provides a fixed historical record of a particular business activity for that interval. Other administrators do a full rebuild of their data rather than performing incremental updates.

Refresh Method Descriptions

Table: Refresh Methods For Cube Materialized Views describes the refresh methods that are supported on cube materialized views.You can specify the COMPLETE, FAST, or FORCE methods for refreshing a cube. The materialized view subsystem invokes two additional methods, FAST_PCT and FAST_SOLVE. They are not separate choices.

Table 7-1 Refresh Methods For Cube Materialized Views

Refresh Method	Description
`COMPLETE`	Deletes and recreates the cube. This option supports arbitrarily complex mappings from the source tables to the cube.
`FAST`	Loads and re-aggregates only changed values, based on the materialized view logs or, after direct path loading, on the ALL_SUMDELTA data dictionary view. The source for the refresh is the incremental differences that have been captured in the materialized view logs, rather than the original mapped sources. These differences are used to incrementally rebuild the cube. Only cells that are affected by the changed values are re-aggregated. This option supports only simple mappings for cube materialized views, that is, where no expressions (other than table.column), views, or aggregations occur in the query defining the mapping. The materialized view subsystem determines whether to perform a `FAST` or a `FAST_PCT` refresh. See the Oracle Database Data Warehousing Guide for information about the methodology.
`FAST_PCT`	Loads and re-aggregates data only from changed partitions. This method works best when the source table and the cube are partitioned on the same dimension. `FAST_PCT` does not use change logs. The materialized view subsystem determines whether to perform a `FAST` or a `FAST_PCT` refresh. See the Oracle Database Data Warehousing Guide for information about the methodology.
`FAST_SOLVE`	Loads and re-aggregates only changed values, based on the original mapped data source. `FAST_SOLVE` is a type of refresh only for cube materialized views. It incrementally re-aggregates the cube even when the refresh source is the original mapped source instead of the materialized view logs. The aggregation subsystem identifies the differences and then incrementally re-aggregates the cube. This option is supported for arbitrarily complex mappings from the source tables to the cube. To discover whether a `FAST_SOLVE` refresh has occurred, review the `CUBE_BUILD_LOG` table as shown in "Fast Solve Refreshes". Or review the `LAST_REFRESH_TYPE` column of `ALL_MVIEW`; a `FAST_SOLVE` refresh appears as `FAST_CS`.
`FORCE`	Loads and re-aggregates values using the best method possible. When a `COMPLETE` refresh is not necessary, the materialized view system first attempts a `FAST` refresh. If it cannot `FAST` refresh a cube materialized view, it performs a `FAST_SOLVE` refresh.

Fast Solve Refreshes

The build log lists the CLEAR LEAVES command when the FAST SOLVE method was used. Example: Identifying a FAST SOLVE Refresh shows the rows of CUBE_BUILD_LOG concerned with building UNITS_CUBE.