MERGE process in SAP BW

In SAP BW systems running on SAP HANA database there is a process called merge. Data being written into (BW) objects/tables are first saved in uncompressed delta index. Merge process is a transfer of uncompressed delta index table data to performance-optimized storages. In order to fully leverage those performance-optimized storages the merge process must be started in regular intervals. In case the data would be just staying in the delta index storages there would be an increased memory consumption (delta storages tables may become very large) that could lead to system instability.

The merge it-self can be executed based on several reasons. There is a term - merge motivations that is a mechanism by which a delta merge operation is triggered. From SAP HANA DB depending on how the merge process is triggered there are following types of the merge process:

Delta merge - operation in the column store of the SAP HANA database that moves changes collected in the delta storage to the read-optimized main storage.

Smart merge – delta merge operation triggered by a request from an application. In SAP BW this term is referred as DataStore smart merge.

Hard merge – delta merge operation triggered manually by SQL statement.

Forced merge – delta merge operation triggered manually by SQL statement that passes an optional parameter to execute the delta merge regardless of system resource availability.

Memory-only merge - delta merge operation triggered manually by SQL statement that passes an optional parameter to omit the final step of the delta merge, that is, persisting main storage to disk.

Auto merge – automatically executed merge by database system.

 

Smart Delta merge in SAP BW trigger can be set on DTP level. In the DTP maintenance t-code RSDTP on Update tab there is a checkbox called 'Trigger Database Merge'. There is also a process type in Process Chains called 'Trigger the delta merge' that can be used to trigger the merge process.

Hard delta merge after request deletion in aDSO can be driven by RSADMIN table parameter called 'RSDSO_HARD_MERGE_AFTER_REQDEL'. It can be set to X to enable hard delta merge after the request deletion.

 

More information:

SAP HANA Delta Merge Related To SAP BW/4HANA

3481267 - Hard delta merge after request deletion