Triggering a Delta Merge (When Using the SAP HANA Database)

If you are using an SAP HANA database, data modifications are initially saved in a delta storage that is optimized for write access. However, most of the data is saved in a highly compressed format in the main storage, which is optimized in terms of required memory space and read performance. A delta merge is used to transfer modifications from the delta storage to the main storage. First, an asynchronous check is performed for see whether a delta merge is required. If a threshold value is exceeded, the merge is carried out in the delta storage. When a read access is executed, the data is read from the main storage and the delta storage and the results are merged together.

Procedure

The check and the delta merge - if data has been changed during Data Warehouse load processes - are either performed automatically by the system or must be triggered manually. This depends on the relevant object type:

Object Type	Delta Merge
Standard DataStore Object SAP HANA-Optimized DataStore Object	After activation, an automatic check is run to see whether a delta merge can be performed. This also applies to DataStore objects that belong to a semantically partitioned object.

Persistent Staging Area (PSA)	After data has been written to the PSA, an automatic check is run to see whether a delta merge can be performed.
Write-Optimized DataStore Object Standard InfoCube SAP HANA-Optimized InfoCube	The check and the delta merge are not performed automatically. This also applies to objects that belong to a semantically partitioned object. The data transfer process (DTP) has an Update tab that contains the Trigger Database Merge checkbox. This controls the delta merge, once the DTP request has been successfully processed. By default, this checkbox is selected. Note In certain exceptional cases, performing a delta merge after processing a DTP request is not recommended, due to load balancing issues. In these cases only, we recommend that you deselect the checkbox in the DTP and use the Trigger for DB Merge process type to trigger the delta merge. For example, data from multiple sources is loaded into an object. The delta merge check is only performed at the end of the entire loading process. If you use the Trigger for DB Merge process type, proceed as follows: You are in the plan view of the process chain where you want to include the process. The process type Trigger for DB Merge is available in the Load Process and Post-Processing process category. Drag and drop the Trigger for DB Merge process type into an appropriate place in the process chain. A dialog box appears. To create a new process variant, choose (Create). In the next screen, enter a name and a description for the process variant and choose (Continue). On the process variant maintenance screen, specify the type and name of the object for the delta merge. Save your entries and return to the plan view of the process chain. Link the process to the required loading processes. For more information, see Creating Process Chains. End of the note. Note Make sure that a delta merge is definitely triggered by either the DTP or the process type. If no delta merge is performed, the data remains in the delta storage and over time this leads to suboptimal memory use and read performance. End of the note.