SAP HANA Delta Merge in SAP BW/4HANA

The column store uses efficient compression algorithms that keep all relevant application
data in memory. Write operations on this compressed data are costly, as they require
reorganizing the storage structure. SAP has addressed this challenge by separating these tables into Main Storage (read-optimized, sorted columns) and Delta Storages (write-optimized, non
sorted columns or rows). All changes go into a separate area called the delta storage.

Delta storage exists only in main memory. Only delta log entries are written to the persistence
layer when delta entries are inserted. There is a regular database activity which merges the
delta storage into the main storage. This activity is called Delta Merge.

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.

The check and the delta merge, if data has been changed during Data Warehouse load processes, are either performed automatically by the system or triggered manually.

This depends on the following object types:

● 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. This checkbox is selected in the default setting.
● For DataStore objects (advanced), the Trigger Database Merge checkbox is selected by
default. After activation, an automatic check is run to ascertain whether a delta merge can
be performed.

Triggering Delta Merge via Process Chain

In exceptional cases, performing a delta merge after processing a DTP request is not recommended, due to load balancing issues. In this case, an alternative method triggers the delta merge via a process type.

In exceptional cases only, which result in load distribution problems, we recommend
deselecting the checkbox in the DTP and using the Trigger Delta Merge process type to trigger
the delta merge. An example of this is an object that data is loaded into from multiple sources.
The delta merge check is only performed at the end of the entire loading process.

Note:
Make sure that the DTP or process type always triggers a delta merge. If no delta merge takes place, the data remains in delta storage. Over time, this results in storage problems and has a negative impact on read performance.

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 as shown in the table.

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 Delta Merge checkbox. This controls the delta merge, once the DTP request has
been processed. This checkbox is selected in the default setting.