Understanding SAP HANA Delta Merge in SAP BW/4HANA

SAP HANA is a powerful in-memory database that offers high performance, scalability, and flexibility for data warehousing and analytics. However, to achieve these benefits, SAP HANA requires a special mechanism to handle data modifications efficiently. This mechanism is called delta merge.

In this blog post, you will learn what delta merge is, how it works, and how it affects SAP BW/4HANA applications. You will also learn how to monitor and trigger delta merge operations using the SAP HANA cockpit and SQL statements.

What is delta merge?

Delta merge is a process that transfers data modifications from a temporary storage (delta storage) to a permanent storage (main storage) in SAP HANA column tables. Column tables are the default and recommended table type for SAP HANA, as they enable fast data compression, parallel processing, and advanced analytics.

However, column tables are not optimized for write operations, such as insert, update, or delete. These operations are expensive and time-consuming, as they require reorganizing and recompressing the entire column. To avoid this overhead, SAP HANA uses a delta storage that is optimized for write operations. The delta storage is a row-based table that stores the data modifications temporarily until they are merged into the main storage.

The delta merge operation is responsible for transferring the data from the delta storage to the main storage. During this operation, SAP HANA performs the following tasks:

It merges the new and modified rows from the delta storage with the existing rows in the main storage.
It deletes the rows that are marked for deletion in the main storage.
It reorganizes and recompresses the columns in the main storage.
It persists the changes to disk.

The delta merge operation improves the read performance, memory consumption, and data consistency of SAP HANA column tables. However, it also consumes CPU and memory resources, and may cause locks or conflicts with concurrent transactions. Therefore, it is important to understand when and how to perform delta merge operations in SAP HANA.

How does delta merge work?

Delta merge can be performed automatically or manually in SAP HANA. The automatic delta merge is triggered by SAP HANA based on certain criteria, such as:

The size or number of records in the delta storage exceeds a threshold.
The ratio of deleted records in the main storage exceeds a threshold.
The table is accessed by a query that requires a consistent view of the data.
The table is involved in a replication or backup scenario.

The manual delta merge can be triggered by the user or by an application using SQL statements or SAP HANA cockpit apps. For example, you can use the following SQL statement to perform a manual delta merge on a table:

MERGE DELTA OF <table_name>;

1 2	MERGE DELTA OF ;

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You can also use parameters or options to customize the delta merge operation, such as:

SMART_MERGE: Enables or disables the smart merge feature that optimizes the delta merge based on cost functions configured in indexserver.ini file.
PART: Specifies the partition number of a partitioned table to perform a delta merge on a specific partition.
HISTORY: Performs a delta merge on the history-delta storage of a temporal table into its history-main storage.
FORCE REBUILD: Forces a delta merge even if the delta storage is empty and no deleted rows exist in the main storage. This option can be used to create a new main storage that matches the latest table definition.

For more information on the syntax and options of the MERGE DELTA statement, see MERGE DELTA Statement (Data Manipulation).

How does delta merge affect SAP BW/4HANA?

SAP BW/4HANA is an application that runs on top of SAP HANA and leverages its capabilities for data warehousing and analytics. SAP BW/4HANA uses column tables to store its objects, such as ADSOs, InfoCubes, or CompositeProviders. Therefore, SAP BW/4HANA applications are affected by delta merge operations in SAP HANA.

Delta merge can have positive or negative impacts on SAP BW/4HANA applications depending on when and how it is performed. Some of these impacts are:

Delta merge can improve the query performance of SAP BW/4HANA objects by reducing the scan time and memory consumption of column tables.
Delta merge can reduce the data load time of SAP BW/4HANA objects by avoiding unnecessary merges during data loading processes.
Delta merge can ensure data consistency and quality of SAP BW/4HANA objects by applying data modifications and validations to column tables.
Delta merge can cause performance degradation or errors of SAP BW/4HANA applications if it is performed at inappropriate times or with inappropriate settings. For example, delta merge can cause locks or conflicts with concurrent data loading or query processes, or consume excessive CPU or memory resources.

Therefore, it is recommended to monitor and manage delta merge operations in SAP HANA carefully to optimize the performance and reliability of SAP BW/4HANA applications.

How to monitor and trigger delta merge?

You can monitor and trigger delta merge operations in SAP HANA using the SAP HANA cockpit or SQL statements. The SAP HANA cockpit provides graphical tools and apps to view and manage delta merge operations in SAP HANA. Some of these tools and apps are:

The Delta Merge Overview app: This app shows the status and details of delta merge operations for all column tables in the system. You can filter, sort, and group the tables by various criteria, such as schema, table name, partition number, merge duration, or merge type. You can also perform actions on the tables, such as triggering a manual delta merge, opening the table definition, or viewing the table details.
The Table Details app: This app shows the detailed information and statistics of a specific column table in the system. You can view the properties, partitions, indexes, columns, data distribution, and delta merge history of the table. You can also perform actions on the table, such as triggering a manual delta merge, opening the table definition, or viewing the table logs.
The Merge Monitor app: This app shows the current and historical information of delta merge operations in the system. You can view the status, duration, start time, end time, and error messages of delta merge operations. You can also filter and sort the operations by various criteria, such as schema, table name, partition number, or merge type.

You can also use SQL statements to monitor and trigger delta merge operations in SAP HANA. Some of these statements are:

The MERGE DELTA statement: This statement performs a manual delta merge on a column table or a partition of a column table. You can use parameters or options to customize the delta merge operation.
The M_DELTA_MERGE_STATISTICS view: This view shows the statistics of delta merge operations for all column tables in the system. You can query this view to get information such as schema name, table name, partition number, merge type, merge duration, or error code.
The M_CS_TABLES view: This view shows the information of all column tables in the system. You can query this view to get information such as schema name, table name, partition number, record count, memory size, delta size, or deleted records.

For more information on how to use these tools and statements to monitor and trigger delta merge operations in SAP HANA, see The Delta Merge Operation and MERGE DELTA Statement (Data Manipulation).

Conclusion

Delta merge is a key mechanism that enables SAP HANA to handle data modifications efficiently in column tables. Delta merge transfers data from a temporary delta storage to a permanent main storage in SAP HANA column tables. Delta merge improves the read performance, memory consumption, and data consistency of SAP HANA column tables. However, it also consumes CPU and memory resources and may cause locks or conflicts with concurrent transactions.

Delta merge affects SAP BW/4HANA applications that run on top of SAP HANA and use column tables to store their objects. Delta merge can have positive or negative impacts on SAP BW/4HANA applications depending on when and how it is performed. Therefore, it is important to monitor and manage delta merge operations in SAP HANA carefully to optimize the performance and reliability of SAP BW/4HANA applications.

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