Types Of Delta Merge in SAP HANA Database | Techrelam

 

Types Of Delta Merge in SAP HANA Database.

There are Four types of delta Merge Operation in SAP HANA Database.

1. Auto Merge
2. Smart Merge
3. Hard and Forced Merges
4. Critical Merge 

Auto Merge:

 

The standard method for initiating a merge in SAP HANA is the auto merge. A system process called mergedog periodically checks the column store tables that are loaded locally. For each individual table or single partition of a split table, it determines whether or not a merge is necessary based on certain criteria (for example, size of delta storage, available memory, time since last merge, and others). If the active parameter in the mergedog section of the indexserver.ini file is set to yes, auto merge is active. 

Note: You can activate or deactivate auto merge for an individual table in the TABLES (SYS) system view. Change the value in the AUTO_MERGE_ON column to TRUE or FALSE. 

Smart Merge:

SAP HANA provides a function that allows the system to check whether or not a delta merge makes sense. This function is called smart merge. For example, if an application starts loading relatively large data volumes, a delta merge during the load can have a negative impact both on the load performance and on other system users. Therefore, the application can disable the auto merge for those tables being loaded and send a hint to the database to merge once the load has completed. When the application issues a smart merge hint to the database to trigger a merge, the database evaluates the criteria that determine whether or not a merge is necessary. If the criteria are met, the merge is executed. If the criteria are not met, the database takes no further action. Only a subsequent hint from the application triggers another evaluation of the criteria.

If the active parameter in the smart_merge_enabled section of the indexserver.ini file is set to yes, smart merge is active. Caution: For tables that you want to merge with the smart merge, disable the auto merge. Otherwise, the auto merge and smart merge may interfere with each other. 

Hard and Forced Merges:

     You can trigger the delta merge operation for a table manually by executing the SQL statement MERGE DELTA OF "" . This is called a hard merge. It causes the database to execute the delta merge for the table either immediately if sufficient system resources are available, or as soon as sufficient system resources become available. The hard merge is therefore subject to the merge token control mechanism. If you want the merge to take place immediately regardless of system resource availability, you can pass an optional parameter. A forced merge may be useful in a situation where there is a heavy system load, but a small table needs to be merged. It is also useful if a missed merge of a certain table negatively impacts system performance. To execute a forced merge, execute the SQL statement MERGE DELTA OF '' WITH PARAMETERS ('FORCED_MERGE' = 'ON') .

Critical Merge:

  To keep the system stable, the database can trigger a critical merge. For example, when auto merge is disabled and no smart merge hints are sent to the system, the size of the delta storage can grow too large for a successful delta merge to occur. The system initiates a critical merge automatically when a certain threshold is passed.