The column store is the part of the SAP HANA database that manages data organized in columns in-memory. Tables created as column tables are stored here.
Delta Merge Operation
The purpose of the delta merge operation is to move changes collected in the delta storage to the read-optimized main storage. After the delta merge operation, the content of the main storage is persisted to disk. Its compression is recalculated and optimized, if necessary. The delta merge operation can be triggered by several events and is executed asynchronously.
A further result of the delta merge operation is the truncation of the delta log. The delta storage structure itself exists only in-memory and is not persisted to disk. The column store creates its logical redo log entries for all operations executed on the delta storage. This log is called the delta log. If a system restart occurs, the delta log entries are replayed to rebuild the in-memory delta storages. After the changes in the delta storage are merged into the main storage, the delta log file is truncated by removing those entries that were written before the merge operation.
Data that is in the delta storage can only be fully loaded or unloaded. Partial loading is not possible. Therefore, if a delta merge has not been performed and the table's entire data is in the delta storage, the table is either fully loaded or unloaded.