How delta works in HANA

Hi All
Can anybody explain how the delta mechanism works in HANA,to be precise if there are any changes to the existing records how they are handled .How HANA classifies the current information and past information.
Thanks for your inputs.
Santosh

varada santosh wrote:
Hi Lars
>
> My question is not about loading the delta loading mechanism which is at the application level ,I am more concern about the database layer , once the data is loaded how the existing records will be modified if the same record was already there and some changes happened to the existing data, how it differentiates which one to consider
>
> Also whether it overwrites the existing data or it keeps both records with time factors to differntiate past and current information.
>
> Thanks
Hello Santosh,
in the column store all modifications of data are written to the delta log of the table first.
If rows from the main storage (called "main index") have been changed or deleted, then these main index entries are made invisible to other transactions (the multiversion concurrency does this visibility handling of data) and the data from the delta log is used thereof.
Once the delta and main indexes get merged, the old main index entry will be removed and only the current version of the row will be present.
This is out it works for common column tables.
If you use "history"-tables, then the old versions of the rows are never actually removed but made (in-)visible depending on the timestamp from when these rows have been inserted first.
This is as much as I can tell about this. For more information on this topic, please consider joining the HANA core development team
regards,
Lars

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