Table Size Mystery

[Former Member]

Hello,

We have a table with 2.6 B records which its size on disk is 433G We are trying to copy this table to another system and after only 93M records its size is already 105G. The only diferent is that we disabled the delta merge to the new table.

How can it be??

Thanks,

Amir

[lbreddemann]

Alright, this seems to be pretty straight forward.

SAP HANA handles new or changed data in a two step approach.

First it saves all data into a insert-only data structure called "Delta Storage" (delta store).

This Delta Storage can handle mass inserts or data changes very well, but at the expense of storage space, as no compression of the data is performed.

The actual data compression is then performed in the second step, when the data gets transferred into the "Main Storage". This in turn is a highly compressed but unchangeable data structure. Once built up, the Main Storage stays as it is.

The process that transfers the data from the Delta to the Main Storage is the "Delta Merge". In short it takes the existing Main Storage and the Delta Storage as input and builds a new combined Main Storage from those.

This new Main Storage is then highly compressed, too.

As you see, it boils down to "No delta merge -> no compression -> high memory usage".

All that is of course described at length in the SAP HANA documentation and in 's and my book ().

- Lars

[former_member182967]

Hello Amir,

Can you try to execute a delta merge on the table by right clicking on the table in HANA Studio and choosing the option 'Perform Delta Merge...'., and then check the usage?

Regards,

Ning Tong