Hi,
Couple of points:
- Projection after Union can help you with filtering the data by rows. Similar to where condition, but at the lower level. It depends on how you define the projection filter. If the projection filter is based on the calculated column, then the filtering will happen in Calc engine, which may have a negative impact on the performance. Please refer to the document here on Query pruning.
- Your understanding is correct. HANA performs the column stripping, which means the columns which are not selected for reporting are not queried. But the Projection can also help in optimizing the result set further by pushing the filters.
- Filter criteria push down depends upon how the filtering is implemented. As mentioned, if there is a calculated column on which the filtering is to be performed, then it is not pushed down. You can check this in the Query visual plan.
Hope this helps.
Regards,
Ravi