Is it possible to somehow overwrite existing counters-fields when COPY FROM data (from CSV),
or completely delete rows from the database?
When I COPY FROM data to existing rows, the counters are summarized.
I can’t completely DELETE these rows as well:
although it seems that the rows are deleted, when you re-COPY FROM the data from CSV,
the counters-fields continue to increase.
You can't set counters to the specific value - for them the only supported operation is either increase, or decrease. To set them to specific value you need either to decrease it to its current value, and then increase to desired value, but this will require that you read value. Or you need to delete corresponding cells (or whole row), and perform increase operation using desired number.
The second approach could be implemented easier, but will require that you first generate a file with CQL DELETE commands based on the content of your CSV file, and then use COPY FROM - if nobody increased values since deletion, then counters will get correct values.