cassandra-loader | Delimited file loader for Cassandra | CSV Processing library

The primary key of your table is numligne, date.

Since the data in your csv file is not unique according to the same primary key, even if you do inserts, cassandra just updates those entries.

To give you an example if at line 43 you have the combination 33,2017-12-07,...this will be inserted. If at line 2000 you have the same combination, when this insert will be run, Cassandra will actually do an update, since that key is already in the database.

Both INSERT and UPDATE operations are upsert operation. Some further reading about INSERT and UPDATE commands.

In order to avoid this you could define another primary key so each line would have a unique key or you could write your own loader that would insert using IF NOT EXISTS so it inserts rows only if they don't exist (see the link for INSERT command, paragraph Inserting a row only if it does not already exist).

Cassandra provides its own COPY command, but

The process verifies the PRIMARY KEY and updates existing records.

After checking the code of the tool that you are using, I can see that the INSERT command being used there is not using IF NOT EXISTS so it will also update if the key already exists.