Cassandra cql: how to select the LAST n rows from a table

Question

I want to verify that rows are getting added to the table. What cql statement would show the last n rows from the table below?

Table description be

Answer 1

You didn't specify last n "by what".

To get the last N per id:

SELECT * FROM option_data WHERE ts=1 ORDER BY id DESC LIMIT N;

ORDER BY clause can only be applied to the second column in a compound primary key. If you need to query by time you will need to think about your data model a little more.

If your queries are most often "last N", you might consider writing something like this:

CREATE TABLE time_series (
    id text,
    t timeuuid,
    data text,
    PRIMARY KEY (id, t)
) WITH CLUSTERING ORDER BY (t DESC)

... where 'id' is your time series id. The CLUSTERING ORDER reverses the order of timeuuid 't', causing the cells to be stored in a natural order for your query.

With this, you would get the last five events as follows:

SELECT * FROM time_series WHERE id='stream id' LIMIT 5;

There is a lot of information out there for time series in Cassandra. I suggest reading some of the more recent articles on the matter.