Difference between UPDATE and INSERT in Cassandra?

Question

What is the difference between UPDATE and INSERT when executing CQL against Cassandra?

It looks like there used to be no difference, but now the documentation says that INSERT does not support counters while UPDATE does.

Is there a "preferred" method to use? Or are there cases where one should be used over the other?

Thanks so much!

Answer 1

Counter Columns in Cassandra couldn't be set to an arbitrary value: they can only be incremented or decremented by any arbitrary value.

For this reason, INSERT doesn't support Counter Column because you cannot "insert" a value into a Counter Column. You can only UPDATE them (increment or decrement) by some value. Here's how you would update a Counter column.

    UPDATE ... SET name1 = name1 + <value> 

You asked:

Is there a "preferred" method to use? Or are there cases where one should be used over the other?

Yes. If you are inserting values to the database, you can use INSERT. If the column doesn't exists, it will be created for you. Otherwise, INSERT's effect is similar to UPDATE. INSERT is useful when you don't have a pre-designed schema (Dynamic Column Family, i.e. insert anything, anytime). If you are designing the schema before hand (Static Column Family, similar to RDMS) and know each column, then you can use UPDATE.

Answer 2

There is a subtle difference. Inserted records via INSERT remain if you set all non-key fields to null. Records inserted via UPDATE go away if you set all non-key fields to null.

Try this:

CREATE TABLE T (
  pk int,
  f1 int,
  PRIMARY KEY (pk)
);

INSERT INTO T (pk, f1) VALUES (1, 1);
UPDATE T SET f1=2 where pk=2;
SELECT * FROM T;

Returns:

 pk | f1
----+----
  1 |  1
  2 |  2

Now, update each row setting f1 to null.

UPDATE T SET f1 = null WHERE pk = 1;
UPDATE T SET f1 = null WHERE pk = 2;
SELECT * FROM T;

Note that row 1 remains, while row 2 is removed.

 pk | f1
----+------
  1 | null

If you look at these using Cassandra-cli, you will see a different in how the rows are added.

I'd sure like to know whether this is by design or a bug and see this behavior documented.

Answer 3

Another subtle difference (i'm starting to believe cql is a terrible interface to cassandra, full of subtleties and caveats due to using similar SQL syntax but slightly different semantics) is with setting TTLs on existing data. With UPDATE you cannot update the TTL of the keys, even if the new actual values are equal to the old values. The solution is to INSERT the new row instead, with the new TTL already set

Answer 4

Regarding the subtle difference highlighted by billbaird (I'm unable to comment on that post directly) where a row created by an update operation will be deleted if all non-key fields are null:

That is expected behavior and not a bug based on the bug report at https://issues.apache.org/jira/browse/CASSANDRA-11805 (which was closed as "Not A Problem")

I ran into this myself when using Spring Data for the first time. I was using the save(T entity) method of a repository, but no row was being created. it turned out Spring Data was using an UPDATE because it determined that the object wasn't 'new' (not sure that test for 'isNew' makes sense here), and I happened to be testing with entities that only had the key fields set.

For this Spring Data case, the Cassandra-specific repository interfaces do provide an insert method that appear to consistently use an INSERT if that behavior is desired instead (though Spring's documentation doesn't document these details sufficiently either).