Cassandra Wide Row/Dynamic Columns

Question

I'm new to NoSQL; so, I'm trying to understand some of the Cassandra concepts that I can't really get from the dozens of sources that I have studied.

1. Should I consider wide row, and dynamic columns as synonyms; or are they 2 different concepts?
2. Am I correct in thinking of columns of collection types as wide row?
3. It seems to me that wide row is a concept from earlier versions of Cassandra, and can be created only through Thrift API; whereas collection types are the modern versions of wide rows.
4. Are collection types still limited to 64k elements? Or that after CQL 3, that limitation has been removed?

Stocky

Answer 1

A common misunderstanding is that CQL does not support dynamic columns or wide rows. On the contrary, CQL was designed to support everything you can do with the Thrift model, but make it easier and more accessible.

Let's take a look at the below cql table.

CREATE TABLE data (
  sensor_id int,
  collected_at timestamp,
  volts float,
  PRIMARY KEY (sensor_id, collected_at)
);

And insert some data

sensor_id | collected_at             | volts
----------+--------------------------+-------
   1      | 2013-06-05 15:11:00-0500 |   3.1
   1      | 2013-06-05 15:11:10-0500 |   4.3
   1      | 2013-06-05 15:11:20-0500 |   5.7
   2      | 2013-06-05 15:11:00-0500 |   3.2
   3      | 2013-06-05 15:11:00-0500 |   3.3
   3      | 2013-06-05 15:11:10-0500 |   4.3

Here clustering column collected_at is similar to Thrift dynamic column.(Q.1)

If we look at the internal structure of this table

RowKey: 1
=> (cell=2013-06-05 15:11:00-0500, value=3.1, timestamp=1370463146717000)
=> (cell=2013-06-05 15:11:10-0500, value=4.3, timestamp=1370463282090000)
=> (cell=2013-06-05 15:11:20-0500, value=5.7, timestamp=1370463282093000)
-------------------
RowKey: 2
=> (cell=2013-06-05 15:11:00-0500, value=3.2, timestamp=1370463332361000)
-------------------
RowKey: 3
=> (cell=2013-06-05 15:11:00-0500, value=3.3, timestamp=1370463332365000)
=> (cell=2013-06-05 15:11:10-0500, value=4.3, timestamp=1370463332368000)

You can see that the clustering column collected_at makes this table table wide row (Q.1).

So we can say that if a table have one or more clustering key, we can called that table wide row.

Let's take another example :

CREATE TABLE example (
    key1 text PRIMARY KEY,
    map1 map<text,text>,
    list1 list<text>,
    set1 set<text>
);

Insert a data :

 key1 | list1             | map1                                         | set1
------+-------------------+----------------------------------------------+-----------------------
 john | ['doug', 'scott'] | {'doug': '555-1579', 'patricia': '555-4326'} | {'patricia', 'scott'}

Now take a look at the internal structure :

RowKey: john
=> (column=, value=, timestamp=1374683971220000)
=> (column=map1:doug, value='555-1579', timestamp=1374683971220000)
=> (column=map1:patricia, value='555-4326', timestamp=1374683971220000)
=> (column=list1:26017c10f48711e2801fdf9895e5d0f8, value='doug', timestamp=1374683971220000)
=> (column=list1:26017c12f48711e2801fdf9895e5d0f8, value='scott', timestamp=1374683971220000)
=> (column=set1:'patricia', value=, timestamp=1374683971220000)
=> (column=set1:'scott', value=, timestamp=1374683971220000)

You can see that map key and set value stored as dynamic column and map value and list value stored as the value of that column. It's similar to wide row (Q.2)

And the last one : Collection type map key and set size is limited to 64k.

• Collection (List): collection limit: ~2 billion (2^31); values size: 65535 (216-1)
• Collection (Set): collection limit: ~2 billion (2^31); values size: 65535 (216-1)
• Collection (Map): collection limit: ~2 billion (2^31); number of keys: 65535 (216-1); values size: 65535 (216-1)

Source :
https://www.datastax.com/blog/2013/06/does-cql-support-dynamic-columns-wide-rows https://teddyma.gitbooks.io/learncassandra/content/model/cql_and_data_structure.html http://docs.datastax.com/en/cql/3.3/cql/cql_reference/refLimits.html

Hypertonic