Before I`ve found good explanation of keys in Cassandra:
Difference between partition key, composite key and clustering key in Cassandra?.
Now I am reading about partitioner and there I can see term "row key". What is the row key? How can I list it with CQL?
The row key is just another name for the PRIMARY KEY. It is the combination of all the partition and clustering fields, and it will map to just one row of data in a table. So when you do a read or write to a particular row key, it will access just one row.
In terms of the partitioner, that only uses the partition key fields, and it generates a token hash value that determines which node in a cluster the partition will be stored on. Individual rows are stored within partitions, so if there are no clustering columns, then the partition will hold a single row and the row key would be the same as the partition key.
If you have clustering columns, then you can store multiple rows within a partition and the row key will be the partition key plus the clustering key.
Related
Good Evening,
my problem is, that my recent understanding for partition and primary key is, that the partition key is to distribute the data between the nodes, and the primary ALWAYS contains the partition key. I want to create a partition key to cluster the data with duplicate partition keys and in these clusters I want to have a primary key for unique rows. In my first understanding of Cassandra, it could be possible if can take apart the partition and primary key. Is this possible?
An example to ease my idea:
country
state
unique_id
USA
TEXAS
123
USA
TEXAS
114
country and state as the partition key and the unique id as the primary key.
If I create the primary key like this: PRIMARY KEY ((country, state,unique_id)) I can't filter without using the unique_id but I want e.g. a query like SELECT unique_id FROM table WHERE state = 'Texas' and country = 'USA'.
If I create the primary key in this way: PRIMARY KEY ((country, state)), it obviously overwrites the data every time one entry gets inserted with the same country and state that's why I need the unique primary key.
Primary key always includes the partition key, that's always a first item in the primary key. Partition key could consist out of multiple columns, that's why you have brackets around first item in your example. I believe that in your case, primary key should be as following:
PRIMARY KEY ((country, state),unique_id)
In this case, partition key is a combination of country + state, and then inside that partition you will have unique IDs that will be used to select specific items. General syntax for primary key is:
partition key, clustering column1, clustering column2, ...
where partition key could be either:
column - single column
(column1, column2, ...) - multiple columns
We want to create table in cassandra keyspace and decided to make only one column as primary key (As a result, that column is the partition key with no clustering key).
For example in 'sample' table, only 'id' column is primary key and so every partition has only one row.
create table sample(id int primary key, name text);
What are the Advantages and Disadvantages when take every row as one partition?
I'm trying to understand the scenario when no clustering key is specified in a table definition.
If a table has only a partition key and no clustering key, what order the rows under the same partition are stored in? Is it even allowed to have multiple rows under the same partition when no clustering key exists? I tried searching for it online but couldn't get a clear explanation.
I got the below explanation from Cassandra user group so posting it here in case someone else is looking for the same info:
"Note that a table always has a partition key, and that if the table has
no clustering columns, then every partition of that table is only
comprised of a single row (since the primary key uniquely identifies
rows and the primary key is equal to the partition key if there is no
clustering columns)."
http://cassandra.apache.org/doc/latest/cql/ddl.html#the-partition-key
I have a table that contains words with their frequencies in Cassandra table(word, frequenc0y, frequency1).
Can I make the primary key consisting of only word as Partition key? if not, can I use word also as Clustering key?
You have no constraints, except that you cannot reuse your columns, so you can put only a single column in your PRIMARY KEY definition, that will specifically be your PARTITION KEY.
BTW, if that's all you need, and depending on your UPDATE capabilities on the frequency0 and frequency1 columns, that could be a job for counter columns. Have a look at the official documentation about counters:
Creating a counter table
Using a counter
HTH.
I'm reading documentation on the Datastax site at http://www.datastax.com/documentation/cassandra/1.2/cassandra/cql_reference/create_table_r.html
and I see:
"When you use a composite partition key, Cassandra treats the columns in nested parentheses as partition keys and stores columns of a row on more than one node. "
The example given is:
CREATE TABLE Cats (
block_id uuid,
breed text,
color text,
short_hair boolean,
PRIMARY KEY ((block_id, breed), color, short_hair)
);
I understand how the cluster columns (in this case, color and short_hair) work in regard to how they are actually stored on disk as contiguous "columns" for the given row. What I don't understand is the line "...stores columns of a row on more than one node". Is this right?
For a given block_id and breed, doesn't this composite key just make a partition key similar to "block_id + breed", in which case the columns/clusters would be in the same row, whose physical location is determined by the partition key (block_id + breed) ?
Or is there some kind of splitting in this row going on because the primary key is based on two fields?
EDIT:
I think Richard's answer below is probably right, but I've also come across this in the Datastax documentation for 1.2 which enforces the first quote I posted:
"composite partition key - Stores columns of a row on more than one node using partition keys declared in nested parentheses of the PRIMARY KEY definition of a table."
Why would it say using plural partition key*s*... The fields that make up the composite key make up the only row key, as far as I know, and they are all used to make the key.
Then they say, the columns of a row can be split, which to me means a single row (with a given partition key) could have its columns split up on different nodes, which would mean the fields of the composite key are being handled separately.
Still a little confused on the Datastax documentation and whether it's actually right.
I think what it means is that rows with the same block_id are stored on different nodes. As you say, the partition key is like "block_id + breed", so columns with the same block_id but different breed will in general be stored on different nodes. But columns with the same block_id and breed will be stored on the same node.
Basically, the nodes that store a partition are found by a function of the partition key only. Whether it is composite or not, nothing else can join together or split rows.