如何区分cassandra里primary key,partition key,cluster key,clustering key

此文来自 stackoverflow, 有时候自己也搞混, mark 一下：

There is a lot of confusion around this, I will try to make it as simple as possible.

The primary key is a general concept to indicate one or more columns used to retrieve data from a Table.

The primary key may be SIMPLE

 create table stackoverflow (
      key text PRIMARY KEY,
      data text      
  );

That means that it is made by a single column.

But the primary key can also be COMPOSITE (aka COMPOUND), generated from more columns.

 create table stackoverflow (
      key_part_one text,
      key_part_two int,
      data text,
      PRIMARY KEY(key_part_one, key_part_two)      
  );

In a situation of COMPOSITE primary key, the "first part" of the key is called PARTITION KEY (in this example key_part_one is the partition key) and the second part of the key is theCLUSTERING KEY (key_part_two)

Please note that the both partition and clustering key can be made by more columns

 create table stackoverflow (
      k_part_one text,
      k_part_two int,
      k_clust_one text,
      k_clust_two int,
      k_clust_three uuid,
      data text,
      PRIMARY KEY((k_part_one,k_part_two), k_clust_one, k_clust_two, k_clust_three)      
  );

Behind these names ...

• The Partition Key is responsible for data distribution accross your nodes.  //partition key 在cassandra内部先转换为primary key 的hash, 根据hash 找到对应Node的token , 这样就直接来到了需要的nodes 节点上
• The Clustering Key is responsible for data sorting within the partition.
• The Primary Key is equivalent to the Partition Key in a single-field-key table.
• The Composite/Compund Key is just a multiple-columns key

Further usage information: DATASTAX DOCUMENTATION

---

EDIT due to further requests
Small usage and content examples
SIMPLE  KEY:

insert into stackoverflow (key, data) VALUES ('han', 'solo');
select * from stackoverflow where key='han';

table content

key | data
----+------
han | solo

COMPOSITE/COMPOUND KEY can retrieve "wide rows"

insert into stackoverflow (key_part_one, key_part_two, data) VALUES ('ronaldo', 9, 'football player');
insert into stackoverflow (key_part_one, key_part_two, data) VALUES ('ronaldo', 10, 'ex-football player');
select * from stackoverflow where key_part_one = 'ronaldo';

table content

 key_part_one | key_part_two | data
--------------+--------------+--------------------
      ronaldo |            9 |    football player
      ronaldo |           10 | ex-football player

But you can query with all key ...

select * from stackoverflow where key_part_one = 'ronaldo' and key_part_two  = 10;

query output

 key_part_one | key_part_two | data
--------------+--------------+--------------------
      ronaldo |           10 | ex-football player

注意：上面select的例子是针对一下table： 

 create table stackoverflow (
      key_part_one text,
      key_part_two int,
      data text,
      PRIMARY KEY(key_part_one, key_part_two)      
  );

而不是： 

 create table stackoverflow (
      k_part_one text,
      k_part_two int,
      k_clust_one text,
      k_clust_two int,
      k_clust_three uuid,
      data text,
      PRIMARY KEY((k_part_one,k_part_two), k_clust_one, k_clust_two, k_clust_three)      
  );

Important note: the partition key is the minimum-specifier needed to perform a query using where clause. If you have a composite partition key, like the following： //特别注意： 当使用where时， primary key 是 cassandra 处理的最小单元。

eg: PRIMARY KEY((col1, col2), col10, col4))

You can perform query only passing at least both col1 and col2, these are the 2 columns that defines the partition key. The "general" rule to make query is you have to pass at least all partition key columns, then you can add each key in the order they're set.

so the valid queries are (excluding secondary indexes)

• col1 and col2
• col1 and col2 and col10
• col1 and col2 and col10 and col 4

invalid:

• col1 and col2 and col4
• anything that does not contain both col1 and col2