Cassandra 2.1-2.0 CQL 3.1指南：数据建模与索引详解

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Cassandra CQL 3.1 是针对 Apache Cassandra 数据库 2.1 和早期版本（如 2.0.x）设计的查询语言文档。Cassandra 的一致性查询语言 (CQL) 是一种面向列的数据库查询语言，它允许开发者以结构化的方式与 Cassandra 进行交互，包括数据建模、查询、索引和数据操作。 **1. CQL 数据模型**： CQL 的核心在于其数据模型，它支持列族式存储，每个表由一组列族组成，每个列族有自己的键空间和列族名称。这使得数据可以高效地进行范围查询和并行处理。 **2. 数据建模示例**： CQL 提供了丰富的灵活性，例如在设计表时可以使用复合键（compound keys），包括简单键（单个字段）和复合键（多个字段组合），以适应复杂的数据关系。音乐服务是一个常用示例，通过复合键可以组织艺术家、专辑和歌曲等信息。 **3. 集合列（Collections）**： CQL 支持在表中使用集合（collections），如 list、set 和 map。这对于存储关联数据非常有用，如歌曲列表或艺术家的朋友列表。如何添加、更新集合以及使用索引进行过滤和检索都有详细的说明。 **4. 索引**： CQL 3.1 强调了索引的重要性，不仅用于加速查询性能，还能决定数据的查询策略。选择何时使用索引取决于数据访问模式，比如当需要频繁过滤或排序数据时，索引就显得尤为重要。 **5. 与遗留应用集成**：为了支持现有系统迁移或混合使用，文档还介绍了如何在 CQL 中处理和利用来自旧系统的数据，以及如何逐步将应用程序从旧接口迁移到 CQL。 **6. CQL 查询使用**：文档详细展示了如何在终端工具 `cqlsh` 中执行查询，包括创建和修改键空间、表格、插入数据、查询系统表获取元数据等基本操作。此外，还介绍了 tab 完成功能以提高交互效率。 Cassandra CQL 3.1 是一个全面的指南，涵盖了从基础数据模型到高级查询技巧，对于开发人员来说，是理解和使用 Cassandra 进行高效数据管理的重要资源。掌握这个版本的 CQL，可以有效地构建、维护和优化 Cassandra 集群中的数据结构和查询性能。

Using CQL

Using tab completion

You can use tab completion to see hints about how to complete a cqlsh command. Some platforms, such

as Mac OSX, do not ship with tab completion installed. You can use easy_install to install tab completion

capabilities on Mac OSX:

easy_install readline

Creating and updating a keyspace

Creating a keyspace is the CQL counterpart to creating an SQL database, but a little different. The

Cassandra keyspace is a namespace that defines how data is replicated on nodes. Typically, a cluster has

one keyspace per application. Replication is controlled on a per-keyspace basis, so data that has different

replication requirements typically resides in different keyspaces. Keyspaces are not designed to be used

as a significant map layer within the data model. Keyspaces are designed to control data replication for a

set of tables.

When you create a keyspace, you specify a strategy class for replicating keyspaces. Using the

SimpleStrategy class is fine for evaluating Cassandra. For production use or for use with mixed workloads,

use the NetworkTopologyStrategy class.

To use NetworkTopologyStrategy for evaluation purposes using, for example, a single node cluster, specify

the default data center name. To determine the default data center name, use the nodetool status

command. On Linux, for example, in the installation directory:

$ bin/nodetool status

The output is:

Datacenter: datacenter1

=======================

Status=Up/Down

|/ State=Normal/Leaving/Joining/Moving

-- Address Load Tokens Owns (effective) Host ID Rack

UN 127.0.0.1 41.62 KB 256 100.0% 75dcca8f... rack1

To use NetworkTopologyStrategy for production use, you need to change the default snitch, SimpleSnitch,

to a network-aware snitch, define one or more data center names in the snitch properties file, and use the

data center name(s) to define the keyspace; otherwise, Cassandra will fail to complete any write request,

such as inserting data into a table, and log this error message:

Unable to complete request: one or more nodes were unavailable.

You cannot insert data into a table in keyspace that uses NetworkTopologyStrategy unless you define the

data center names in the snitch properties file or you use a single data center named datacenter1.

Example of creating a keyspace

About this task

To query Cassandra, you first create and use a keyspace. You can chose an arbitrary data center name

and register the name in the properties file of the snitch. Alternatively, if you use a cluster in a single data

center, simply use the default data center name in OS Cassandra, for example datacenter1 and skip

registering the name in the properties file.

Procedure

1. Create a keyspace.

Using CQL

CREATE TYPE mykeyspace.fullname (

firstname text,

lastname text

);

4. Create a table for storing user data in columns of type fullname and address. Use the frozen keyword in

the definition of the user-defined type column.

CREATE TABLE mykeyspace.users (

id uuid PRIMARY KEY,

name frozen <fullname>,

direct_reports set<frozen <fullname>>, // a collection set

addresses map<text, frozen <address>> // a collection map

);

5. Insert a user's name into the fullname column.

INSERT INTO mykeyspace.users (id, name) VALUES

(62c36092-82a1-3a00-93d1-46196ee77204, {firstname: 'Marie-Claude',

lastname: 'Josset'});

6. Insert an address labeled home into the table.

UPDATE mykeyspace.users SET addresses = addresses + {'home': { street: '191

Rue St. Charles', city: 'Paris', zip_code: 75015, phones: {'33 6 78 90 12

34'}}} WHERE id=62c36092-82a1-3a00-93d1-46196ee77204;

7. Retrieve the full name of a user.

SELECT name FROM mykeyspace.users WHERE

id=62c36092-82a1-3a00-93d1-46196ee77204;

Using the column name of a user-defined type retrieves all fields: firstname and lastname.

name

-------------------------------------------------

{firstname: 'Marie-Claude', lastname: 'Josset'}

Using dot notation, you can retrieve a component of the user-defined type column, for example just the

last name.

SELECT name.lastname FROM mykeyspace.users WHERE

id=62c36092-82a1-3a00-93d1-46196ee77204;

name.lastname

---------------

Josset

8. Filter data on a column of a user-defined type. Create an index and then run a conditional query. In

Cassandra 2.1.x, you need to list all components of the name column in the WHERE clause.

CREATE INDEX on mykeyspace.users (name);

SELECT id FROM mykeyspace.users WHERE name = {firstname: 'Marie-Claude',

lastname: 'Josset'};

Output is:

--------------------------------------

62c36092-82a1-3a00-93d1-46196ee77204

9. Insert names of people who report to Marie-Claude. Use the UPDATE command. Insert the name of a

person who reports to another manager using the INSERT command.

When using the frozen keyword, you cannot update parts of a user-defined type value. The entire value

must be overwritten. Cassandra treats the value of a frozen, user-defined type like a blob.

UPDATE mykeyspace.users SET direct_reports = { ( 'Naoko', 'Murai'),

( 'Sompom', 'Peh') } WHERE id=62c36092-82a1-3a00-93d1-46196ee77204;

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