cassandra，简单查询

Posted 2023-03-22

【中文标题】cassandra，简单查询

【英文标题】：cassandra, simple query

【发布时间】：2011-07-19 15:25:37

【问题描述】：

我从 cassandra 开始，但我有些难以理解查询的工作原理。

我有这个

[default@Keyspace1] get Users[jsmith];                    
=> (column=adress, value=London, timestamp=1311087089506000)
=> (column=age, value=*, timestamp=1311086896514000)
=> (column=first, value=John, timestamp=1311086884652000)
=> (column=last, value=Smith, timestamp=1311086891619000)
Returned 4 results.

在 SQL 中我可以做到：SELECT last FROM Keyspace1 WHERE first="John"

但在 cassandra 语言中？

我正在尝试这样的事情，但我承认我对 sintax 完全感到困惑。

[default@Keyspace1] keyspace1.get("column_family","John","last")
... 

【问题讨论】：

基本上我想转换一个 sql 查询。等价物。

【参考方案1】：

[default@***] set Users['jsmith']['address']=London;
Value inserted.
[default@***] set Users['jsmith']['age']=42;
Value inserted.
[default@***] set Users['jsmith']['first']=john;
Value inserted.
[default@***] set Users['jsmith']['last']=smith;
Value inserted.
[default@***] get Users['jsmith']['last'];
=> (column=last, value=smith, timestamp=1311090805707000)

但是，您必须知道行键。做这种搜索会很困难； look at this thread, they discuss the same.

---

更新（感谢@jbellis的提示）

似乎在 Cassandra 0.7 和更新版本中添加了很多东西。我一直在研究 0.6。所以，是的，你可以做到。

在 Cassandra 0.7 中，您可以

[default@unknown] connect localhost/9160;
Connected to: "Test Cluster" on localhost/9160
[default@unknown] use ***;
[default@***] create column family IndexedUsers with 
comparator=UTF8Type and column_metadata=[
column_name:address, validation_class:UTF8Type, 
column_name:age, validation_class:UTF8Type, 
column_name:first, validation_class:UTF8Type, index_type:KEYS, 
column_name:last, validation_class:UTF8Type];
79d6ebce-b279-11e0-9ddf-e700f669bcfc
Waiting for schema agreement...
... schemas agree across the cluster
[default@***] set IndexedUsers['jsmith']['age']=42;
Value inserted.
[default@***] set IndexedUsers['jsmith']['address']='London';
Value inserted.
[default@***] set IndexedUsers['jsmith']['first']='John';
Value inserted.
[default@***] set IndexedUsers['jsmith']['last']='Smith';
Value inserted.
[default@***] get IndexedUsers where first='John';
-------------------
RowKey: jsmith
=> (column=address, value=London, timestamp=1311129646984000)
=> (column=age, value=42, timestamp=1311129627578000)
=> (column=first, value=John, timestamp=1311129676977000)
=> (column=last, value=Smith, timestamp=1311129698125000)

1 Row Returned.

但不是

[default@***] get IndexedUsers[last] where first='John';
Syntax error at position 23: missing EOF at 'where'

因为：

[default@***] help get;
get <cf>['<key>'];
get <cf>['<key>']['<col>'] (as <type>)*;
get <cf>['<key>']['<super>'];
get <cf>['<key>'][<function>];
get <cf>['<key>'][<function>(<super>)][<function>(<col>)];
get <cf> where <column> = <value> [and <column> > <value> and ...] [limit <integer>];
Default LIMIT is 100. Available operations: =, >, >=, <, <=

CQL 似乎是一个更好的选择。它是 Cassandra 0.8 版本的一部分。 Refer this。在那里你可以：

CREATE INDEX [index_name] ON <column_family> (column_name);

然后

SELECT [FIRST N] [REVERSED]  col_name1, col_name2 FROM <COLUMN FAMILY> 
[USING <CONSISTENCY>] [WHERE <CLAUSE>] [LIMIT N];

Refer complete CQL commands here.

【讨论】：

不是这样，从两个版本开始就可以在 Cassandra 中创建索引了。

【参考方案2】：

这篇文章解释了如何在 Cassandra 中创建索引并执行您想要的查询：http://www.datastax.com/dev/blog/whats-new-cassandra-07-secondary-indexes