nosql - 获取 Cassandra 行键

Question

假设 Cassandra 数据存储有 20 行，行键名为"r1".. "r20"。

问题：

• 如何获取前十行（r1to r10）的行键？

• 如何获取接下来十行的行键（r11to r20）？

我正在寻找 Cassandra 的类比：

SELECT row_key FROM table LIMIT 0, 10;
SELECT row_key FROM table LIMIT 10, 10;

Answer 1

看一眼：

list<KeySlice> get_range_slices(keyspace, column_parent, predicate, range, consistency_level)

您的 KeyRange 元组在哪里 (start_key, end_key) == (r1, r10)

Answer 2

根据我的测试，行没有顺序（与列不同）。CQL 3.0.0 可以检索行键但不能区分（应该有一种我不知道的方法）。我的情况是我不知道我的键范围是什么，所以我尝试用 Hector 和 Thrift 检索所有键，然后对键进行排序。使用 CQL 3.0.0 对 100000 列 200 行进行的性能测试约为 500 毫秒，Hector 约为 100 毫秒，thrift 约为 50 毫秒。我的 Row 键是整数。赫克托代码如下：

public void qureyRowkeys(){
    myCluster = HFactory.getOrCreateCluster(CLUSTER_NAME, "127.0.0.1:9160");
    ConfigurableConsistencyLevel ccl = new ConfigurableConsistencyLevel();
    ccl.setDefaultReadConsistencyLevel(HConsistencyLevel.ONE);
    myKeyspace = HFactory.createKeyspace(KEYSPACE_NAME, myCluster, ccl);
    RangeSlicesQuery<Integer, Composite, String> rangeSlicesQuery = HFactory.createRangeSlicesQuery(myKeyspace, IntegerSerializer.get(), 
            CompositeSerializer.get(), StringSerializer.get());
    long start = System.currentTimeMillis();
    QueryResult<OrderedRows<Integer, Composite, String>> result =
      rangeSlicesQuery.setColumnFamily(CF).setKeys(0, -1).setReturnKeysOnly().execute();
    OrderedRows<Integer, Composite, String> orderedRows = result.get();
    ArrayList<Integer> list = new ArrayList<Integer>();
    for(Row<Integer, Composite, String> row: orderedRows){
        list.add(row.getKey());
    }

    System.out.println((System.currentTimeMillis()-start));
    Collections.sort(list);
    for(Integer i: list){
        System.out.println(i);
    }
}

这是节俭代码：

public void retreiveRows(){
    try {
        transport = new TFramedTransport(new TSocket("localhost", 9160));
        TProtocol protocol = new TBinaryProtocol(transport);
        client = new Cassandra.Client(protocol);
        transport.open();
        client.set_keyspace("prefdb");
        ColumnParent columnParent = new ColumnParent("events"); 
        SlicePredicate predicate = new SlicePredicate();
        predicate.setSlice_range(new SliceRange(ByteBuffer.wrap(new byte[0]), ByteBuffer.wrap(new byte[0]), false, 1));              
        KeyRange keyRange = new KeyRange();  //Get all keys
        keyRange.setStart_key(new byte[0]);
        keyRange.setEnd_key(new byte[0]);
        long start = System.currentTimeMillis();
        List<KeySlice> keySlices = client.get_range_slices(columnParent, predicate, keyRange, ConsistencyLevel.ONE);
        ArrayList<Integer> list = new ArrayList<Integer>();
        for (KeySlice ks : keySlices) {
                 list.add(ByteBuffer.wrap(ks.getKey()).getInt());
        }    
        Collections.sort(list);
        System.out.println((System.currentTimeMillis()-start));
        for(Integer i: list){
            System.out.println(i);
        }

        transport.close();
    } catch (Exception e) {
        e.printStackTrace();

    }
}

Answer 3

首先cassandra.yaml在cassandra1.1.o的版本中进行修改，这里设置如下：

partitioner: org.apache.cassandra.dht.ByteOrderedPartitioner

其次，你应该定义如下：

create keyspace DEMO with placement_strategy =
  'org.apache.cassandra.locator.SimpleStrategy' and
  strategy_options = [{replication_factor:1}];

use DEMO;

create column family Users with comparator = AsciiType and
  key_validation_class = LongType and
  column_metadata = [
    {
      column_name: aaa,
      validation_class: BytesType
    },{
      column_name: bbb,
      validation_class: BytesType
    },{
      column_name: ccc,
      validation_class: BytesType
    }
  ];

最后可以在cassandra中插入数据，实现范围查询。