Question

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

问题：

如何获取前十行（r1到r10）的行键？
如何获取接下来十行（r11到r20）的行键？

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

看看：

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

你的KeyRange元组是（start_key，end_key）==（r1，r10）

Answer 2

根据我的测试，没有行的顺序（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式）。我的情况我不知道我的键范围是什么，所以我试图用Hector和Thrift检索所有键，并在以后对键进行排序。使用CQL 3.0.0对100000列200行的性能测试约为500毫秒，Hector约为100，节约约50毫秒。这里的我的行键是整数。赫克托尔代码如下：

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Thrift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

您应该首先修改cassandra1.1.o版本中的cassandra.yaml，您应该在其中进行如下设置：

partitioner: org.apache.cassandra.dht.ByteOrderedPartitioner

其次，您应该定义如下：

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，您可以将数据插入cassandra并实现范围查询。

获取Cassandra行键

3 个答案: