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最新点查询

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最新点查询

最新点查询是时序数据库 Apache IoTDB 中提供的一种特殊查询。它返回指定时间序列中时间戳最大的数据点,即一条序列的最新状态。

在物联网数据分析场景中,此功能尤为重要。为了满足了用户对设备实时监控的需求,Apache IoTDB 对最新点查询进行了缓存优化,能够提供毫秒级的返回速度。

SQL 语法:

select last <Path> [COMMA <Path>]* from < PrefixPath > [COMMA < PrefixPath >]* <whereClause> [ORDER BY TIMESERIES (DESC | ASC)?]

其含义是: 查询时间序列 prefixPath.path 中最近时间戳的数据。

  • whereClause 中当前只支持时间过滤条件,任何其他过滤条件都将会返回异常。当缓存的最新点不满足过滤条件时,IoTDB 需要从存储中获取结果,此时性能将会有所下降。

  • 结果集为四列的结构:

    +----+----------+-----+--------+
    |Time|timeseries|value|dataType|
    +----+----------+-----+--------+
    
  • 可以使用 ORDER BY TIMESERIES (DESC | ASC) 指定结果集按照序列名降序/升序排列。

示例 1: 查询 root.ln.wf01.wt01.status 的最新数据点

IoTDB> select last status from root.ln.wf01.wt01
+-----------------------------+------------------------+-----+--------+
|                         Time|              timeseries|value|dataType|
+-----------------------------+------------------------+-----+--------+
|2017-11-07T23:59:00.000+08:00|root.ln.wf01.wt01.status|false| BOOLEAN|
+-----------------------------+------------------------+-----+--------+
Total line number = 1
It costs 0.000s

示例 2: 查询 root.ln.wf01.wt01 下 status,temperature 时间戳大于等于 2017-11-07T23:50:00 的最新数据点。

IoTDB> select last status, temperature from root.ln.wf01.wt01 where time >= 2017-11-07T23:50:00
+-----------------------------+-----------------------------+---------+--------+
|                         Time|                   timeseries|    value|dataType|
+-----------------------------+-----------------------------+---------+--------+
|2017-11-07T23:59:00.000+08:00|     root.ln.wf01.wt01.status|    false| BOOLEAN|
|2017-11-07T23:59:00.000+08:00|root.ln.wf01.wt01.temperature|21.067368|  DOUBLE|
+-----------------------------+-----------------------------+---------+--------+
Total line number = 2
It costs 0.002s

示例 3: 查询 root.ln.wf01.wt01 下所有序列的最新数据点,并按照序列名降序排列。

IoTDB> select last * from root.ln.wf01.wt01 order by timeseries desc;
+-----------------------------+-----------------------------+---------+--------+
|                         Time|                   timeseries|    value|dataType|
+-----------------------------+-----------------------------+---------+--------+
|2017-11-07T23:59:00.000+08:00|root.ln.wf01.wt01.temperature|21.067368|  DOUBLE|
|2017-11-07T23:59:00.000+08:00|     root.ln.wf01.wt01.status|    false| BOOLEAN|
+-----------------------------+-----------------------------+---------+--------+
Total line number = 2
It costs 0.002s

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