IoTDB-Quality

# Sample

# Usage

This function is used to sample the input series, that is, select a specified number of data points from the input series and output them. Currently, two sampling methods are supported: Reservoir sampling randomly selects data points. All of the points have the same probability of being sampled. Isometric sampling selects data points at equal index intervals.

Name: SAMPLE

Input Series: Only support a single input series. The type is arbitrary.

Parameters:

  • method: The method of sampling, which is 'reservoir' or 'isometric'. By default, reservoir sampling is used.
  • k: The number of sampling, which is a positive integer. By default, it's 1.

Output Series: Output a single series. The type is the same as the input. The length of the output series is k. Each data point in the output series comes from the input series.

Note: If k is greater than the length of input series, all data points in the input series will be output.

# Examples

# Reservoir Sampling

When method is 'reservoir' or the default, reservoir sampling is used. Due to the randomness of this method, the output series shown below is only a possible result.

Input series:

+-----------------------------+---------------+
|                         Time|root.test.d1.s1|
+-----------------------------+---------------+
|2020-01-01T00:00:01.000+08:00|            1.0|
|2020-01-01T00:00:02.000+08:00|            2.0|
|2020-01-01T00:00:03.000+08:00|            3.0|
|2020-01-01T00:00:04.000+08:00|            4.0|
|2020-01-01T00:00:05.000+08:00|            5.0|
|2020-01-01T00:00:06.000+08:00|            6.0|
|2020-01-01T00:00:07.000+08:00|            7.0|
|2020-01-01T00:00:08.000+08:00|            8.0|
|2020-01-01T00:00:09.000+08:00|            9.0|
|2020-01-01T00:00:10.000+08:00|           10.0|
+-----------------------------+---------------+

SQL for query:

select sample(s1,'method'='reservoir','k'='5') from root.test.d1

Output series:

+-----------------------------+------------------------------------------------------+
|                         Time|sample(root.test.d1.s1, "method"="reservoir", "k"="5")|
+-----------------------------+------------------------------------------------------+
|2020-01-01T00:00:02.000+08:00|                                                   2.0|
|2020-01-01T00:00:03.000+08:00|                                                   3.0|
|2020-01-01T00:00:05.000+08:00|                                                   5.0|
|2020-01-01T00:00:08.000+08:00|                                                   8.0|
|2020-01-01T00:00:10.000+08:00|                                                  10.0|
+-----------------------------+------------------------------------------------------+

# Isometric Sampling

When method is 'isometric', isometric sampling is used.

Input series is the same as above, the SQL for query is shown below:

select sample(s1,'method'='isometric','k'='5') from root.test.d1

Output series:

+-----------------------------+------------------------------------------------------+
|                         Time|sample(root.test.d1.s1, "method"="isometric", "k"="5")|
+-----------------------------+------------------------------------------------------+
|2020-01-01T00:00:01.000+08:00|                                                   1.0|
|2020-01-01T00:00:03.000+08:00|                                                   3.0|
|2020-01-01T00:00:05.000+08:00|                                                   5.0|
|2020-01-01T00:00:07.000+08:00|                                                   7.0|
|2020-01-01T00:00:09.000+08:00|                                                   9.0|
+-----------------------------+------------------------------------------------------+

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