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<img src="https://user-images.githubusercontent.com/16079446/78784824-9f41ae00-79d8-11ea-9920-0825e081cae0.png" style="width:100%;max-width:800px;max-height:600px;margin-left:auto;margin-right:auto;display:block;"></p> <p><code>select temperature FROM root.ln.wf01.wt01 where time &gt;= 1 and time &lt;= 38</code></p> <table><thead><tr><th>Time</th> <th>root.ln.wf01.wt01.temperature</th></tr></thead> <tbody><tr><td>1</td> <td>21</td></tr> <tr><td>3</td> <td>23</td></tr> <tr><td>5</td> <td>25</td></tr> <tr><td>20</td> <td>26</td></tr> <tr><td>27</td> <td>29</td></tr> <tr><td>28</td> <td>30</td></tr> <tr><td>30</td> <td>40</td></tr></tbody></table> <p>When we use Previous fill, even though the data between D and M is from the future, we should also use value of point D to fill them.</p> <p><code>SELECT last_value(temperature) as last_temperature FROM root.ln.wf01.wt01 GROUP BY([8, 39), 5m) FILL (int32[previous])</code></p> <table><thead><tr><th>Time</th> <th>last_temperature</th></tr></thead> <tbody><tr><td>8</td> <td>25</td></tr> <tr><td>13</td> <td>25</td></tr> <tr><td>18</td> <td>26</td></tr> <tr><td>23</td> <td>29</td></tr> <tr><td>28</td> <td>40</td></tr> <tr><td>33</td> <td>40</td></tr> <tr><td>38</td> <td>40</td></tr></tbody></table> <p>However, When we use NONLASTPREVIOUS fill, because data between D and M is from the future, we won't fill them and still return null.</p> <p><code>SELECT last_value(temperature) as last_temperature FROM root.ln.wf01.wt01 GROUP BY([8, 39), 5m) FILL (int32[PREVIOUSUNTILLAST])</code></p> <table><thead><tr><th>Time</th> <th>last_temperature</th></tr></thead> <tbody><tr><td>8</td> <td>25</td></tr> <tr><td>13</td> <td>25</td></tr> <tr><td>18</td> <td>26</td></tr> <tr><td>23</td> <td>29</td></tr> <tr><td>28</td> <td>40</td></tr> <tr><td>33</td> <td>null</td></tr> <tr><td>38</td> <td>null</td></tr></tbody></table> <h2 id="core-query-logic"><a href="#core-query-logic" class="header-anchor">#</a> Core query logic</h2> <p>We maintain two primary variable in <code>GroupByFillDataSet</code></p> <div class="language- line-numbers-mode"><pre class="language-text"><code>// the first value for each time series
private Object[] previousValue;
// last timestamp for each time series
private long[] lastTimeArray;
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><h3 id="previousvalue"><a href="#previousvalue" class="header-anchor">#</a> <code>previousValue</code></h3> <p><code>previousValue</code> maintain the previous value before current time interval for each time series and we initialize it by calling the <code>initPreviousParis</code> method in the constructor of <code>GroupByFillDataSet</code>.</p> <div class="language- line-numbers-mode"><pre class="language-text"><code>  private void initPreviousParis(QueryContext context, GroupByFillPlan groupByFillPlan)
          throws StorageEngineException, IOException, QueryProcessException {
    previousValue = new Object[paths.size()];
    for (int i = 0; i &lt; paths.size(); i++) {
      Path path = paths.get(i);
      TSDataType dataType = dataTypes.get(i);
      IFill fill = new PreviousFill(dataType, groupByEngineDataSet.getStartTime(), -1L);
      fill.constructReaders(path, groupByFillPlan.getAllMeasurementsInDevice(path.getDevice()), context);

      TimeValuePair timeValuePair = fill.getFillResult();
      if (timeValuePair == null || timeValuePair.getValue() == null) {
        previousValue[i] = null;
      } else {
        previousValue[i] = timeValuePair.getValue().getValue();
      }
    }
  }
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br></div></div><p><code>initPreviousParis</code> construct a Single point supplementary null query for each time series and the parameter <code>queryTime</code> is set to the start time of group by query and the parameter <code>beforeRange</code> is set to -1.</p> <h3 id="lasttimearray"><a href="#lasttimearray" class="header-anchor">#</a> <code>lastTimeArray</code></h3> <p><code>lastTimeArray</code> maintain the last timestamp for each time series and is used in PREVIOUSUNTILLAST fill way. initialize it by calling the <code>initLastTimeArray</code> method in the constructor of <code>GroupByFillDataSet</code>.</p> <div class="language- line-numbers-mode"><pre class="language-text"><code>  private void initLastTimeArray(QueryContext context)
      throws IOException, StorageEngineException, QueryProcessException {
    lastTimeArray = new long[paths.size()];
    Arrays.fill(lastTimeArray, Long.MAX_VALUE);
    for (int i = 0; i &lt; paths.size(); i++) {
      TimeValuePair lastTimeValuePair =
          LastQueryExecutor.calculateLastPairForOneSeries(paths.get(i), dataTypes.get(i), context);
      if (lastTimeValuePair.getValue() != null) {
        lastTimeArray[i] = lastTimeValuePair.getTimestamp();
      }
    }
  }
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br></div></div><p><code>initPreviousParis</code> construct a last query for each time series.</p> <h3 id="the-process-of-filling"><a href="#the-process-of-filling" class="header-anchor">#</a> The process of filling</h3> <p>The logic of filling is in the <code>nextWithoutConstraint</code> method:</p> <div class="language- line-numbers-mode"><pre class="language-text"><code>protected RowRecord nextWithoutConstraint() throws IOException {

    // get group by result without filling through groupByEngineDataSet
    RowRecord rowRecord = groupByEngineDataSet.nextWithoutConstraint();

    // judge whether each time series is needed to be filled
    for (int i = 0; i &lt; paths.size(); i++) {
      Field field = rowRecord.getFields().get(i);
      // current group by result is null
      if (field.getDataType() == null) {
        // the previous value is not null and (fill type is not previous until last or now time is before last time)
        if (previousValue[i] != null
            &amp;&amp; (!((PreviousFill) fillTypes.get(dataTypes.get(i))).isUntilLast()
            || rowRecord.getTimestamp() &lt;= lastTimeArray[i])) {
          rowRecord.getFields().set(i, Field.getField(previousValue[i], dataTypes.get(i)));
        }
      } else {
        // current group by result is not null，no need to fill
        // use now value update previous value
        previousValue[i] = field.getObjectValue(field.getDataType());
      }
    }
    return rowRecord;
  }
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