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# Aggregation query

The main logic of the aggregation query is in AggregateExecutor

* org.apache.iotdb.db.query.executor.AggregationExecutor

## Aggregation query without value filter

For aggregate queries without value filters, the results are obtained by the `executeWithoutValueFilter()` method and a dataSet is constructed. First use the `mergeSameSeries()` method to merge aggregate queries for the same time series. For example: if you need to calculate count(s1), sum(s2), count(s3), sum(s1), you need to calculate two aggregation values of s1, then the pathToAggrIndexesMap result will be: s1-> 0, 3; s2-> 1; s3-> 2.

Then you will get `pathToAggrIndexesMap`, where each entry is an aggregate query of series, so you can calculate its aggregate value `aggregateResults` by calling the `groupAggregationsBySeries()` method.  Before you finally create the result set, you need to restore its order to the order of the user query.  Finally use the `constructDataSet()` method to create a result set and return it.

The `groupAggregationsBySeries ()` method is explained in detail below.  First create an `IAggregateReader`:
```
IAggregateReader seriesReader = new SeriesAggregateReader(
        pathToAggrIndexes.getKey(), tsDataType, context, QueryResourceManager.getInstance()
        .getQueryDataSource(seriesPath, context, timeFilter), timeFilter, null);
```

For each entry (that is, series), first create an aggregate result `AggregateResult` for each aggregate query. Maintain a boolean list `isCalculatedList`, corresponding to whether each `AggregateResult` has been calculated. Record the remaining number of functions to be calculated in `remainingToCalculate`.  The list of boolean values and this count value will make some aggregate functions (such as `FIRST_VALUE`) not need to continue the entire loop process after obtaining the result.

Next, update `AggregateResult` according to the usage method of `aggregateReader` introduced in Section 5.2:

```
while (aggregateReader.hasNextChunk()) {
  if (aggregateReader.canUseCurrentChunkStatistics()) {
    Statistics chunkStatistics = aggregateReader.currentChunkStatistics();
    
    // do some aggregate calculation using chunk statistics
    ...
    
    aggregateReader.skipCurrentChunk();
    continue;
  }
	  
  while (aggregateReader.hasNextPage()) {
	 if (aggregateReader.canUseCurrentPageStatistics()) {
	   Statistics pageStatistic = aggregateReader.currentPageStatistics();
	   
	   // do some aggregate calculation using page statistics
      ...
	   
	   aggregateReader.skipCurrentPage();
	   continue;
	 } else {
	 	BatchData batchData = aggregateReader.nextPage();
	 	// do some aggregate calculation using batch data
      ...
	 }	 
  }
}
```

It should be noted that before updating each result, you need to first determine whether it has been calculated (using the isCalculatedList list); after each update, call the isCalculatedAggregationResult () method to update the boolean values in the list  .  If all values in the list are true, that is, the value of `remainingToCalculate` is 0, it proves that all aggregate function results have been calculated and can be returned.
```
if (Boolean.FALSE.equals(isCalculatedList.get(i))) {
  AggregateResult aggregateResult = aggregateResultList.get(i);
  ... // update
  if (aggregateResult.isCalculatedAggregationResult()) {
    isCalculatedList.set(i, true);
    remainingToCalculate--;
    if (remainingToCalculate == 0) {
      return aggregateResultList;
    }
  }
}
```

When using `overlapedPageData` to update, since each batch function result will traverse this batchData, you need to call the` resetBatchData () `method to point the pointer to its starting position, so that the next function can traverse.

## Aggregated query with value filter
For an aggregate query with a value filter, obtain the results through the `executeWithoutValueFilter()` method and build a dataSet.  First create a `timestampGenerator` based on the expression, then create a `SeriesReaderByTimestamp` for each time series and place it in the `readersOfSelectedSeries` list; create an aggregate result for each query as `AggregateResult`, and place it in the `aggregateResults` list.

After initialization is complete, call the `aggregateWithValueFilter()` method to update the result:
```
while (timestampGenerator.hasNext()) {
  // Generate timestamps
  long[] timeArray = new long[aggregateFetchSize];
  int timeArrayLength = 0;
  for (int cnt = 0; cnt < aggregateFetchSize; cnt++) {
    if (!timestampGenerator.hasNext()) {
      break;
    }
    timeArray[timeArrayLength++] = timestampGenerator.next();
  }

  // Calculate aggregate results using timestamps
  for (int i = 0; i < readersOfSelectedSeries.size(); i++) {
    aggregateResults.get(i).updateResultUsingTimestamps(timeArray, timeArrayLength,
      readersOfSelectedSeries.get(i));
    }
  }
```
