Aggregations can be provided at ingestion time as part of the ingestion spec as a way of summarizing data before it enters Druid. Aggregations can also be specified as part of many queries at query time.

Available aggregations are:

`count`

computes the count of Druid rows that match the filters.

```
{ "type" : "count", "name" : <output_name> }
```

Please note the count aggregator counts the number of Druid rows, which does not always reflect the number of raw events ingested. This is because Druid can be configured to roll up data at ingestion time. To count the number of ingested rows of data, include a count aggregator at ingestion time, and a longSum aggregator at query time.

`longSum`

aggregatorcomputes the sum of values as a 64-bit, signed integer

```
{ "type" : "longSum", "name" : <output_name>, "fieldName" : <metric_name> }
```

`name`

– output name for the summed value
`fieldName`

– name of the metric column to sum over

`doubleSum`

aggregatorComputes and stores the sum of values as 64-bit floating point value. Similar to `longSum`

```
{ "type" : "doubleSum", "name" : <output_name>, "fieldName" : <metric_name> }
```

`floatSum`

aggregatorComputes and stores the sum of values as 32-bit floating point value. Similar to `longSum`

and `doubleSum`

```
{ "type" : "floatSum", "name" : <output_name>, "fieldName" : <metric_name> }
```

`doubleMin`

aggregator`doubleMin`

computes the minimum of all metric values and Double.POSITIVE_INFINITY

```
{ "type" : "doubleMin", "name" : <output_name>, "fieldName" : <metric_name> }
```

`doubleMax`

aggregator`doubleMax`

computes the maximum of all metric values and Double.NEGATIVE_INFINITY

```
{ "type" : "doubleMax", "name" : <output_name>, "fieldName" : <metric_name> }
```

`floatMin`

aggregator`floatMin`

computes the minimum of all metric values and Float.POSITIVE_INFINITY

```
{ "type" : "floatMin", "name" : <output_name>, "fieldName" : <metric_name> }
```

`floatMax`

aggregator`floatMax`

computes the maximum of all metric values and Float.NEGATIVE_INFINITY

```
{ "type" : "floatMax", "name" : <output_name>, "fieldName" : <metric_name> }
```

`longMin`

aggregator`longMin`

computes the minimum of all metric values and Long.MAX_VALUE

```
{ "type" : "longMin", "name" : <output_name>, "fieldName" : <metric_name> }
```

`longMax`

aggregator`longMax`

computes the maximum of all metric values and Long.MIN_VALUE

```
{ "type" : "longMax", "name" : <output_name>, "fieldName" : <metric_name> }
```

First and Last aggregator cannot be used in ingestion spec, and should only be specified as part of queries.

Note that queries with first/last aggregators on a segment created with rollup enabled will return the rolled up value, and not the last value within the raw ingested data.

`doubleFirst`

aggregator`doubleFirst`

computes the metric value with the minimum timestamp or 0 if no row exist

```
{
"type" : "doubleFirst",
"name" : <output_name>,
"fieldName" : <metric_name>
}
```

`doubleLast`

aggregator`doubleLast`

computes the metric value with the maximum timestamp or 0 if no row exist

```
{
"type" : "doubleLast",
"name" : <output_name>,
"fieldName" : <metric_name>
}
```

`floatFirst`

aggregator`floatFirst`

computes the metric value with the minimum timestamp or 0 if no row exist

```
{
"type" : "floatFirst",
"name" : <output_name>,
"fieldName" : <metric_name>
}
```

`floatLast`

aggregator`floatLast`

computes the metric value with the maximum timestamp or 0 if no row exist

```
{
"type" : "floatLast",
"name" : <output_name>,
"fieldName" : <metric_name>
}
```

`longFirst`

aggregator`longFirst`

computes the metric value with the minimum timestamp or 0 if no row exist

```
{
"type" : "longFirst",
"name" : <output_name>,
"fieldName" : <metric_name>
}
```

`longLast`

aggregator`longLast`

computes the metric value with the maximum timestamp or 0 if no row exist

```
{
"type" : "longLast",
"name" : <output_name>,
"fieldName" : <metric_name>,
}
```

Computes an arbitrary JavaScript function over a set of columns (both metrics and dimensions are allowed). Your JavaScript functions are expected to return floating-point values.

```
{ "type": "javascript",
"name": "<output_name>",
"fieldNames" : [ <column1>, <column2>, ... ],
"fnAggregate" : "function(current, column1, column2, ...) {
<updates partial aggregate (current) based on the current row values>
return <updated partial aggregate>
}",
"fnCombine" : "function(partialA, partialB) { return <combined partial results>; }",
"fnReset" : "function() { return <initial value>; }"
}
```

**Example**

```
{
"type": "javascript",
"name": "sum(log(x)*y) + 10",
"fieldNames": ["x", "y"],
"fnAggregate" : "function(current, a, b) { return current + (Math.log(a) * b); }",
"fnCombine" : "function(partialA, partialB) { return partialA + partialB; }",
"fnReset" : "function() { return 10; }"
}
```

JavaScript-based functionality is disabled by default. Please refer to the Druid JavaScript programming guide for guidelines about using Druid's JavaScript functionality, including instructions on how to enable it.

Computes the cardinality of a set of Druid dimensions, using HyperLogLog to estimate the cardinality. Please note that this aggregator will be much slower than indexing a column with the hyperUnique aggregator. This aggregator also runs over a dimension column, which means the string dimension cannot be removed from the dataset to improve rollup. In general, we strongly recommend using the hyperUnique aggregator instead of the cardinality aggregator if you do not care about the individual values of a dimension.

```
{
"type": "cardinality",
"name": "<output_name>",
"fields": [ <dimension1>, <dimension2>, ... ],
"byRow": <false | true> # (optional, defaults to false),
"round": <false | true> # (optional, defaults to false)
}
```

Each individual element of the "fields" list can be a String or DimensionSpec. A String dimension in the fields list is equivalent to a DefaultDimensionSpec (no transformations).

The HyperLogLog algorithm generates decimal estimates with some error. "round" can be set to true to round off estimated values to whole numbers. Note that even with rounding, the cardinality is still an estimate. The "round" field only affects query-time behavior, and is ignored at ingestion-time.

When setting `byRow`

to `false`

(the default) it computes the cardinality of the set composed of the union of all dimension values for all the given dimensions.

- For a single dimension, this is equivalent to

```
SELECT COUNT(DISTINCT(dimension)) FROM <datasource>
```

- For multiple dimensions, this is equivalent to something akin to

```
SELECT COUNT(DISTINCT(value)) FROM (
SELECT dim_1 as value FROM <datasource>
UNION
SELECT dim_2 as value FROM <datasource>
UNION
SELECT dim_3 as value FROM <datasource>
)
```

When setting `byRow`

to `true`

it computes the cardinality by row, i.e. the cardinality of distinct dimension combinations.
This is equivalent to something akin to

```
SELECT COUNT(*) FROM ( SELECT DIM1, DIM2, DIM3 FROM <datasource> GROUP BY DIM1, DIM2, DIM3 )
```

**Example**

Determine the number of distinct countries people are living in or have come from.

```
{
"type": "cardinality",
"name": "distinct_countries",
"fields": [ "country_of_origin", "country_of_residence" ]
}
```

Determine the number of distinct people (i.e. combinations of first and last name).

```
{
"type": "cardinality",
"name": "distinct_people",
"fields": [ "first_name", "last_name" ],
"byRow" : true
}
```

Determine the number of distinct starting characters of last names

```
{
"type": "cardinality",
"name": "distinct_last_name_first_char",
"fields": [
{
"type" : "extraction",
"dimension" : "last_name",
"outputName" : "last_name_first_char",
"extractionFn" : { "type" : "substring", "index" : 0, "length" : 1 }
}
],
"byRow" : true
}
```

Uses HyperLogLog to compute the estimated cardinality of a dimension that has been aggregated as a "hyperUnique" metric at indexing time.

```
{
"type" : "hyperUnique",
"name" : <output_name>,
"fieldName" : <metric_name>,
"isInputHyperUnique" : false,
"round" : false
}
```

"isInputHyperUnique" can be set to true to index pre-computed HLL (Base64 encoded output from druid-hll is expected). The "isInputHyperUnique" field only affects ingestion-time behavior, and is ignored at query-time.

The HyperLogLog algorithm generates decimal estimates with some error. "round" can be set to true to round off estimated values to whole numbers. Note that even with rounding, the cardinality is still an estimate. The "round" field only affects query-time behavior, and is ignored at ingestion-time.

For more approximate aggregators, please see theta sketches.

A filtered aggregator wraps any given aggregator, but only aggregates the values for which the given dimension filter matches.

This makes it possible to compute the results of a filtered and an unfiltered aggregation simultaneously, without having to issue multiple queries, and use both results as part of post-aggregations.

*Note:* If only the filtered results are required, consider putting the filter on the query itself, which will be much faster since it does not require scanning all the data.

```
{
"type" : "filtered",
"filter" : {
"type" : "selector",
"dimension" : <dimension>,
"value" : <dimension value>
}
"aggregator" : <aggregation>
}
```