druid/docs/content/Post-aggregations.md

---
layout: doc_page
---
Post-aggregations are specifications of processing that should happen on aggregated values as they come out of Druid. If you include a post aggregation as part of a query, make sure to include all aggregators the post-aggregator requires.

There are several post-aggregators available.

### Arithmetic post-aggregator

The arithmetic post-aggregator applies the provided function to the given fields from left to right. The fields can be aggregators or other post aggregators.

Supported functions are `+`, `-`, `*`, and `/`

The grammar for an arithmetic post aggregation is:

```json
postAggregation : {
  "type"  : "arithmetic",
  "name"  : <output_name>,
  "fn"    : <arithmetic_function>,
  "fields": [<post_aggregator>, <post_aggregator>, ...]
}
```

### Field accessor post-aggregator

This returns the value produced by the specified [aggregator](Aggregations.html).

`fieldName` refers to the output name of the aggregator given in the [aggregations](Aggregations.html) portion of the query.

```json
{ "type" : "fieldAccess", "fieldName" : <aggregator_name> }
```

### Constant post-aggregator

The constant post-aggregator always returns the specified value.

```json
{ "type"  : "constant", "name"  : <output_name>, "value" : <numerical_value> }
```

### Example Usage

In this example, let’s calculate a simple percentage using post aggregators. Let’s imagine our data set has a metric called "total".

The format of the query JSON is as follows:

```json
{
  ...
  "aggregations" : [
    { "type" : "count", "name" : "rows" },
    { "type" : "doubleSum", "name" : "tot", "fieldName" : "total" }
  ],
  "postAggregations" : {
    "type"   : "arithmetic",
    "name"   : "average",
    "fn"     : "*",
    "fields" : [
       { "type"   : "arithmetic",
         "name"   : "div",
         "fn"     : "/",
         "fields" : [
           { "type" : "fieldAccess", "name" : "tot", "fieldName" : "tot" },
           { "type" : "fieldAccess", "name" : "rows", "fieldName" : "rows" }
         ]
       },
       { "type" : "constant", "name": "const", "value" : 100 }
    ]
  }
  ...
}

```