Transform data with mathematic operations

Flux supports mathematic expressions in data transformations. This article describes how to use Flux arithmetic operators to “map” over data and transform values using mathematic operations.

If you’re just getting started with Flux queries, check out the following:

• Get started with Flux for a conceptual overview of Flux and parts of a Flux query.
• Execute queries to discover a variety of ways to run your queries.

Basic mathematic operations

// Examples executed using the Flux REPL
> 9 + 9
18
> 22 - 14
8
> 6 * 5
30
> 21 / 7
3

See Flux read-eval-print-loop (REPL).

Error: type error: float != int

100 // Parsed as an integer
100.0 // Parsed as a float

// Example evaluations
> 20 / 8
2

> 20.0 / 8.0
2.5

Custom mathematic functions

Flux lets you create custom functions that use mathematic operations. View the examples below.

Custom multiplication function

multiply = (x, y) => x * y

multiply(x: 10, y: 12)
// Returns 120

Custom percentage function

percent = (sample, total) => (sample / total) * 100.0

percent(sample: 20.0, total: 80.0)
// Returns 25.0

Transform values in a data stream

To transform multiple values in an input stream, your function needs to:

• Handle piped-forward data.
• Each operand necessary for the calculation exists in each row (see Pivot vs join below).
• Use the map() function to iterate over each row.

The example multiplyByX() function below includes:

• A tables parameter that represents the input data stream (<-).
• An x parameter which is the number by which values in the _value column are multiplied.
• A map() function that iterates over each row in the input stream. It uses the with operator to preserve existing columns in each row. It also multiples the _value column by x.

multiplyByX = (x, tables=<-) =>
  tables
    |> map(fn: (r) => ({
        r with
        _value: r._value * x
      })
    )

data
  |> multiplyByX(x: 10)

Examples

Convert bytes to gigabytes

To convert active memory from bytes to gigabytes (GB), divide the active field in the mem measurement by 1,073,741,824.

The map() function iterates over each row in the piped-forward data and defines a new _value by dividing the original _value by 1073741824.

from(bucket: "db/rp")
  |> range(start: -10m)
  |> filter(fn: (r) =>
    r._measurement == "mem" and
    r._field == "active"
  )
  |> map(fn: (r) => ({
      r with
      _value: r._value / 1073741824
    })
  )

You could turn that same calculation into a function:

bytesToGB = (tables=<-) =>
  tables
    |> map(fn: (r) => ({
        r with
        _value: r._value / 1073741824
      })
    )

data
  |> bytesToGB()

Include partial gigabytes

Because the original metric (bytes) is an integer, the output of the operation is an integer and does not include partial GBs. To calculate partial GBs, convert the _value column and its values to floats using the float() function and format the denominator in the division operation as a float.

bytesToGB = (tables=<-) =>
  tables
    |> map(fn: (r) => ({
        r with
        _value: float(v: r._value) / 1073741824.0
      })
    )

Calculate a percentage

To calculate a percentage, use simple division, then multiply the result by 100.

> 1.0 / 4.0 * 100.0
25.0

For an in-depth look at calculating percentages, see Calculate percentages.

Pivot vs join

To query and use values in mathematical operations in Flux, operand values must exists in a single row. Both pivot() and join() will do this, but there are important differences between the two:

Pivot is more performant

pivot() reads and operates on a single stream of data. join() requires two streams of data and the overhead of reading and combining both streams can be significant, especially for larger data sets.

Use join for multiple data sources

Use join() when querying data from different buckets or data sources.

Pivot fields into columns for mathematic calculations

data
  |> pivot(rowKey:["_time"], columnKey: ["_field"], valueColumn: "_value")
  |> map(fn: (r) => ({ r with
    _value: (r.field1 + r.field2) / r.field3 * 100.0
  }))

Join multiple data sources for mathematic calculations

import "sql"
import "influxdata/influxdb/secrets"

pgUser = secrets.get(key: "POSTGRES_USER")
pgPass = secrets.get(key: "POSTGRES_PASSWORD")
pgHost = secrets.get(key: "POSTGRES_HOST")

t1 = sql.from(
  driverName: "postgres",
  dataSourceName: "postgresql://${pgUser}:${pgPass}@${pgHost}",
  query:"SELECT id, name, available FROM exampleTable"
)

t2 = from(bucket: "db/rp")
  |> range(start: -1h)
  |> filter(fn: (r) =>
    r._measurement == "example-measurement" and
    r._field == "example-field"
  )

join(tables: {t1: t1, t2: t2}, on: ["id"])
  |> map(fn: (r) => ({ r with _value: r._value_t2 / r.available_t1 * 100.0 }))