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Merge pull request #804 from influxdata/flux/onEmpty-warning 

Added warning about keeping empty tables on first filter call
pull/807/head
Scott Anderson 2020-03-09 12:00:44 -06:00 committed by GitHub
parent 6b767677bc 9a13ffe9c9
commit 62388fac76
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1 changed files with 19 additions and 3 deletions
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content/v2.0/reference/flux/stdlib/built-in/transformations/filter.md
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@ -49,13 +49,21 @@ Defines the behavior for empty tables.
Potential values are `keep` and `drop`. Potential values are `keep` and `drop`.
Defaults to `drop`. Defaults to `drop`.
_**Data type:** String_
##### drop ##### drop
Empty tables are dropped. Tables without rows are dropped.
##### keep ##### keep
Empty tables are output to the next transformation. Tables without rows are output to the next transformation.
_**Data type:** String_ {{% warn %}}
Keeping empty tables with your first `filter()` function can have severe performance
costs since it retains empty tables from your entire data set.
For higher performance, use your first `filter()` function to do basic filtering,
then keep empty tables on subsequent `filter()` calls with smaller data sets.
_[See the example below](#keep-empty-tables-when-filtering)._
{{% /warn %}}
## Examples ## Examples
@ -84,6 +92,14 @@ from(bucket:"example-bucket")
|> filter(fn: (r) => r._value > 50.0 and r._value < 65.0 ) |> filter(fn: (r) => r._value > 50.0 and r._value < 65.0 )
``` ```
##### Keep empty tables when filtering
```js
from(bucket: "example-bucket")
|> range(start: -1h)
|> filter(fn: (r) => r._measurement == "events" and r._field == "open")
|> filter(fn: (r) => r.doorId =~ /^2.*/, onEmpty: "keep")
```
<hr style="margin-top:4rem"/> <hr style="margin-top:4rem"/>
##### Related InfluxQL functions and statements: ##### Related InfluxQL functions and statements: