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Add docs for RegisteredDimensionExtractionFn

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Charles Allen 2016-03-28 13:27:49 -07:00
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docs/content/querying/dimensionspecs.md
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@ -308,6 +308,26 @@ This allows distinguishing between a null dimension and a lookup resulting in a
For example, specifying `{"":"bar","bat":"baz"}` with dimension values `[null, "foo", "bat"]` and replacing missing values with `"oof"` will yield results of `["bar", "oof", "baz"]`.
Omitting the empty string key will cause the missing value to take over. For example, specifying `{"bat":"baz"}` with dimension values `[null, "foo", "bat"]` and replacing missing values with `"oof"` will yield results of `["oof", "oof", "baz"]`.
### Registered Lookup Extraction Function
While it is recommended that the [lookup dimension spec](#lookup-dimensionspecs) be used whenever possible, any lookup that is registered for use as a lookup dimension spec can be used as a dimension extraction.
The specification for dimension extraction using dimension specification named lookups is formatted as per the following example:
```json
{
"type":"registeredLookup",
"lookup":"some_lookup_name",
"retainMissingValue":true,
"injective":false
}
```
All the flags for [lookup extraction function](#lookup-extraction-function) apply here as well.
In general, the dimension specification should be used. This dimension **extraction** implementation is made available for testing, validation, and transitioning from dimension extraction to the dimension specification style lookups.
There is also a chance that a feature uses dimension extraction in such a way that it is not applied to dimension specification lookups. Such a scenario should be brought to the attention of the development mailing list.
### Cascade Extraction Function
Provides chained execution of extraction functions.