druid/docs/content/querying/dimensionspecs.md

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layout: doc_page
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# Transforming Dimension Values

The following JSON fields can be used in a query to operate on dimension values.

## DimensionSpec

`DimensionSpec`s define how dimension values get transformed prior to aggregation.

### Default DimensionSpec

Returns dimension values as is and optionally renames the dimension.

```json
{ "type" : "default", "dimension" : <dimension>, "outputName": <output_name> }
```

### Extraction DimensionSpec

Returns dimension values transformed using the given [extraction function](#extraction-functions).

```json
{
  "type" : "extraction",
  "dimension" : <dimension>,
  "outputName" :  <output_name>,
  "extractionFn" : <extraction_function>
}
```

## Extraction Functions

Extraction functions define the transformation applied to each dimension value.

Transformations can be applied to both regular (string) dimensions, as well
as the special `__time` dimension, which represents the current time bucket
according to the query [aggregation granularity](../querying/granularities.html).

**Note**: for functions taking string values (such as regular expressions),
`__time` dimension values will be formatted in [ISO-8601 format](https://en.wikipedia.org/wiki/ISO_8601)
before getting passed to the extraction function.

### Regular Expression Extraction Function

Returns the first matching group for the given regular expression.
If there is no match, it returns the dimension value as is.

```json
{
  "type" : "regex", "expr" : <regular_expression>,
  "replaceMissingValues" : true,
  "replaceMissingValuesWith" : "foobar"
}
```

For example, using `"expr" : "(\\w\\w\\w).*"` will transform
`'Monday'`, `'Tuesday'`, `'Wednesday'` into `'Mon'`, `'Tue'`, `'Wed'`.

If the `replaceMissingValues` property is true, the extraction function will transform dimension values that do not match the regex pattern to a user-specified String. Default value is `false`.

The `replaceMissingValuesWith` property sets the String that unmatched dimension values will be replaced with, if `replaceMissingValues` is true. If `replaceMissingValuesWith` is not specified, unmatched dimension values will be replaced with nulls.

For example, if `expr` is `"(a\w+)"` in the example JSON above, a regex that matches words starting with the letter `a`, the extraction function will convert a dimension value like `banana` to `foobar`.


### Partial Extraction Function

Returns the dimension value unchanged if the regular expression matches, otherwise returns null.

```json
{ "type" : "partial", "expr" : <regular_expression> }
```

### Search Query Extraction Function

Returns the dimension value unchanged if the given [`SearchQuerySpec`](../querying/searchqueryspec.html)
matches, otherwise returns null.

```json
{ "type" : "searchQuery", "query" : <search_query_spec> }
```

### Substring Extraction Function

Returns a substring of the dimension value starting from the supplied index and of the desired length. If the desired
length exceeds the length of the dimension value, the remainder of the string starting at index will be returned. 
If index is greater than the length of the dimension value, null will be returned.

```json
{ "type" : "substring", "index" : 1, "length" : 4 }
```

The length may be omitted for substring to return the remainder of the dimension value starting from index, 
or null if index greater than the length of the dimension value.

```json
{ "type" : "substring", "index" : 3 }
```


### Time Format Extraction Function

Returns the dimension value formatted according to the given format string, time zone, and locale.

For `__time` dimension values, this formats the time value bucketed by the
[aggregation granularity](../querying/granularities.html)

For a regular dimension, it assumes the string is formatted in
[ISO-8601 date and time format](https://en.wikipedia.org/wiki/ISO_8601).

* `format` : date time format for the resulting dimension value, in [Joda Time DateTimeFormat](http://www.joda.org/joda-time/apidocs/org/joda/time/format/DateTimeFormat.html).
* `locale` : locale (language and country) to use, given as a [IETF BCP 47 language tag](http://www.oracle.com/technetwork/java/javase/java8locales-2095355.html#util-text), e.g. `en-US`, `en-GB`, `fr-FR`, `fr-CA`, etc.
* `timeZone` : time zone to use in [IANA tz database format](http://en.wikipedia.org/wiki/List_of_tz_database_time_zones), e.g. `Europe/Berlin` (this can possibly be different than the aggregation time-zone)

```json
{ "type" : "timeFormat",
  "format" : <output_format>,
  "timeZone" : <time_zone> (optional),
  "locale" : <locale> (optional) }
```

For example, the following dimension spec returns the day of the week for Montréal in French:

```json
{
  "type" : "extraction",
  "dimension" : "__time",
  "outputName" :  "dayOfWeek",
  "extractionFn" : {
    "type" : "timeFormat",
    "format" : "EEEE",
    "timeZone" : "America/Montreal",
    "locale" : "fr"
  }
}
```

### Time Parsing Extraction Function

Parses dimension values as timestamps using the given input format,
and returns them formatted using the given output format.

Note, if you are working with the `__time` dimension, you should consider using the
[time extraction function instead](#time-format-extraction-function) instead,
which works on time value directly as opposed to string values.

Time formats are described in the
[SimpleDateFormat documentation](http://icu-project.org/apiref/icu4j/com/ibm/icu/text/SimpleDateFormat.html)

```json
{ "type" : "time",
  "timeFormat" : <input_format>,
  "resultFormat" : <output_format> }
```


### Javascript Extraction Function

Returns the dimension value, as transformed by the given JavaScript function.

For regular dimensions, the input value is passed as a string.

For the `__time` dimension, the input value is passed as a number
representing the number of milliseconds since January 1, 1970 UTC.

Example for a regular dimension

```json
{
  "type" : "javascript",
  "function" : "function(str) { return str.substr(0, 3); }"
}
```

```json
{
  "type" : "javascript",
  "function" : "function(str) { return str + '!!!'; }",
  "injective" : true
}
```

A property of `injective` specifies if the javascript function preserves uniqueness. The default value is `false` meaning uniqueness is not preserved

Example for the `__time` dimension:

```json
{
  "type" : "javascript",
  "function" : "function(t) { return 'Second ' + Math.floor((t % 60000) / 1000); }"
}
```

### Lookup extraction function
Explicit lookups allow you to specify a set of keys and values to use when performing the extraction

```json
{
  "type":"lookup",
  "lookup":{
    "type":"map",
    "map":{"foo":"bar", "baz":"bat"}
  },
  "retainMissingValue":true,
  "injective":true
}
```

```json
{
  "type":"lookup",
  "lookup":{
    "type":"map",
    "map":{"foo":"bar", "baz":"bat"}
  },
  "retainMissingValue":false,
  "injective":false,
  "replaceMissingValueWith":"MISSING"
}
```

```json
{
  "type":"lookup",
  "lookup":{"type":"namespace","namespace":"some_lookup"},
  "replaceMissingValueWith":"Unknown",
  "injective":false
}
```

```json
{
  "type":"lookup",
  "lookup":{"type":"namespace","namespace":"some_lookup"},
  "retainMissingValue":true,
  "injective":false
}
```

A lookup can be of type `namespace` or `map`. A `map` lookup is passed as part of the query. A `namespace` lookup is populated on all the nodes which handle queries as per [lookups](../querying/lookups.html)

A property of `retainMissingValue` and `replaceMissingValueWith` can be specified at query time to hint how to handle missing values. Setting `replaceMissingValueWith` to `""` has the same effect of setting it to `null` or omitting the property. Setting `retainMissingValue` to true will use the dimension's original value if it is not found in the lookup. The default values are `replaceMissingValueWith = null` and `retainMissingValue = false` which causes missing values to be treated as missing.
 
It is illegal to set `retainMissingValue = true` and also specify a `replaceMissingValueWith`

A property of `injective` specifies if optimizations can be used which assume there is no combining of multiple names into one. For example: If ABC123 is the only key that maps to SomeCompany, that can be optimized since it is a unique lookup. But if both ABC123 and DEF456 BOTH map to SomeCompany, then that is NOT a unique lookup. Setting this value to true and setting `retainMissingValue` to FALSE (the default) may cause undesired behavior.

A null dimension value can be mapped to a specific value by specifying the empty string as the key.
This allows distinguishing between a null dimension and a lookup resulting in a null.
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"]`.

### Filtering DimensionSpecs
These are only valid for multi-valued dimensions. If you have a row in druid that has a multi-valued dimension with values ["v1", "v2", "v3"] and you send a groupBy/topN query grouping by that dimension with [query filter](filter.html) for value "v1". In the response you will get 3 rows containing "v1", "v2" and "v3". This behavior might be unintuitive for some use cases.

It happens because `query filter` is internally used on the bitmaps and only used to match the row to be included in the query result processing. With multivalued dimensions, "query filter" behaves like a contains check, which will match the row with dimension value ["v1", "v2", "v3"]. Please see the section on "Multi-value columns" in [segment](../design/segments.html) for more details.
Then groupBy/topN processing pipeline "explodes" all multi-valued dimensions resulting 3 rows for "v1", "v2" and "v3" each.

In addition to "query filter" which efficiently selects the rows to be processed, you can use the filtering dimension spec to filter for specific values within the values of a multi-valued dimension. These dimensionSpecs take a delegate DimensionSpec and a filtering criteria. From the "exploded" rows, only rows matching the given filtering criteria are returned in the query result.

The following filtered dimension spec acts as a whiltelist or blacklist for values as per the "isWhitelist" attribute value.
```json
{ "type" : "listFiltered", "delegate" : <dimensionSpec>, "values": <array of strings>, "isWhitelist": <optional attribute for true/false, default is true> }
```

Following filtered dimension spec retains only the values matching regex. Note that `listFiltered` is faster than this and one should use that for whitelist or blacklist usecase.
```json
{ "type" : "regexFiltered", "delegate" : <dimensionSpec>, "pattern": <java regex pattern> }
```

For more details and examples, see [multi-valued dimensions](multi-valued-dimensions.html).