ICU collation keyword field

Collations are used for sorting documents in a language-specific word order. The icu_collation_keyword field type is available to all indices and will encode the terms directly as bytes in a doc values field and a single indexed token just like a standard Keyword Field.

Defaults to using DUCET collation, which is a best-effort attempt at language-neutral sorting.

Below is an example of how to set up a field for sorting German names in “phonebook” order:

PUT my-index-000001
{
  "mappings": {
    "properties": {
      "name": {   
        "type": "text",
        "fields": {
          "sort": {  
            "type": "icu_collation_keyword",
            "index": false,
            "language": "de",
            "country": "DE",
            "variant": "@collation=phonebook"
          }
        }
      }
    }
  }
}

GET /my-index-000001/_search 
{
  "query": {
    "match": {
      "name": "Fritz"
    }
  },
  "sort": "name.sort"
}

	The `name` field uses the `standard` analyzer, and so supports full text queries.
	The `name.sort` field is an `icu_collation_keyword` field that will preserve the name as a single token doc values, and applies the German “phonebook” order.
	An example query which searches the `name` field and sorts on the `name.sort` field.

Parameters for ICU collation keyword fields

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The following parameters are accepted by icu_collation_keyword fields:

`doc_values`	Should the field be stored on disk in a column-stride fashion, so that it can later be used for sorting, aggregations, or scripting? Accepts `true` (default) or `false`.
`index`	Should the field be searchable? Accepts `true` (default) or `false`.
`null_value`	Accepts a string value which is substituted for any explicit `null` values. Defaults to `null`, which means the field is treated as missing.
`ignore_above`	Strings longer than the `ignore_above` setting will be ignored. Checking is performed on the original string before the collation. The `ignore_above` setting can be updated on existing fields using the PUT mapping API. By default, there is no limit and all values will be indexed.
`store`	Whether the field value should be stored and retrievable separately from the `_source` field. Accepts `true` or `false` (default).
`fields`	Multi-fields allow the same string value to be indexed in multiple ways for different purposes, such as one field for search and a multi-field for sorting and aggregations.

Collation options

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strength: The strength property determines the minimum level of difference considered significant during comparison. Possible values are : primary, secondary, tertiary, quaternary or identical. See the ICU Collation documentation for a more detailed explanation for each value. Defaults to tertiary unless otherwise specified in the collation.
decomposition: Possible values: no (default, but collation-dependent) or canonical. Setting this decomposition property to canonical allows the Collator to handle unnormalized text properly, producing the same results as if the text were normalized. If no is set, it is the user’s responsibility to ensure that all text is already in the appropriate form before a comparison or before getting a CollationKey. Adjusting decomposition mode allows the user to select between faster and more complete collation behavior. Since a great many of the world’s languages do not require text normalization, most locales set no as the default decomposition mode.

The following options are expert only:

alternate: Possible values: shifted or non-ignorable. Sets the alternate handling for strength quaternary to be either shifted or non-ignorable. Which boils down to ignoring punctuation and whitespace.
case_level: Possible values: true or false (default). Whether case level sorting is required. When strength is set to primary this will ignore accent differences.
case_first: Possible values: lower or upper. Useful to control which case is sorted first when the case is not ignored for strength tertiary. The default depends on the collation.
numeric: Possible values: true or false (default) . Whether digits are sorted according to their numeric representation. For example the value egg-9 is sorted before the value egg-21.
variable_top: Single character or contraction. Controls what is variable for alternate.
hiragana_quaternary_mode: Possible values: true or false. Distinguishing between Katakana and Hiragana characters in quaternary strength.

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