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fielddata

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`fielddata`

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Most fields are indexed by default, which makes them searchable. The inverted index allows queries to look up the search term in unique sorted list of terms, and from that immediately have access to the list of documents that contain the term.

Sorting, aggregations, and access to field values in scripts requires a different data access pattern. Instead of lookup up the term and finding documents, we need to be able to look up the document and find the terms that it has in a field.

Most fields can use index-time, on-disk doc_values to support this type of data access pattern, but analyzed string fields do not support doc_values.

Instead, analyzed strings use a query-time data structure called fielddata. This data structure is built on demand the first time that a field is used for aggregations, sorting, or is accessed in a script. It is built by reading the entire inverted index for each segment from disk, inverting the term ↔︎ document relationship, and storing the result in memory, in the JVM heap.

Loading fielddata is an expensive process so, once it has been loaded, it remains in memory for the lifetime of the segment.

Fielddata can fill up your heap space

`fielddata.format`

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For analyzed string fields, the fielddata format controls whether fielddata should be enabled or not. It accepts: disabled and paged_bytes (enabled, which is the default). To disable fielddata loading, you can use the following mapping:

PUT my_index
{
  "mappings": {
    "my_type": {
      "properties": {
        "text": {
          "type": "string",
          "fielddata": {
            "format": "disabled" 
          }
        }
      }
    }
  }
}

The text field cannot be used for sorting, aggregations, or in scripts.

Fielddata and other datatypes

Historically, other field datatypes also used fielddata, but this has been replaced by index-time, disk-based doc_values.

`fielddata.loading`

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This per-field setting controls when fielddata is loaded into memory. It accepts three options:

`lazy`	Fielddata is only loaded into memory when it is needed. (default)
`eager`	Fielddata is loaded into memory before a new search segment becomes visible to search. This can reduce the latency that a user may experience if their search request has to trigger lazy loading from a big segment.
`eager_global_ordinals`	Loading fielddata into memory is only part of the work that is required. After loading the fielddata for each segment, Elasticsearch builds the Global ordinals data structure to make a list of all unique terms across all the segments in a shard. By default, global ordinals are built lazily. If the field has a very high cardinality, global ordinals may take some time to build, in which case you can use eager loading instead.

Global ordinals

Global ordinals is a data-structure on top of fielddata and doc values, that maintains an incremental numbering for each unique term in a lexicographic order. Each term has a unique number and the number of term A is lower than the number of term B. Global ordinals are only supported on string fields.

Fielddata and doc values also have ordinals, which is a unique numbering for all terms in a particular segment and field. Global ordinals just build on top of this, by providing a mapping between the segment ordinals and the global ordinals, the latter being unique across the entire shard.

Global ordinals are used for features that use segment ordinals, such as sorting and the terms aggregation, to improve the execution time. A terms aggregation relies purely on global ordinals to perform the aggregation at the shard level, then converts global ordinals to the real term only for the final reduce phase, which combines results from different shards.

Global ordinals for a specified field are tied to all the segments of a shard, while fielddata and doc values ordinals are tied to a single segment. which is different than for field data for a specific field which is tied to a single segment. For this reason global ordinals need to be entirely rebuilt whenever a once new segment becomes visible.

The loading time of global ordinals depends on the number of terms in a field, but in general it is low, since it source field data has already been loaded. The memory overhead of global ordinals is a small because it is very efficiently compressed. Eager loading of global ordinals can move the loading time from the first search request, to the refresh itself.

`fielddata.filter`

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Fielddata filtering can be used to reduce the number of terms loaded into memory, and thus reduce memory usage. Terms can be filtered by frequency or by regular expression, or a combination of the two:

Filtering by frequency

The frequency filter allows you to only load terms whose document frequency falls between a min and max value, which can be expressed an absolute number (when the number is bigger than 1.0) or as a percentage (eg 0.01 is 1% and 1.0 is 100%). Frequency is calculated per segment. Percentages are based on the number of docs which have a value for the field, as opposed to all docs in the segment.

Small segments can be excluded completely by specifying the minimum number of docs that the segment should contain with min_segment_size:

PUT my_index
{
  "mappings": {
    "my_type": {
      "properties": {
        "tag": {
          "type": "string",
          "fielddata": {
            "filter": {
              "frequency": {
                "min": 0.001,
                "max": 0.1,
                "min_segment_size": 500
              }
            }
          }
        }
      }
    }
  }
}

Filtering by regex

Terms can also be filtered by regular expression - only values which match the regular expression are loaded. Note: the regular expression is applied to each term in the field, not to the whole field value. For instance, to only load hashtags from a tweet, we can use a regular expression which matches terms beginning with #:

PUT my_index
{
  "mappings": {
    "my_type": {
      "properties": {
        "tweet": {
          "type": "string",
          "analyzer": "whitespace",
          "fielddata": {
            "filter": {
              "regex": {
                "pattern": "^#.*"
              }
            }
          }
        }
      }
    }
  }
}

These filters can be updated on an existing field mapping and will take effect the next time the fielddata for a segment is loaded. Use the Clear Cache API to reload the fielddata using the new filters.

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