119 lines
5.0 KiB
Plaintext
119 lines
5.0 KiB
Plaintext
[[fielddata]]
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=== `fielddata`
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Most fields are <<mapping-index,indexed>> by default, which makes them
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searchable. The inverted index allows queries to look up the search term in
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unique sorted list of terms, and from that immediately have access to the list
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of documents that contain the term.
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Sorting, aggregations, and access to field values in scripts requires a
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different data access pattern. Instead of lookup up the term and finding
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documents, we need to be able to look up the document and find the terms that
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it has in a field.
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Most fields can use index-time, on-disk <<doc-values,`doc_values`>> to support
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this type of data access pattern, but `text` fields do not support `doc_values`.
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Instead, `text` strings use a query-time data structure called
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`fielddata`. This data structure is built on demand the first time that a
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field is used for aggregations, sorting, or is accessed in a script. It is built
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by reading the entire inverted index for each segment from disk, inverting the
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term ↔︎ document relationship, and storing the result in memory, in the
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JVM heap.
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Loading fielddata is an expensive process so it is disabled by default. Also,
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when enabled, once it has been loaded, it remains in memory for the lifetime of
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the segment.
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[WARNING]
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.Fielddata can fill up your heap space
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==============================================================================
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Fielddata can consume a lot of heap space, especially when loading high
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cardinality `text` fields. Most of the time, it doesn't make sense
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to sort or aggregate on `text` fields (with the notable exception
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of the
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<<search-aggregations-bucket-significantterms-aggregation,`significant_terms`>>
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aggregation). Always think about whether a <<keyword,`keyword`>> field (which can
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use `doc_values`) would be a better fit for your use case.
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==============================================================================
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TIP: The `fielddata.*` settings must have the same settings for fields of the
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same name in the same index. Its value can be updated on existing fields
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using the <<indices-put-mapping,PUT mapping API>>.
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[[global-ordinals]]
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.Global ordinals
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*****************************************
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Global ordinals is a data-structure on top of fielddata and doc values, that
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maintains an incremental numbering for each unique term in a lexicographic
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order. Each term has a unique number and the number of term 'A' is lower than
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the number of term 'B'. Global ordinals are only supported on string fields.
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Fielddata and doc values also have ordinals, which is a unique numbering for all terms
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in a particular segment and field. Global ordinals just build on top of this,
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by providing a mapping between the segment ordinals and the global ordinals,
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the latter being unique across the entire shard.
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Global ordinals are used for features that use segment ordinals, such as
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sorting and the terms aggregation, to improve the execution time. A terms
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aggregation relies purely on global ordinals to perform the aggregation at the
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shard level, then converts global ordinals to the real term only for the final
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reduce phase, which combines results from different shards.
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Global ordinals for a specified field are tied to _all the segments of a
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shard_, while fielddata and doc values ordinals are tied to a single segment.
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which is different than for field data for a specific field which is tied to a
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single segment. For this reason global ordinals need to be entirely rebuilt
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whenever a once new segment becomes visible.
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The loading time of global ordinals depends on the number of terms in a field, but in general
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it is low, since it source field data has already been loaded. The memory overhead of global
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ordinals is a small because it is very efficiently compressed. Eager loading of global ordinals
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can move the loading time from the first search request, to the refresh itself.
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*****************************************
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[[field-data-filtering]]
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==== `fielddata_frequency_filter`
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Fielddata filtering can be used to reduce the number of terms loaded into
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memory, and thus reduce memory usage. Terms can be filtered by _frequency_:
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The frequency filter allows you to only load terms whose term frequency falls
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between a `min` and `max` value, which can be expressed an absolute
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number (when the number is bigger than 1.0) or as a percentage
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(eg `0.01` is `1%` and `1.0` is `100%`). Frequency is calculated
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*per segment*. Percentages are based on the number of docs which have a
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value for the field, as opposed to all docs in the segment.
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Small segments can be excluded completely by specifying the minimum
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number of docs that the segment should contain with `min_segment_size`:
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[source,js]
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--------------------------------------------------
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PUT my_index
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{
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"mappings": {
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"my_type": {
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"properties": {
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"tag": {
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"type": "text",
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"fielddata": {
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"filter": {
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"frequency": {
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"min": 0.001,
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"max": 0.1,
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"min_segment_size": 500
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}
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}
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}
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}
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}
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}
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}
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}
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--------------------------------------------------
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// AUTOSENSE
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