70 lines
2.2 KiB
Plaintext
70 lines
2.2 KiB
Plaintext
[[search-request-rescore]]
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=== Rescoring
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Rescoring can help to improve precision by reordering just the top (eg
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100 - 500) documents returned by the
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<<search-request-query,`query`>> and
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<<search-request-post-filter,`post_filter`>> phases, using a
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secondary (usually more costly) algorithm, instead of applying the
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costly algorithm to all documents in the index.
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A `rescore` request is executed on each shard before it returns its
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results to be sorted by the node handling the overall search request.
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Currently the rescore API has only one implementation: the query
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rescorer, which uses a query to tweak the scoring. In the future,
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alternative rescorers may be made available, for example, a pair-wise rescorer.
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*Note:* the `rescore` phase is not executed when
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<<search-request-search-type,`search_type`>> is set
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to `scan` or `count`.
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==== Query rescorer
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The query rescorer executes a second query only on the Top-K results
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returned by the <<search-request-query,`query`>> and
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<<search-request-post-filter,`post_filter`>> phases. The
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number of docs which will be examined on each shard can be controlled by
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the `window_size` parameter, which defaults to
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<<search-request-from-size,`from` and `size`>>.
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The scores from the original query and the rescore query are combined
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linearly to produce the final `_score` for each document. The relative
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importance of the original query and of the rescore query can be
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controlled with the `query_weight` and `rescore_query_weight`
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respectively. Both default to `1`.
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For example:
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[source,js]
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--------------------------------------------------
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curl -s -XPOST 'localhost:9200/_search' -d '{
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"query" : {
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"match" : {
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"field1" : {
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"operator" : "or",
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"query" : "the quick brown",
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"type" : "boolean"
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}
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}
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},
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"rescore" : {
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"window_size" : 50,
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"query" : {
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"rescore_query" : {
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"match" : {
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"field1" : {
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"query" : "the quick brown",
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"type" : "phrase",
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"slop" : 2
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}
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}
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},
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"query_weight" : 0.7,
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"rescore_query_weight" : 1.2
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}
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}
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}
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'
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--------------------------------------------------
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