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Docs: More search speed advices. (#24802)

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Adrien Grand 2017-06-01 17:23:22 +02:00 committed by GitHub
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16
docs/reference/how-to/search-speed.asciidoc
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@ -310,3 +310,19 @@ setting.
WARNING: Loading data into the filesystem cache eagerly on too many indices or WARNING: Loading data into the filesystem cache eagerly on too many indices or
too many files will make search _slower_ if the filesystem cache is not large too many files will make search _slower_ if the filesystem cache is not large
enough to hold all the data. Use with caution. enough to hold all the data. Use with caution.
[float]
=== Map identifiers as `keyword`
When you have numeric identifiers in your documents, it is tempting to map them
as numbers, which is consistent with their json type. However, the way that
Elasticsearch indexes numbers optimizes for `range` queries while `keyword`
fields are better at `term` queries. Since identifiers are never used in `range`
queries, they should be mapped as a `keyword`.
[float]
=== Use index sorting to speed up conjunctions
<<index-modules-index-sorting,Index sorting>> can be useful in order to make
conjunctions faster at the cost of slightly slower indexing. Read more about it
in the <<index-modules-index-sorting-conjunctions,index sorting documentation>>.

24
docs/reference/index-modules/index-sorting.asciidoc
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@ -105,3 +105,27 @@ Index sorting supports the following settings:
[WARNING] [WARNING]
Index sorting can be defined only once at index creation. It is not allowed to add or update Index sorting can be defined only once at index creation. It is not allowed to add or update
a sort on an existing index. a sort on an existing index.
// TODO: Also document how index sorting can be used to early-terminate
// sorted search requests when the total number of matches is not needed
[[index-modules-index-sorting-conjunctions]]
=== Use index sorting to speed up conjunctions
Index sorting can be useful in order to organize Lucene doc ids (not to be
conflated with `_id`) in a way that makes conjunctions (a AND b AND ...) more
efficient. In order to be efficient, conjunctions rely on the fact that if any
clause does not match, then the entire conjunction does not match. By using
index sorting, we can put documents that do not match together, which will
help skip efficiently over large ranges of doc IDs that do not match the
conjunction.
This trick only works with low-cardinality fields. A rule of thumb is that
you should sort first on fields that both have a low cardinality and are
frequently used for filtering. The sort order (`asc` or `desc`) does not
matter as we only care about putting values that would match the same clauses
close to each other.
For instance if you were indexing cars for sale, it might be interesting to
sort by fuel type, body type, make, year of registration and finally mileage.