Elasticsearch－4種內置分析器

新手需要特別注意ES中內置的分析器，否則查詢結果很可能不是預期的，分析器它直接影響到Search與Index操作。

Built-in Analyzers

However, Elasticsearch also ships with prepackaged analyzers that you can use directly. We list the most important ones next and, to demonstrate the difference in behavior, we show what terms each would produce from this string:

"Set the shape to semi-transparent by calling set_trans(5)"

Standard analyzer

The standard analyzer is the default analyzer that Elasticsearch uses. It is the best general choice for analyzing text that may be in any language. It splits the text on word boundaries, as defined by theUnicode Consortium, and removes most punctuation. Finally, it lowercases all terms. It would produce

set, the, shape, to, semi, transparent, by, calling, set_trans, 5

Simple analyzer

The simple analyzer splits the text on anything that isn’t a letter, and lowercases the terms. It would produce

set, the, shape, to, semi, transparent, by, calling, set, trans

Whitespace analyzer

The whitespace analyzer splits the text on whitespace. It doesn’t lowercase. It would produce

Set, the, shape, to, semi-transparent, by, calling, set_trans(5)

Language analyzers

Language-specific analyzers are available for many languages. They are able to take the peculiarities of the specified language into account. For instance, the english analyzer comes with a set of Englishstopwords (common words like and or the that don’t have much impact on relevance), which it removes. This analyzer also is able to stem English words because it understands the rules of English grammar.

The english analyzer would produce the following:

set, shape, semi, transpar, call, set_tran, 5

Note how transparent, calling, and set_trans have been stemmed to their root form.

4種分詞器有各自的特徵，根據實際查詢需要指定適合的分詞器，否則會有意想不到的查詢結果。當需要爲字段指定分詞器時，可以使用mapping。

Specifying Analyzersedit

When Elasticsearch detects a new string field in your documents, it automatically configures it as a full-textstring field and analyzes it with the standard analyzer.

You don’t always want this. Perhaps you want to apply a different analyzer that suits the language your data is in. And sometimes you want a string field to be just a string field—to index the exact value that you pass in, without any analysis, such as a string user ID or an internal status field or tag.

To achieve this, we have to configure these fields manually by specifying the mapping.

官方文檔：https://www.elastic.co/guide/en/elasticsearch/guide/current/analysis-intro.html

當然，Elasticsearch也支持通過在映射文件中添加一個setting部分，自定義分析器或設置默認分析器。