ElasticSearch之分詞器edge_ngram和ngram的區別

ElasticSearch一看就懂之分詞器edge_ngram和ngram的區別
1 year ago
edge_ngram和ngram是ElasticSearch自帶的兩個分詞器，一般設置索引映射的時候都會用到，設置完步長之后，就可以直接給解析器analyzer的tokenizer賦值使用。
這里，我們統一用字符串來做分詞示例：
字符串

1. edge_ngram分詞器，分詞結果如下：
   {
   "tokens": [
   {
   "token": "字",
   "start_offset": 0,
   "end_offset": 1,
   "type": "word",
   "position": 0
   },
   {
   "token": "字符",
   "start_offset": 0,
   "end_offset": 2,
   "type": "word",
   "position": 1
   },
   {
   "token": "字符串",
   "start_offset": 0,
   "end_offset": 3,
   "type": "word",
   "position": 2
   }
   ]
   }
2. ngram分詞器，分詞結果如下：
   {
   "tokens": [
   {
   "token": "字",
   "start_offset": 0,
   "end_offset": 1,
   "type": "word",
   "position": 0
   },
   {
   "token": "字符",
   "start_offset": 0,
   "end_offset": 2,
   "type": "word",
   "position": 1
   },
   {
   "token": "字符串",
   "start_offset": 0,
   "end_offset": 3,
   "type": "word",
   "position": 2
   },
   {
   "token": "符",
   "start_offset": 1,
   "end_offset": 2,
   "type": "word",
   "position": 3
   },
   {
   "token": "符串",
   "start_offset": 1,
   "end_offset": 3,
   "type": "word",
   "position": 4
   },
   {
   "token": "串",
   "start_offset": 2,
   "end_offset": 3,
   "type": "word",
   "position": 5
   }
   ]
   }
   一目了然，看明白了嗎？簡單理解來說：edge_ngram的分詞器，就是從首字開始，按步長，逐字符分詞，直至最終結尾文字；ngram呢，就不僅是從首字開始，而是逐字開始按步長，逐字符分詞。
   具體應用呢？如果必須首字匹配的情況，那么用edge_ngram自然是最佳選擇，如果需要文中任意字符的匹配，ngram就更為合適了。