Oracle全文检索方面的研究(全3)(2)

ctx_ddl.set_attribute(german_lexer,mixed_case,yes);

ctx_ddl.set_attribute(german_lexer,alternate_spelling,german);

ctx_ddl.create_preference(japanese_lexer,japanese_vgram_lexer);

ctx_ddl.create_preference(global_lexer, multi_lexer);

ctx_ddl.add_sub_lexer(global_lexer,default,english_lexer);

ctx_ddl.add_sub_lexer(global_lexer,german,german_lexer,ger);

ctx_ddl.add_sub_lexer(global_lexer,japanese,japanese_lexer,jpn);

end;

create index globalx on globaldoc(text) indextype is ctxsys.context

parameters (lexer global_lexer language column lang);

3.3.3 chinese_vgram_lexer 和chinese_lexer

basic_lexer 只能识别出被空格、标点和回车符分隔出来的部分，如果要对中文内容进行索引的话，就必须使用chinese_vgram_lexer 或是chinese_lexer

Chinese_lexer 相比chinese_vgram_lexer 有如下的优点：

产生的索引更小

更好的查询响应时间

产生更接近真实的索引切词，使得查询精度更高

支持停用词

因为chinese_lexer 采用不同的算法来标记tokens, 建立索引的时间要比chinese_vgram_lexer

长.

字符集：支持al32utf8，zhs16cgb231280，zhs16gbk，zhs32gb18030，zht32euc，zht16big5

zht32tris， zht16mswin950，zht16hkscs，utf8

--建立chinese lexer

Begin

ctx_ddl.create_preference(my_chinese_vgram_lexer, chinese_vgram_lexer);

ctx_ddl.create_preference(my_chinese_lexer, chinese_lexer);

End;

-- chinese_vgram_lexer

Create index ind_m_lex1 on my_lex(docs) indextype is ctxsys.context Parameters (lexer foo.my_chinese_vgram_lexer);

Select * from my_lex t where contains(docs, 中国) > 0;

-- chinese_lexer

drop   index ind_m_lex1 force; 

Create index ind_m_lex2 on my_lex(docs) indextype is ctxsys.context

Parameters (lexer ctxsys.my_chinese_lexer);

Select * from my_lex t where contains(docs, 中国) > 0;

3.3.4 User_lexer

Use USER_LEXER to plug in your own language-specific lexing solution. This enables you to

define lexers for languages that are not supported by Oracle Text. It also enables you to define a

new lexer for a language that is supported but whose lexer is inappropriate for your application.

3.3.5 Default_lexer

如果数据库在建立的时候指定的是中文则default_lexer 为chinese_vgram_lexer，如果是英文，则default_lexer 为basic_lexer

3.3.6 Query_procedure

This callback stored procedure is called by Oracle Text as needed to tokenize words in the query.

A space-delimited group of characters (excluding the query operators) in the query will be

identified by Oracle Text as a word.

3.3.7 参考脚本

--建立basic_lexer

begin

ctx_ddl.create_preference(mylex, BASIC_LEXER);

ctx_ddl.set_attribute (mylex, printjoins, _-); --保留_ -符号

ctx_ddl.set_attribute (mylex,mixed_case,yes); --区分大小写

end;

create index indx_m_lex on my_lex(docs) indextype is ctxsys.context parameters(lexer