NLS_SORT = { BINARY | linguistic_definition}NLS_SORT specifies the collating sequence for character value comparison in various SQL operators and clauses, for example, ORDER BY, GROUP BY, comparison conditions (=, <>, <=, >=), IN, BETWEEN, LIKE, MIN/MAX, GREATEST/LEAST, and INSTR.
If the value is BINARY, then comparison is based directly on byte values in the binary encoding of the character values being compared. The ordering depends on the character set of the compared values, which is either the database character set (for VARCHAR2, CHAR, LONG, and CLOB) or the national character set (for NVARCHAR2, NCHAR, and NCLOB).
If the value is a named linguistic sort, then comparison is defined by this sort. A linguistic sort uses various rules to achieve ordering expected by speakers of one or more natural languages. This is usually the same ordering that is used in dictionaries and/or telephone directories in those languages.
The BINARY comparison is faster and uses less resources than any linguistic comparison but for text in a natural language, it does not provide ordering expected by users.
The value of NLS_SORT affects execution plans of queries. Because a standard index cannot be used as a source of values sorted in a linguistic order, an explicit sort operation must usually be performed instead of an index range scan. A functional index on the NLSSORT function may be defined to provide values sorted in a linguistic order and reintroduce the index range scan to the execution plan.
下面做个测试：
首先来看看可用的中文排序方法：
返回可用的中文排序方法
SCHINESE_PINYIN_M -- 按照拼音排序
SCHINESE_STROKE_M -- 按照笔划（第一顺序）、部首（第二顺序）排序
SCHINESE_RADICAL_M -- 按照部首（第一顺序）、笔划（第二顺序）排序
下面来做测试。
create index emp_ename_idx on emp(ename);
explain plan for select * from emp where ename='Mike';
select * from table(dbms_xplan.display);
可以看到oracle会走索引emp_ename_idx。