NLS_COMP和NLS_SORT参数

Oracle默认是采用binary进行排序，这对于例如中文的排序来说，是不恰当的。使用这两个参数可以指定排序的方法，比如拼音或是，要注意可能会引起性能问题。
解决方法是使用NLSSORT函数来建立一个函数索引。

NLS_COMP = { BINARY | LINGUISTIC | ANSI }
BINARY
Normally, comparisons in the WHERE clause and in PL/SQL blocks is binary unless you specify theNLSSORT function.
LINGUISTIC
Comparisons for all SQL operations in the WHERE clause and in PL/SQL blocks should use the linguistic sort specified in theNLS_SORT parameter. To improve the performance, you can also define a linguistic index on the column for which you want linguistic comparisons.
ANSI
A setting of ANSI is for backwards compatibility; in general, you should setNLS_COMP to LINGUISTIC

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：
select value from v$nls_valid_values where parameter='SORT' and value like '%SCHINESE%';
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

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NLS_SORT specifies the collating sequence for ORDER BY queries.

 

• If the value is BINARY, then the collating sequence for ORDER BY queries is based on the numeric value of characters (a binary sort that requires less system overhead).
• If the value is a named linguistic sort, sorting is based on the order of the defined linguistic sort. Most (but not all) languages supported by the NLS_LANGUAGE parameter also support a linguistic sort with the same name.

  Note:

  Setting NLS_SORT to anything other than BINARY causes a sort to use a full table scan, regardless of the path chosen by the optimizer. BINARY is the exception because indexes are built according to a binary order of keys. Thus the optimizer can use an index to satisfy the ORDER BY clause when NLS_SORT is set to BINARY. If NLS_SORT is set to any linguistic sort, the optimizer must include a full table scan and a full sort in the execution plan.

 

You must use the NLS_SORT operator with comparison operations if you want the linguistic sort behavior.

 

根据上文中标红部分的注视，如果NLS_SORT不是设置为"Binary"，那么就会引起全表扫描，是不会使用索引的.

NLS_COMP specifies the collation behavior of the database session.

Values:

• BINARY

Normally, comparisons in the WHERE clause and in PL/SQL blocks is binary unless you specify the NLSSORT function.

• LINGUISTIC

Comparisons for all SQL operations in the WHERE clause and in PL/SQL blocks should use the linguistic sort specified in the NLS_SORT parameter. To improve the performance, you can also define a linguistic index on the column for which you want linguistic comparisons.

• ANSI

A setting of ANSI is for backwards compatibility; in general, you should set NLS_COMP to LINGUISTIC

《FROM:http://www.linuxidc.com/Linux/2012-09/70931.htm   http://www.cnblogs.com/yanrongpi/archive/2008/04/11/1149144.html》