Oracle 参数 NLS_SORT 官方解释，作用，如何配置最优化建议

For example, NLS_SORT specifies the collating sequence for character value comparison in these SQL operators and clauses: 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 telephone directories in those languages.

The exact operators and query clauses that obey the NLS_SORT parameter depend on the value of the NLS_COMP parameter. If an operator or clause does not obey the NLS_SORT value, as determined by NLS_COMP, the collation used is BINARY.

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.