不要随便将varchar2列修改成nvarchar2()

最新推荐文章于 2025-06-14 16:14:24 发布

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当使用不同的数据类型绑定查询时，可能导致全表扫描和隐式转换问题，表现为Predicate Information中出现SYS_OP_C2C过滤操作。解决方法包括创建基于函数的索引或确保绑定变量和列数据类型一致。对于大规模转换，谨慎使用nvarchar2，可通过参数设置或索引来避免性能下降。

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总结教训，不要想抄近路，不要搞偏门，不要以为自己很聪明，不要听rd，不要相信测试
SYMPTOMS

You are executing a query using bind variables.
The binding occurs via an application (eg. .NET, J2EE ) using a “string” variable to bind.
The query is incorrectly performing a full table/index scan instead of an unique/range index scan.
When looking at advanced explain plan, sqltxplain or 10053 trace, you notice that the “Predicate Information” shows is doing a “filter(SYS_OP_C2C)”.

e.g select * from table(dbms_xplan.display_cursor(&sql_id,null,‘ADVANCED’));

Predicate Information (identified by operation id):

1 - filter(SYS_OP_C2C(“COL1”)=:B1) <=== filter operation occurring
CHANGES

CAUSE
The bind variable “string” is using a different datatype to the column that is being queried.
This means that an implicit conversion of the data is required to execute the query. SYS_OP_C2C is the implicit function which is used to convert the column between nchar and char.

SOLUTION

Create a function based index on the column.

e.g create index <index_name> on <table_name> (SYS_OP_C2C());
OR
2. Ensure that your bind “string” datatype and column datatype are the same.
A java example where this can occurs is when defaultNChar=TRUE. This will cause strings to bind as NVARCHAR2 causing the predicate that are subset datatypes to be converted to NVARCHAR2.
e.g. -Doracle.jdbc.defaultNChar=true
true

修改后的问题，如果你只是修改1，2个字段，那么你可以通过加索引方式解决，如果你修改的是全部的字段，你可以通过设置参数，解决，否则，你就不应该修改为nvarchar2()。