解决EXP-00091: Exporting questionable statistics.

今天，在查看客户的exp日志的时候，发现有如下错误：
...
EXP-00091: Exporting questionable statistics.
EXP-00091: Exporting questionable statistics.
. exporting synonyms
. exporting views
. exporting stored procedures
. exporting operators
...

错误的描述：
EXP-00091 Exporting questionable statistics
  
Cause: Export was able to export statistics, but the statistics may not be useable. The statistics are questionable because one or more of the following happened during export: a row error occurred, client character set or NCHARSET does not match with the server, a query clause was specified on export, only certain partitions or subpartitions were exported, or a fatal error occurred while processing a table.
  
Action: To export non-questionable statistics, change the client character set or NCHARSET to match the server, export with no query clause, or export complete tables. If desired, import parameters can be supplied so that only non-questionable statistics will be imported, and all questionable statistics will be recalculated.

从export日志中可以看出，导出时字符集为US7ASCII，而DB用的是AL32UTF8。
这样的话，该dmp文件在import的时候会有一次字符集转换工作。
Connected to: Oracle Database 10g Enterprise Edition Release 10.2.0.5.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options
Export done in US7ASCII character set and AL16UTF16 NCHAR character set
server uses AL32UTF8 character set (possible charset conversion)

推断是客户端的NLS_LANG设置有问题，发现NLS_LANG的值设为American，改成NLS_LANG=American_America.AL32UTF8后，无任何警告信息。

Connected to: Oracle Database 10g Enterprise Edition Release 10.2.0.5.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options
Export done in AL32UTF8 character set and AL16UTF16 NCHAR character set
. exporting pre-schema procedural objects and actions