解決EXP-00091: Exporting questionable statistics.

今天，在查看客戶的exp日誌的時候，發現有以下錯誤：
...
EXP-00091: Exporting questionable statistics.
EXP-00091: Exporting questionable statistics.
. exporting synonyms
. exporting views
. exporting stored procedures
. exporting operators
...

錯誤的描述：
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.

從export日誌中能夠看出，導出時字符集爲US7ASCII，而DB用的是AL32UTF8。
這樣的話，該dmp文件在import的時候會有一次字符集轉換工做。
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)

推斷是客戶端的NLS_LANG設置有問題，發現NLS_LANG的值設爲American，改爲NLS_LANG=American_America.AL32UTF8後，無任何警告信息。

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