對一個DBA或需使用exp,imp的普通用戶來說，在我們做exp的過程中可能經常會遇到EXP－00091 Exporting
questionable statistics.這樣的EXP信息，其實它就是exp的error
message，它產生的原因是因為我們exp工具所在的環境變量中的
NLS_LANG與DB中的
NLS_CHARACTERSET不
一致。但需說明的是，exp-91這個error
message對所生成的dump檔沒有影響，生成的dump檔還可以正常的imp（個人體會，不知道有沒有錯），雖然它對我們的dump檔沒有影響，我
個人還是不想它出現，大家也有同感吧， 。。下面我們就讓它消失吧。。我們一起來
step 01 查看DB中的NLS_CHARACTERSET的值（提供兩種方法）：
select * from nls_database_parameters t where t.parameter='NLS_CHARACTERSET'
or
select * from v$nls_parameters where parameter='NLS_CHARACTERSET';
SQL> select * from v$nls_parameters where parameter='NLS_CHARACTERSET';
PARAMETER VALUE
----------------------- ----------------------------------------------
NLS_CHARACTERSET ZHT16BIG5
step 02 根據step 01查出的NLS_CHARACTERSET（ZHT16BIG5）來設定exp的環境變量：
WINNT> set NLS_LANG=AMERICAN_AMERICA.ZHT16BIG5
LINUX> export NLS_LANG=AMERICAN_AMERICA.ZHT16BIG5
附上exp-91的oracle error message 解決方案說明：
oerr exp 91
00091, 00000, "Exporting questionable statistics."
// *Cause: Export was able export statistics, but the statistics may not be
// usuable. 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,
// 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.
From: http://vongates.itpub.net/post/2553/18625