遇到EXP－00091 Exporting questionable statistics.这样的EXP信息，它是exp的错误信息，

产生原因是用户所在的环境变量中的NLS_LANG与DB中的NLS_CHARACTERSET不一致。

step 01　查看DB中的NLS_CHARACTERSET的值（提供两种方法）：

select * from nls_database_parameters t where t.parameter='NLS_CHARACTERSET'

SQL> select * from v$nls_parameters where parameter='NLS_CHARACTERSET';

PARAMETER　　　　 VALUE

-----------------------　　----------------------------------------------

NLS_CHARACTERSETZHS16GBK

step 02　根据step 01查出的NLS_CHARACTERSET（ZHS16GBK）来设定exp的环境变量：

WINNT> set NLS_LANG=AMERICAN_AMERICA.ZHS16GBK

LINUX> export NLS_LANG=AMERICAN_AMERICA.ZHS16GBK

附上exp-91的oracle error message 解决方案说明：

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.