EXP-00091:Exporting questionable statistics.这样的EXP信息，其实它就是exp的error message，它產生的原因是因为我们exp工具所在的环境变量中的NLS_LANG与DB中的NLS_CHARACTERSET不一致。但需说明的是，exp-91这个error message对所生成的dump档没有影响，生成的dump档还可以正常的imp:

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'
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　根据step 01查出的NLS_CHARACTERSET（ZHT16BIG5）来设定exp的环境变量：

WINNT> set NLS_LANG=AMERICAN_AMERICA.ZHT16BIG5
LINUX> export NLS_LANG=AMERICAN_AMERICA.ZHT16BIG5

SQL> select * from nls_database_parameters t where t.parameter='NLS_CHARACTERSET';

PARAMETER                      VALUE
------------------------------ --------------------------------------------------
NLS_CHARACTERSET               AL32UTF8
WINNT> set NLS_LANG=AMERICAN_AMERICA.AL32UTF8

set NLS_LANG=SIMPLIFIED CHINESE_CHINA.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.

转自:http://fronkx.blog.hexun.com/21952318_d.html