查了下，又是字符集的问题。
[oracle@DC214QB2X-2950 ~]$ oerr exp 00091
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.
[oracle@DC214QB2X-2950 ~]$ 
这个error产生的原因是因为我们exp所在的环境变量中的NLS_LANG与DB中的NLS_CHARACTERSET不一致。
1. 我们来查看DB中NLS_CHARACTERSET的值（提供两种方法）：
select * from nls_database_parameters t where t.parameter='NLS_CHARACTERSET'
select * from v$nls_parameters where parameter='NLS_CHARACTERSET'
输出结果：


2.查看当前主机的NLS_LANG环境变量的值
[oracle@DC214QB2X-2950 ~]$ echo $NLS_LANG
american_america.zhs16gbk
NLS_LANG的输出为american_america.zhs16gbk。 ------与DB中的NLS_CHARACTERSET不同。

3. 根据第一步中查出的NLS_CHARACTERSET 来设定exp的环境变量。
[oracle@DC214QB2X-2950 ~]$ export NLS_LANG=AMERICAN_AMERICA.AL32UTF8
[oracle@DC214QB2X-2950 ~]$ echo $NLS_LANG
AMERICAN_AMERICA.AL32UTF8

4. 设置完成后再尽心exp导出操作没有报错。