exp导出数据时，输出日志会报EXP-00091: Exporting questionable statistics的错误，特别是对于一些crontab中执行的exp脚本。
造成该错误提示的原因通常都是导出端nls_lang参数未设定，或者设置有误，解决方式也很简单：
a.查询数据库中的字符集：
SQL> select * from v$nls_parameters where parameter='NLS_CHARACTERSET';
PARAMETER VALUE
--------------------------- -----------------
NLS_CHARACTERSET ZHS16GBK
b.设置nls_lang变量
export NLS_LANG=AMERICAN_AMERICA.ZHS16GBK
然后再重新执行exp即可。
附，官方文档中对EXP-00091错误的描述及解决方案：
EXP-00091: 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.