对一个DBA或需使用exp,imp的普通用户来说，在我们做exp的过程中可能经常会遇到EXP－00091 Exporting questionable statistics.这样的EXP信息，其实它就是exp的error message，它产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。
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
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.