EXP，IMP导数据时，导出时操作系统的字符集与数据库的NLS_CHARACTERSET 字符语言不一致时会报EXP-00091.可以先查出数据库的NLS_CHARACTERSET ,在操作系统上set/export NLS_LANG 再导就不会出这个问题了

---please set NLS_LANG before IMP/DMP

1.first ,select NLS_CHARACTERSET from database
sql>conn /as sysdba
SQL> select * from props$;

NAME
------------------------------------------------------------
VALUE$
--------------------------------------------------------------------------------
COMMENT$
--------------------------------------------------------------------------------
NLS_NUMERIC_CHARACTERS
.,
Numeric characters

NLS_CHARACTERSET
ZHS16CGB231280
Character set

2.set NLS_LANG value in OS:
in AIX:
export NLS_LANG=AMERICAN_AMERICA.ZHS16CGB231280

in windows:
c:> set  NLS_LANG=AMERICAN_AMERICA.ZHS16CGB231280

3.exp or imp your data .

exp user/password@sid file=E:\dnft_ora_back\ddd.dmp log=E:\dnft_ora_back\ll.log wner=llconsistent=Y

exp user/password@sidfile=/oradata/II.dmp log=/oradata/ddd.log wner=llconsistent=Y

imp user/password@sid fromuser=ll touser=ll file=z:\ll.dmp log=z:\llimp.log

 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.