最近在做exp的过程中经常会碰见EXP-00091 Exporting questionable statistics的错误. 确切的说是个警告，那么他产生的原因主要就是我们系统环境变量中的NLS_LANG和数据库中的NLS_CHARACTERSET设置不一致造成的。但是EXP-00091这个对我们所生成的dump影响不大，可以正常的做imp，但是对于有技术强迫症的自己仍然希望他能消失。

解决这个方法还是分两步走。
step 01　查看数据库中的NLS_CHARACTERSET的值

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    WE8ISO8859P1

step 02　根据我们在step 01查出的NLS_CHARACTERSET(WE8ISO8859P1)，来设定操作系统的环境变量:NLS_LANG

$ export NLS_LANG=AMERICAN_AMERICA.WE8ISO8859P1

这个时候我们在exp的时候就不会再出现EXP-00091这个错误了

附上exp-91的oracle error message：

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.