在我们做exp的过程中可能经常会遇到EXP-00091: Exporting questionable statistics.
查看EXP-00091的oracle error message 解决方案说明：
$oerr exp 00091
00091, 00000, "Exporting questionable statistics."
// *Cause: Export was able export statistics, but the statistics may not be
// usuable. The statistics are questionable because one or more of
// the following happened during export: a row error occurred, client
// character set or NCHARSET does not match with the server, a query
// clause was specified on export, only certain partitions or
// subpartitions were exported, or a fatal error occurred while
// processing a table.
// *Action: To export non-questionable statistics, change the client character
// set or NCHARSET to match the server, export with no query clause,
// export complete tables. If desired, import parameters can be
// supplied so that only non-questionable statistics will be imported,
// and all questionable statistics will be recalculated.
这样的EXP信息，其实它就是exp的error message，它产生的原因是因为我们exp工具所在的环境变量中的NLS_LANG与DB中的NLS_CHARACTERSET不一致。
解决步骤
1. 查看DB中NLS_CHARACTERSET的值：
SQL> select * from v$nls_parameters where parameter='NLS_CHARACTERSET';
PARAMETER VALUE
------------------------ ------------------
NLS_CHARACTERSET ZHS16GBK
2. 查看环境变量中的NLS_LANG:
$ echo $NLS_LANG
3. 根据1中查出来的NLS_CHARACTERSET来设置EXP的环境变量：
$ export NLS_LANG=AMERICAN_AMERICA.ZHS16GBK
$ echo $NLS_LANG
ZHS16GBK --设置与DB中NLS_CHARACTERSET=ZHS16GBK相同
