在EXP导出时，出现EXP-00091报错：
    . . exporting table FRIEND
    …………..
    142480 rows exported
    EXP-00091: Exporting questionable statistics.
    查看EXP-00091的oracle error message 解决方案说明：
    [oracle@MWSG1: ~]$oerr exp 00091
    00091, 00000, “Exporting questionable statistics.”
    // Cause: Export was able export statistics, but the statistics may not be
    // usuable. The statistics are questionable because one or more of
    // the following happened during export: a row error occurred, client
    // character set or NCHARSET does not match with the server, a query
    // clause was specified on export, only certain partitions or
    // subpartitions were exported, or a fatal error occurred while
    // processing a table.
    //     Action: To export non-questionable statistics, change the client character
    // set or NCHARSET to match the server, export with no query clause,
    // export complete tables. If desired, import parameters can be
    // supplied so that only non-questionable statistics will be imported,
    // and all questionable statistics will be recalculated.
    在我们做exp的过程中可能经常会遇到EXP-00091: Exporting questionable statistics.这样的EXP信息，其实它就是exp的error message，它产生的原因是因为我们exp工具所在的环境变量中的NLS_LANG与DB中的NLS_CHARACTERSET不一致。
    ⑴ 我们来查看DB中的NLS_CHARACTERSET的值（提供两种方法）：
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    1. sys@CCDB> select from nls_database_parameters t *where t.parameter=’NLS_CHARACTERSET’;
    3. PARAMETER VALUE
    5. NLS_CHARACTERSET AL32UTF8
    7. 或者
    9. sys@CCDB> select from v$nls_parameters *where parameter=’NLS_CHARACTERSET’;
    11. PARAMETER VALUE
    13. NLS_CHARACTERSET AL32UTF8

    sys@CCDB> select from nls_database_parameters t where t.parameter=’NLS_CHARACTERSET’; PARAMETER VALUE ————————————- ———————- NLS_CHARACTERSET AL32UTF8 或者 sys@CCDB> select from v$nls_parameters where parameter=’NLS_CHARACTERSET’; PARAMETER VALUE ————————————- ———————- NLS_CHARACTERSET AL32UTF8
    ⑵ 查看环境变量中的NLS_LANG：
    [oracle@MWSG1: ~]$echo $NLS_LANG
    AMERICAN_AMERICA.ZHS16CGB231280 —果然与DB中NLS_CHARACTERSET＝AL32UTF8不同。
    ⑶ 根据⑴查出的NLS_CHARACTERSET（AL32UTF8）来设定exp的环境变量：
    WINNT> set NLS_LANG=AMERICAN_AMERICA.AL32UTF8
    [oracle@MWSG1: ~]$export NLS_LANG=AMERICAN_AMERICA.AL32UTF8
    [oracle@MWSG1: ~]$echo $NLS_LANG
    AMERICAN_AMERICA.AL32UTF8 —设置与DB中NLS_CHARACTERSET＝AL32UTF8相同。
    ⑷ 再来EXP导出数据 查看log中是否还有EXP-00091报错？
    . . exporting table FRIEND
    …………………………………………………………………
    ……………………………………….
    1216253 rows exported
    . . exporting table FRIEND_GRP
    128 rows exported
    Export terminated successfully without warnings. —成功导出且没警告。
    EXP-00091问题解决了！