exp scott/tiger file=test.dmp tables=emp query="where empno <7499"
一般情况下，是由于字符集设置问题，这个case下不存在字符集的问题，去掉query后，exp成功。
metalink上的解释
The precalculated optimizer statistics are flagged as questionable at export time if:
1. There are row errors while exporting
2. The client character set or NCHAR character set does not match the server character set or
NCHAR character set
3. A QUERY clause is specified
4. Only certain partitions or subpartitions are exported
To implement the solution, please execute the following steps:
1. Set NLS_LANG on OS, then exp again:
$export NLS_LANG=american_america.ZHS16GBK
$echo $NLS_LANG
$exp system/passwd full=y
Or
2. You can add statistics=none:
$exp system/passwd full=y statistics=none
这些EXP－00091错误不影响dmp文件的有效性，可加 statistics=none屏蔽掉该错误。
[@more@]
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