先说总结：
Windows下oracle的systimestamp函数的最高精度是毫秒，
linux下oracle的systimestamp函数的最高精度是微秒
You could say that SYSTIMESTAMP is "sysdate with time zone information added".
SYSTIMESTAMP is the timestamp on the server machine itself and is obtained on Unix platforms by calling " GetTimeOfDay " and on Windows by calling "GetSystemTime" to get the servers local time.
This means that SYSTIMESTAMP, just like SYSDATE depends on Unix platforms on the UNIX time configuration (= Unix TZ variable) for the Unix session when the database and listener where started.
The precision is platform dependant, on most Unix platforms it's microseconds (10-6) on Windows this is Milliseconds (10-3). The output is defined by NLS_TIMESTAMP_TZ_FORMAT in NLS_SESSION_PARAMETERS.
来源：Timestamps & time zones - Frequently Asked Questions (Doc ID 340512.1)
实验：
