Export provides two methods for exporting table data:
■ Conventional path Export
■ Direct path Export
Conventional path Export uses the SQL SELECT statement to extract data from
tables. Data is read from disk into a buffer cache, and rows are transferred to the
evaluatingbuffer.Thedata,afterpassingexpressionevaluation,istransferredtothe
Export client, which then writes the data into the export file.
Direct path Export is much faster than conventional path Export because data is
read from disk into the buffer cache and rows are transferreddirectly to the Export
client. The evaluating buffer is bypassed. The data is already in the format that
Export expects, thus avoiding unnecessary data conversion. The data is transferred
to the Export client, which then writes the data into the export file.

不通过内存，直接将数据写出到磁盘 ,直接导出数据(通过cached commit scn 加载数据)，数据不需要经过内存整合和检查
这样减少了sql语句处理的过程.否则exp将使用select来读取表中的数据，