该参数用来指定，undo保留时间，以秒为单位 超过该参数指定的时间，undo段的数据被标识为过期，空间可被重用 设定undo_retention sql>alter system set undo_retention=1800 scope=both;
UNDO_RETENTION specifies (in seconds) the amount of committed undo information to retain in the database. You can use UNDO_RETENTION to satisfy queries that require old undo information to rollback changes to produce older images of data blocks. You can set the value at instance startup.The UNDO_RETENTION parameter works best if the current undo tablespace has enough space for the active transactions. If an active transaction needs undo space and the undo tablespace does not have any free space, then the system will start reusing undo space that would have been retained. This may cause long queries to fail. Be sure to allocate enough space in the undo tablespace to satisfy the space requirement for the current setting of this parameter.
超过该参数指定的时间，undo段的数据被标识为过期，空间可被重用
设定undo_retention
sql>alter system set undo_retention=1800 scope=both;