test timestamp

highgo=# select '2013-7-8 23:23:23.9999989'::timestamp(6);
timestamp
----------------------------
2013-07-08 23:23:23.999999
(1 行记录)

highgo=# select '2013-7-8 23:23:23.9999984'::timestamp(6);
timestamp
----------------------------
2013-07-08 23:23:23.999998
(1 行记录)