impala 中 cast 函数进行数据类型的转换导致结果错误？

查找到原因了。
因为udfs函数获取汇率的返回值为double类型，double类型的字段乘以decmial类型的字段，结果数据也为double类型，使用cast函数对double类型进行转换为decimal类型，结果就有可能出现错误的情况。
如下所示：
[master:21000] > select cast(cast(291328.29 as double) as decimal(16,2));
Query: select cast(cast(291328.29 as double) as decimal(16,2))
Query submitted at: 2018-02-27 14:59:22 (Coordinator: http://master:25000)
Query progress can be monitored at: http://master:25000/query_plan?query_id=ee4756fa52bdbeea:dee570ad00000000
+--------------------------------------------------+
| cast(cast(291328.29 as double) as decimal(16,2)) |
+--------------------------------------------------+
| 291328.28 |
+--------------------------------------------------+
Fetched 1 row(s) in 0.02s
[master:21000] > select cast(cast(291328.28 as double) as decimal(16,2));
Query: select cast(cast(291328.28 as double) as decimal(16,2))
Query submitted at: 2018-02-27 15:02:23 (Coordinator: http://master:25000)
Query progress can be monitored at: http://master:25000/query_plan?query_id=2e462958268f7e04:802b9d9f00000000
+--------------------------------------------------+
| cast(cast(291328.28 as double) as decimal(16,2)) |
+--------------------------------------------------+
| 291328.28 |
+--------------------------------------------------+
Fetched 1 row(s) in 0.01s