比较新的版本中，mysql SUM聚合运算的返回值为MYSQL_TYPE_NEWDECIMAL。
目前OB中sum等聚合函数的返回值数据类型依赖于参与运算的列的数据类型。
MySQL中相关问题的bugfix：
https://github.com/ziutek/mymysql/issues/14
https://github.com/sidorares/nodejs-mysql-native/issues/49
'SUM()' field type is MYSQL_TYPE_NEWDECIMAL which was not supported until recently ( fixed with #48 )
I opened #50 for
better error handling in similar problems.
enum enum_field_types { MYSQL_TYPE_DECIMAL, MYSQL_TYPE_TINY,
MYSQL_TYPE_SHORT, MYSQL_TYPE_LONG,
MYSQL_TYPE_FLOAT, MYSQL_TYPE_DOUBLE,
MYSQL_TYPE_NULL, MYSQL_TYPE_TIMESTAMP,
MYSQL_TYPE_LONGLONG,MYSQL_TYPE_INT24,
MYSQL_TYPE_DATE, MYSQL_TYPE_TIME,
MYSQL_TYPE_DATETIME, MYSQL_TYPE_YEAR,
MYSQL_TYPE_NEWDATE, MYSQL_TYPE_VARCHAR,
MYSQL_TYPE_BIT, MYSQL_TYPE_NEWDECIMAL=246,
MYSQL_TYPE_ENUM=247,
MYSQL_TYPE_SET=248,
MYSQL_TYPE_TINY_BLOB=249,
MYSQL_TYPE_MEDIUM_BLOB=250,
MYSQL_TYPE_LONG_BLOB=251,
MYSQL_TYPE_BLOB=252,
MYSQL_TYPE_VAR_STRING=253,
MYSQL_TYPE_STRING=254,
MYSQL_TYPE_GEOMETRY=255
};