原文说明：
SET DATA TYPE
This form changes the type of a column of a table. Indexes and simple table constraints involving the column willbe automatically converted to use the new column type by reparsing the originally supplied expression. The optional COLLATE clause specifies a collation for the new column; if omitted,the collation is the default for the new column type. The optional USING clause specifies how to compute the new column value from the old; if omitted,the default conversion is the same as an assignment cast from old data type to new. A USING clause must be provided if there is no implicit or assignment cast from old to new type.
大致意思是：转换类型的时候有隐含类型转换的时候，会自动转换，如果没有，那么就必须使用using指定一下转换规则。
1. 建表
create table 101(id integer);
2. 插入数据
insert into tb10 select generate_series(1,5);
3. 把id的int变为varchar
postgres=# alter table tb101 alter id type varchar;
ALTER TABLE
因为int转varchar有隐式的转换，故可以自动转换过去。
postgres=# \d tb101
Table "public.tb101"
Column | Type | Modifiers --------+-------------------+-----------
id | character varying |
4. 把id的varchar变为int
postgres=# alter table tb101 alter id type int;
ERROR: column "id" cannot be cast automatically to type integer
HINT: Specify a USING expression to perform the conversion.
在没有隐式的转换下，就需要指定Using来显示的转换。
5. 使用Using进行类型转换
postgres=# alter table tb101 alter id type int using id::int;
ALTER TABLE postgres=# \d tb101 Table "public.tb101" Column | Type | Modifiers --------+---------+----------- id | integer |
