数据类型隐式转换规则

NA：表示不支持隐式类型转换，例如：

create table t_smallint(c1 smallint);
insert into t_smallint select 1;
select * from t_smallint ;
 c1
----
  1
(1 row)

explain verbose select CAST(c1 as timestamp) from t_smallint;
ERROR:  cannot cast type smallint to timestamp without time zone
LINE 1: explain verbose select CAST(c1 as timestamp) from t_smallint...

e : 表示仅支持通过CAST或::语法进行显示类型转换，例如：

create table t_timestamp_without_time_zone(c1 timestamp without time zone);
insert into t_timestamp_without_time_zone values('2021-10-31 08:00:00');
select CAST(c1 as time without time zone) from t_timestamp_without_time_zone;
    c1
----------
 08:00:00
(1 row)

insert into t_timestamp_without_time_zone values('08:00:00'::time without time zone);
ERROR:  column "c1" is of type timestamp without time zone but expression is of type time without time zone
LINE 1: insert into t_timestamp_without_time_zone values('08:00:00':...
                                                         ^
HINT:  You will need to rewrite or cast the expression.

a : 表示除支持e外，还支持隐式向目标列类型赋值（通常指INSERT VALUES赋值或UPDATE SET赋值）。例如：

    create table t_int(c1 integer);
    insert into t_int values(2);
    select cast(c1 as smallint) from t_int;  -- ok
     c1
    ----
      2
    insert into t_int values(3::smallint);  -- ok

i : 表示除支持以上a和e外，还支持其他隐式转换，包括如表达式参数等。例如：

    -- case 1
    CREATE OR REPLACE FUNCTION F_VARCHAR(arg1 VarChar) RETURN void
    IS
    BEGIN
       dbms_output.put_line(arg1);
       RETURN;
    END;

    SELECT F_VARCHAR(cast('10' as CHAR(10))) FROM DUAL;  -- ok

-- case 2
    create table t_varchar(c1 varchar(10));
    insert into t_varchar values(2);
    explain verbose select sum(c1) from t_varchar;
                                QUERY PLAN
    ---------------------------------------------------------------------------
    Aggregate  (cost=43.95..43.96 rows=1 width=32)
      Output: sum((c1)::numeric)
      ->  Seq Scan on public.t_varchar  (cost=0.00..29.40 rows=1940 width=14)
           Output: c1