Question

1) create table

CREATE TABLE ad_position_ad ( 
 p_id           bigint                ,
 p_name         character varying(20) ,
 devid          integer               ,
 ad_type        character varying(20) ,
 platform       character varying(20) ,
 category_id    integer               ,
 config         character varying(20) ,
 is_check       character varying(20) ,
 status         character varying(20) ,
 showing_total  bigint                ,
 click_total    bigint                ,
 user_total     bigint                ,
 income_total   bigint                ,
 online_time    integer               ,
 create_time    integer               ,
 modify_time    integer);

After create the table, we insert some data in the table.

2) Data of the table

mydb=> select count(*) from ad_position_ad;
 count 
-------
   275
(1 row)

mydb=> select distinct config from ad_position_ad;
 config 
--------
 2
 0
 1
(3 rows)

3) convert a varchar column to smallint

mydb=>
alter table ad_position_ad
alter column config type smallint using platform::smallint;
ALTER TABLE

4) check the data again

mydb=> select distinct config from ad_position_ad;
 config 
--------
      3
(1 row)

Notice the value of column config have already changed. Does anybody know about that?
But the following command works fine, and the value does not change:

mydb=>
alter table ad_position_ad
alter column config type smallint using cast(config as smallint);
ALTER TABLE

Answer 1

Edit:

After rewriting the question I finally think I found the real problem. I quote:

alter table ad_position_ad
alter column config type smallint using platform::smallint;

You realize that you ALTER COLUMN config, but use the value of the column platform for it?

Try:

alter table ad_position_ad
alter column config type smallint using config::smallint;