Alter a Column Data Type Postgres

Postgres does't allow int type column to change directly into timezone. To achive this, you have to first change column type to varchar and then change it to timezone.

alter table tableName alter column columnName type varchar(64);

alter table tableName alter column columnName type timestamp with time zone;

Postgres doesn't know how to translate int to timestamp. There are several cases and usually they have different starting date.

1. Create temporary column with timestamp
2. Update table and copy data from old column to temporary column using your own translation
3. Drop old column
4. Rename temporary column.

If you look into documentation, you will find one line syntax with example how to convert unix time integer type:

ALTER [ COLUMN ] column [ SET DATA ] TYPE type [ USING expression ]