Get used to Oracle:
ALTER TABLE Table name ALTER COLUMN Column name New data type [(length)] NULL or NOT NULL
When this modification method is used, in pg:
highgo=# create table p1 (id int,pswd varchar(30),time timestamp);
CREATE TABLE
highgo=# insert into p1 select generate_series(1,500000),md5('random()::text'),clock_timestamp();
Error: The value is too long for variable character types (30)
Will you find that it cannot be added successfully?
highgo=# alter table p1 alter column pswd text NULL;
Error: Syntax error in or near "text"
LINE 1: alter table p1 alter column pswd text NULL;
Let's take a look at the syntax in pg:
highgo=# \h auto
where action is one of:
ADD [ COLUMN ] [ IF NOT EXISTS ] column_name data_type [ COLLATE collation ] [ column_constraint [ ... ] ]
DROP [ COLUMN ] [ IF EXISTS ] column_name [ RESTRICT | CASCADE ]
ALTER [ COLUMN ] column_name [ SET DATA ] TYPE data_type [ COLLATE collation ] [ USING expression ]
ALTER [ COLUMN ] column_name SET DEFAULT expression
highgo=# alter table p1 alter COLUMN pswd type text ;
ALTER TABLE
highgo=# \d p1
Table "public.p1"
Column | Type | Collation | Nullable | Default
--------+-----------------------------+-----------+----------+---------
id | integer | | |
pswd | text | | |
time | timestamp without time zone | | |
success!
Supplement: postgresql modify the field type to array type (text is changed to text[])
grammar:
alter table tablename alter columnname type oldcolumntype USING columnname:: newcolumntype
eg：
alter table dirty_track alter labels type text USING labels::text[];
The above is personal experience. I hope you can give you a reference and I hope you can support me more. If there are any mistakes or no complete considerations, I would like to give you advice.