How to change definition of a Postgres 12 GENERATED column

Question

How to change definition of a Postgres 12 GENERATED column?

CREATE TABLE test (
    id serial PRIMARY KEY,
    val_a int4 NOT NULL DEFAULT 0,
    val_b int4 NOT NULL DEFAULT 0,
    val_sum int4 generated always AS (val_a + val_b) stored
);

I tried

ALTER TABLE test ALTER COLUMN val_sum TYPE int4 generated always AS (val_a + val_b + 1) stored;
ALTER TABLE test ALTER COLUMN val_sum SET generated always AS (val_a + val_b + 1) stored;

but both give syntax errors.

The alternative is to drop & add column, which works, but I wonder how to simply change it like any other column.

Answer 1

I'm not sure this is the correct answer, but reading the docs about ALTER TABLE I would say: No, you can't.

where action is one of:

ADD [ COLUMN ] [ IF NOT EXISTS ] column_name data_type [ COLLATE collation ] [ column_constraint [ ... ] ]

and column_constraint is:

[ CONSTRAINT constraint_name ]
{ NOT NULL |
  NULL |
  CHECK ( expression ) [ NO INHERIT ] |
  DEFAULT default_expr |
  GENERATED ALWAYS AS ( generation_expr ) STORED |
  GENERATED { ALWAYS | BY DEFAULT } AS IDENTITY [ ( sequence_options ) ] |
  UNIQUE index_parameters |
  PRIMARY KEY index_parameters |
  REFERENCES reftable [ ( refcolumn ) ] [ MATCH FULL | MATCH PARTIAL | MATCH SIMPLE ]
    [ ON DELETE referential_action ] [ ON UPDATE referential_action ] }
[ DEFERRABLE | NOT DEFERRABLE ] [ INITIALLY DEFERRED | INITIALLY IMMEDIATE ]

It seems that the only command that allows column_constraint is ADD COLUMN

In fact you can add a constraint name to the computed column:

val_sum int4 CONSTRAINT GA_test generated always AS (val_a + val_b) stored

Answer 2

For now as a possible work around can you drop the column and add it again?

ALTER TABLE test drop COLUMN val_sum;
ALTER TABLE test add COLUMN val_sum int4 generated always AS (val_a + val_b + 1) stored;

Fiddle

Fiddle with test