Postgres 12

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CREATE TABLE t(id INTEGER, foo VARCHAR(10) DEFAULT 'abc')

CREATE TABLE

INSERT INTO t(id) VALUES (1), (2)

INSERT 0 2

SELECT * FROM t

id
foo

1
abc

2
abc

SELECT 2

SELECT column_default
FROM information_schema.columns 
WHERE table_name = 't' AND column_name = 'foo'

column_default

'abc'::character varying

SELECT 1

UPDATE t
SET foo = 'new value'
WHERE foo = (
  SELECT TRIM('''' FROM SPLIT_PART(column_default, '::', 1))
  FROM information_schema.columns 
  WHERE table_name = 't' AND column_name = 'foo'
)

UPDATE 2

SELECT * FROM t

id
foo

1
new value

2
new value

SELECT 2