gistfile1.txt

postgres=# CREATE TABLE t (a int);
CREATE TABLE
postgres=# SELECT * FROM  pg_attrdef;
 oid | adrelid | adnum | adbin 
-----+---------+-------+-------
(0 rows)

postgres=# INSERT INTO t VALUES (1);
INSERT 0 1
postgres=# ALTER TABLE t ADD b int DEFAULT 2;
ALTER TABLE
postgres=# INSERT INTO t VALUES (2);
INSERT 0 1
postgres=# SELECT * FROM  pg_attrdef;
-[ RECORD 1 ]------------------------------------------------------------------------------------------------------------------------------------------------
oid     | 22896
adrelid | 22893
adnum   | 2
adbin   | {CONST :consttype 23 :consttypmod -1 :constcollid 0 :constlen 4 :constbyval true :constisnull false :location 32 :constvalue 4 [ 2 0 0 0 0 0 0 0 ]}

postgres=# SELECT attmissingval FROM pg_attribute WHERE attname = 'b';
 attmissingval 
---------------
 {2}
(1 row)

postgres=# SELECT * FROM t;
 a | b 
---+---
 1 | 2
 2 | 2
(2 rows)

postgres=# ALTER TABLE t ALTER b SET DEFAULT 42;
ALTER TABLE
postgres=# INSERT INTO t VALUES (3);
INSERT 0 1
postgres=# SELECT * FROM  pg_attrdef;
-[ RECORD 1 ]-------------------------------------------------------------------------------------------------------------------------------------------------
oid     | 22897
adrelid | 22893
adnum   | 2
adbin   | {CONST :consttype 23 :consttypmod -1 :constcollid 0 :constlen 4 :constbyval true :constisnull false :location 34 :constvalue 4 [ 42 0 0 0 0 0 0 0 ]}

postgres=# SELECT attmissingval FROM pg_attribute WHERE attname = 'b';
 attmissingval 
---------------
 {2}
(1 row)

postgres=# SELECT * FROM t;
 a | b  
---+----
 1 |  2
 2 |  2
 3 | 42
(3 rows)