Fix broken handling of domains in atthasmissing logic.

If a domain type has a default, adding a column of that type (without
any explicit DEFAULT clause) failed to install the domain's default
value in existing rows, instead leaving the new column null.  This
is unexpected, and it used to work correctly before v11.  The cause
is confusion in the atthasmissing mechanism about which default value
to install: we'd only consider installing an explicitly-specified
default, and then we'd decide that no table rewrite is needed.

To fix, take the responsibility for filling attmissingval out of
StoreAttrDefault, and instead put it into ATExecAddColumn's existing
logic that derives the correct value to fill the new column with.
Also, centralize the logic that determines the need for
default-related table rewriting there, instead of spreading it over
four or five places.

In the back branches, we'll leave the attmissingval-filling code
in StoreAttrDefault even though it's now dead, for fear that some
extension may be depending on that functionality to exist there.
A separate HEAD-only patch will clean up the now-useless code.

Reported-by: jian he <jian.universality@gmail.com>
Author: jian he <jian.universality@gmail.com>
Author: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CACJufxHFssPvkP1we7WMhPD_1kwgbG52o=kQgL+TnVoX5LOyCQ@mail.gmail.com
Backpatch-through: 13

2 files changed, 109 insertions, 0 deletions

diff --git a/src/test/regress/expected/fast_default.out b/src/test/regress/expected/fast_default.out
index 59365dad964..3944812c298 100644
--- a/src/test/regress/expected/fast_default.out
+++ b/src/test/regress/expected/fast_default.out
@@ -245,6 +245,71 @@ SELECT comp();
 (1 row)
 
 DROP TABLE T;
+-- Test domains with default value for table rewrite.
+CREATE DOMAIN domain1 AS int DEFAULT 11;  -- constant
+CREATE DOMAIN domain2 AS int DEFAULT random(min=>10, max=>100);  -- volatile
+CREATE DOMAIN domain3 AS text DEFAULT foo(4);  -- stable
+CREATE DOMAIN domain4 AS text[]
+  DEFAULT ('{"This", "is", "' || foo(4) || '","the", "real", "world"}')::TEXT[];
+CREATE TABLE t2 (a domain1);
+INSERT INTO t2 VALUES (1),(2);
+-- no table rewrite
+ALTER TABLE t2 ADD COLUMN b domain1 default 3;
+SELECT attnum, attname, atthasmissing, atthasdef, attmissingval
+FROM pg_attribute
+WHERE attnum > 0 AND attrelid = 't2'::regclass
+ORDER BY attnum;
+ attnum | attname | atthasmissing | atthasdef | attmissingval 
+--------+---------+---------------+-----------+---------------
+      1 | a       | f             | f         | 
+      2 | b       | t             | t         | {3}
+(2 rows)
+
+-- table rewrite should happen
+ALTER TABLE t2 ADD COLUMN c domain3 default left(random()::text,3);
+NOTICE:  rewriting table t2 for reason 2
+-- no table rewrite
+ALTER TABLE t2 ADD COLUMN d domain4;
+SELECT attnum, attname, atthasmissing, atthasdef, attmissingval
+FROM pg_attribute
+WHERE attnum > 0 AND attrelid = 't2'::regclass
+ORDER BY attnum;
+ attnum | attname | atthasmissing | atthasdef |           attmissingval           
+--------+---------+---------------+-----------+-----------------------------------
+      1 | a       | f             | f         | 
+      2 | b       | f             | t         | 
+      3 | c       | f             | t         | 
+      4 | d       | t             | f         | {"{This,is,abcd,the,real,world}"}
+(4 rows)
+
+-- table rewrite should happen
+ALTER TABLE t2 ADD COLUMN e domain2;
+NOTICE:  rewriting table t2 for reason 2
+SELECT attnum, attname, atthasmissing, atthasdef, attmissingval
+FROM pg_attribute
+WHERE attnum > 0 AND attrelid = 't2'::regclass
+ORDER BY attnum;
+ attnum | attname | atthasmissing | atthasdef | attmissingval 
+--------+---------+---------------+-----------+---------------
+      1 | a       | f             | f         | 
+      2 | b       | f             | t         | 
+      3 | c       | f             | t         | 
+      4 | d       | f             | f         | 
+      5 | e       | f             | f         | 
+(5 rows)
+
+SELECT a, b, length(c) = 3 as c_ok, d, e >= 10 as e_ok FROM t2;
+ a | b | c_ok |               d               | e_ok 
+---+---+------+-------------------------------+------
+ 1 | 3 | t    | {This,is,abcd,the,real,world} | t
+ 2 | 3 | t    | {This,is,abcd,the,real,world} | t
+(2 rows)
+
+DROP TABLE t2;
+DROP DOMAIN domain1;
+DROP DOMAIN domain2;
+DROP DOMAIN domain3;
+DROP DOMAIN domain4;
 DROP FUNCTION foo(INT);
 -- Fall back to full rewrite for volatile expressions
 CREATE TABLE T(pk INT NOT NULL PRIMARY KEY);
diff --git a/src/test/regress/sql/fast_default.sql b/src/test/regress/sql/fast_default.sql
index dc9df78a35d..a21b406e659 100644
--- a/src/test/regress/sql/fast_default.sql
+++ b/src/test/regress/sql/fast_default.sql
@@ -237,6 +237,50 @@ SELECT comp();
 
 DROP TABLE T;
 
+-- Test domains with default value for table rewrite.
+CREATE DOMAIN domain1 AS int DEFAULT 11;  -- constant
+CREATE DOMAIN domain2 AS int DEFAULT random(min=>10, max=>100);  -- volatile
+CREATE DOMAIN domain3 AS text DEFAULT foo(4);  -- stable
+CREATE DOMAIN domain4 AS text[]
+  DEFAULT ('{"This", "is", "' || foo(4) || '","the", "real", "world"}')::TEXT[];
+
+CREATE TABLE t2 (a domain1);
+INSERT INTO t2 VALUES (1),(2);
+
+-- no table rewrite
+ALTER TABLE t2 ADD COLUMN b domain1 default 3;
+
+SELECT attnum, attname, atthasmissing, atthasdef, attmissingval
+FROM pg_attribute
+WHERE attnum > 0 AND attrelid = 't2'::regclass
+ORDER BY attnum;
+
+-- table rewrite should happen
+ALTER TABLE t2 ADD COLUMN c domain3 default left(random()::text,3);
+
+-- no table rewrite
+ALTER TABLE t2 ADD COLUMN d domain4;
+
+SELECT attnum, attname, atthasmissing, atthasdef, attmissingval
+FROM pg_attribute
+WHERE attnum > 0 AND attrelid = 't2'::regclass
+ORDER BY attnum;
+
+-- table rewrite should happen
+ALTER TABLE t2 ADD COLUMN e domain2;
+
+SELECT attnum, attname, atthasmissing, atthasdef, attmissingval
+FROM pg_attribute
+WHERE attnum > 0 AND attrelid = 't2'::regclass
+ORDER BY attnum;
+
+SELECT a, b, length(c) = 3 as c_ok, d, e >= 10 as e_ok FROM t2;
+
+DROP TABLE t2;
+DROP DOMAIN domain1;
+DROP DOMAIN domain2;
+DROP DOMAIN domain3;
+DROP DOMAIN domain4;
 DROP FUNCTION foo(INT);
 
 -- Fall back to full rewrite for volatile expressions