Clarify behavior of adding and altering a column in same ALTER command.

The behavior of something like

ALTER TABLE transactions
  ADD COLUMN status varchar(30) DEFAULT 'old',
  ALTER COLUMN status SET default 'current';

is to fill existing table rows with 'old', not 'current'.  That's
intentional and desirable for a couple of reasons:

* It makes the behavior the same whether you merge the sub-commands
into one ALTER command or give them separately;

* If we applied the new default while filling the table, there would
be no way to get the existing behavior in one SQL command.

The same reasoning applies in cases that add a column and then
manipulate its GENERATED/IDENTITY status in a second sub-command,
since the generation expression is really just a kind of default.
However, that wasn't very obvious (at least not to me; earlier in
the referenced discussion thread I'd thought it was a bug to be
fixed).  And it certainly wasn't documented.

Hence, add documentation, code comments, and a test case to clarify
that this behavior is all intentional.

In passing, adjust ATExecAddColumn's defaults-related relkind check
so that it matches up exactly with ATRewriteTables, instead of being
effectively (though not literally) the negated inverse condition.
The reasoning can be explained a lot more concisely that way, too
(not to mention that the comment now matches the code, which it
did not before).

Discussion: https://postgr.es/m/10365.1558909428@sss.pgh.pa.us

2 files changed, 12 insertions, 0 deletions

diff --git a/src/test/regress/expected/identity.out b/src/test/regress/expected/identity.out
index 7cf4696ec95..1a614b85f99 100644
--- a/src/test/regress/expected/identity.out
+++ b/src/test/regress/expected/identity.out
@@ -409,6 +409,12 @@ ALTER TABLE itest8
   ALTER COLUMN f5 DROP NOT NULL,
   ALTER COLUMN f5 SET DATA TYPE bigint;
 INSERT INTO itest8 VALUES(0), (1);
+-- This does not work when the table isn't empty.  That's intentional,
+-- since ADD GENERATED should only affect later insertions:
+ALTER TABLE itest8
+  ADD COLUMN f22 int NOT NULL,
+  ALTER COLUMN f22 ADD GENERATED ALWAYS AS IDENTITY;
+ERROR:  column "f22" contains null values
 TABLE itest8;
  f1 | f2 | f3 | f4 | f5 
 ----+----+----+----+----
diff --git a/src/test/regress/sql/identity.sql b/src/test/regress/sql/identity.sql
index 685607c90c1..b4cdd21bdd4 100644
--- a/src/test/regress/sql/identity.sql
+++ b/src/test/regress/sql/identity.sql
@@ -269,6 +269,12 @@ ALTER TABLE itest8
 
 INSERT INTO itest8 VALUES(0), (1);
 
+-- This does not work when the table isn't empty.  That's intentional,
+-- since ADD GENERATED should only affect later insertions:
+ALTER TABLE itest8
+  ADD COLUMN f22 int NOT NULL,
+  ALTER COLUMN f22 ADD GENERATED ALWAYS AS IDENTITY;
+
 TABLE itest8;
 \d+ itest8
 \d itest8_f2_seq