Remove arbitrary ALTER TABLE .. ADD COLUMN restriction.

The previous coding prevented ALTER TABLE .. ADD COLUMN from being used
with a non-NULL default in situations where the table's rowtype was being
used elsewhere.  But this is a completely arbitrary restriction since
you could do the same operation in multiple steps (add the column, add
the default, update the table).

Inspired by a patch from Noah Misch, though I didn't use his code.

src/backend/commands/tablecmds.c

@@ -142,6 +142,7 @@ typedef struct AlteredTableInfo
 	List	   *newvals;		/* List of NewColumnValue */
 	bool		new_notnull;	/* T if we added new NOT NULL constraints */
 	bool		new_changeoids; /* T if we added/dropped the OID column */
+	bool		new_changetypes; /* T if we changed column types */
 	Oid			newTableSpace;	/* new tablespace; 0 means no change */
 	/* Objects to rebuild after completing ALTER TYPE operations */
 	List	   *changedConstraintOids;	/* OIDs of constraints to rebuild */
@@ -3378,14 +3379,14 @@ ATRewriteTable(AlteredTableInfo *tab, Oid OIDNewHeap, LOCKMODE lockmode)
 	}
 
 	/*
-	 * If we need to rewrite the table, the operation has to be propagated to
-	 * tables that use this table's rowtype as a column type.
+	 * If we change column data types or add/remove OIDs, the operation has to
+	 * be propagated to tables that use this table's rowtype as a column type.
 	 *
 	 * (Eventually this will probably become true for scans as well, but at
 	 * the moment a composite type does not enforce any constraints, so it's
 	 * not necessary/appropriate to enforce them just during ALTER.)
 	 */
-	if (newrel)
+	if (tab->new_changetypes || tab->new_changeoids)
 		find_composite_type_dependencies(oldrel->rd_rel->reltype,
 										 RelationGetRelationName(oldrel),
 										 NULL);
@@ -6429,6 +6430,7 @@ ATPrepAlterColumnType(List **wqueue,
 		newval->expr = (Expr *) transform;
 
 		tab->newvals = lappend(tab->newvals, newval);
+		tab->new_changetypes = true;
 	}
 	else if (tab->relkind == RELKIND_FOREIGN_TABLE)
 	{

src/test/regress/expected/rowtypes.out

@@ -82,11 +82,14 @@ select * from people;
  (Joe,Blow) | 01-10-1984
 (1 row)
 
--- at the moment this will not work due to ALTER TABLE inadequacy:
+-- the default doesn't need to propagate through to the rowtypes, so this is OK
 alter table fullname add column suffix text default '';
-ERROR:  cannot alter table "fullname" because column "people"."fn" uses its rowtype
--- but this should work:
+alter table fullname drop column suffix;
+-- this one, without a default, is OK too
 alter table fullname add column suffix text default null;
+-- but this should fail, due to ALTER TABLE inadequacy
+alter table fullname alter column suffix set data type integer using null;
+ERROR:  cannot alter table "fullname" because column "people"."fn" uses its rowtype
 select * from people;
      fn      |     bd     
 -------------+------------

src/test/regress/sql/rowtypes.sql

@@ -45,12 +45,16 @@ insert into people values ('(Joe,Blow)', '1984-01-10');
 
 select * from people;
 
--- at the moment this will not work due to ALTER TABLE inadequacy:
+-- the default doesn't need to propagate through to the rowtypes, so this is OK
 alter table fullname add column suffix text default '';
+alter table fullname drop column suffix;
 
--- but this should work:
+-- this one, without a default, is OK too
 alter table fullname add column suffix text default null;
 
+-- but this should fail, due to ALTER TABLE inadequacy
+alter table fullname alter column suffix set data type integer using null;
+
 select * from people;
 
 -- test insertion/updating of subfields