Fix toast table creation.

Instead of using slightly-too-clever heuristics to decide when we must
create a TOAST table, just check whether one is needed every time the
table is altered.  Checking whether a toast table is needed is cheap
enough that we needn't worry about doing it on every ALTER TABLE command,
and the previous coding is apparently prone to accidental breakage:
commit 04e17bae50 broken ALTER TABLE ..
SET STORAGE, which moved some actions from AT_PASS_COL_ATTRS to
AT_PASS_MISC, and commit 6c57239985 broke
ALTER TABLE .. ADD COLUMN by changing the way that adding columns
recurses into child tables.

Noah Misch, with one comment change by me

src/backend/catalog/toasting.c

@@ -59,11 +59,11 @@ AlterTableCreateToastTable(Oid relOid, Datum reloptions)
 	Relation	rel;
 
 	/*
-	 * Grab an exclusive lock on the target table, which we will NOT release
-	 * until end of transaction.  (This is probably redundant in all present
-	 * uses...)
+	 * Grab a DDL-exclusive lock on the target table, since we'll update the
+	 * pg_class tuple.  This is redundant for all present users.  Tuple toasting
+	 * behaves safely in the face of a concurrent TOAST table add.
 	 */
-	rel = heap_open(relOid, AccessExclusiveLock);
+	rel = heap_open(relOid, ShareUpdateExclusiveLock);
 
 	/* create_toast_table does all the work */
 	(void) create_toast_table(rel, InvalidOid, InvalidOid, reloptions);
@@ -103,7 +103,7 @@ BootstrapToastTable(char *relName, Oid toastOid, Oid toastIndexOid)
 /*
  * create_toast_table --- internal workhorse
  *
- * rel is already opened and exclusive-locked
+ * rel is already opened and locked
  * toastOid and toastIndexOid are normally InvalidOid, but during
  * bootstrap they can be nonzero to specify hand-assigned OIDs
  */

src/backend/commands/tablecmds.c

@@ -3022,18 +3022,12 @@ ATRewriteCatalogs(List **wqueue, LOCKMODE lockmode)
 		}
 	}
 
-	/*
-	 * Check to see if a toast table must be added, if we executed any
-	 * subcommands that might have added a column or changed column storage.
-	 */
+	/* Check to see if a toast table must be added. */
 	foreach(ltab, *wqueue)
 	{
 		AlteredTableInfo *tab = (AlteredTableInfo *) lfirst(ltab);
 
-		if (tab->relkind == RELKIND_RELATION &&
-			(tab->subcmds[AT_PASS_ADD_COL] ||
-			 tab->subcmds[AT_PASS_ALTER_TYPE] ||
-			 tab->subcmds[AT_PASS_COL_ATTRS]))
+		if (tab->relkind == RELKIND_RELATION)
 			AlterTableCreateToastTable(tab->relid, (Datum) 0);
 	}
 }

src/test/regress/expected/alter_table.out

@@ -1522,6 +1522,19 @@ ERROR:  composite type recur1 cannot be made a member of itself
 alter table recur1 add column f2 int;
 alter table recur1 alter column f2 type recur2; -- fails
 ERROR:  composite type recur1 cannot be made a member of itself
+-- SET STORAGE may need to add a TOAST table
+create table test_storage (a text);
+alter table test_storage alter a set storage plain;
+alter table test_storage add b int default 0; -- rewrite table to remove its TOAST table
+alter table test_storage alter a set storage extended; -- re-add TOAST table
+select reltoastrelid <> 0 as has_toast_table
+from pg_class
+where oid = 'test_storage'::regclass;
+ has_toast_table 
+-----------------
+ t
+(1 row)
+
 --
 -- lock levels
 --

src/test/regress/input/misc.source

@@ -153,6 +153,12 @@ SELECT * FROM e_star*;
 
 ALTER TABLE a_star* ADD COLUMN a text;
 
+-- That ALTER TABLE should have added TOAST tables.
+SELECT relname, reltoastrelid <> 0 AS has_toast_table
+   FROM pg_class
+   WHERE oid::regclass IN ('a_star', 'c_star')
+   ORDER BY 1;
+
 --UPDATE b_star*
 --   SET a = text 'gazpacho'
 --   WHERE aa > 4;

src/test/regress/output/misc.source

@@ -376,6 +376,17 @@ SELECT * FROM e_star*;
 
 ALTER TABLE a_star* ADD COLUMN a text;
 NOTICE:  merging definition of column "a" for child "d_star"
+-- That ALTER TABLE should have added TOAST tables.
+SELECT relname, reltoastrelid <> 0 AS has_toast_table
+   FROM pg_class
+   WHERE oid::regclass IN ('a_star', 'c_star')
+   ORDER BY 1;
+ relname | has_toast_table 
+---------+-----------------
+ a_star  | t
+ c_star  | t
+(2 rows)
+
 --UPDATE b_star*
 --   SET a = text 'gazpacho'
 --   WHERE aa > 4;

src/test/regress/sql/alter_table.sql

@@ -1133,6 +1133,16 @@ alter table recur1 add column f2 recur2; -- fails
 alter table recur1 add column f2 int;
 alter table recur1 alter column f2 type recur2; -- fails
 
+-- SET STORAGE may need to add a TOAST table
+create table test_storage (a text);
+alter table test_storage alter a set storage plain;
+alter table test_storage add b int default 0; -- rewrite table to remove its TOAST table
+alter table test_storage alter a set storage extended; -- re-add TOAST table
+
+select reltoastrelid <> 0 as has_toast_table
+from pg_class
+where oid = 'test_storage'::regclass;
+
 --
 -- lock levels
 --