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 04e17bae50a73af524731fa11210d5c3f7d8e1f9 broken ALTER TABLE ..
SET STORAGE, which moved some actions from AT_PASS_COL_ATTRS to
AT_PASS_MISC, and commit 6c5723998594dffa5d47c3cf8c96ccf89c033aae broke
ALTER TABLE .. ADD COLUMN by changing the way that adding columns
recurses into child tables.

Noah Misch, with one comment change by me

2 files changed, 7 insertions, 13 deletions

diff --git a/src/backend/catalog/toasting.c b/src/backend/catalog/toasting.c
index 472237c4a0f..85fe57fb2a5 100644
--- a/src/backend/catalog/toasting.c
+++ b/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
  */
diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index 6afebc728f5..d3692754959 100644
--- a/src/backend/commands/tablecmds.c
+++ b/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);
 	}
 }