diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index f976c0e5c7e..fcdb81dd4ea 100644
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -4901,8 +4901,22 @@ ATController(AlterTableStmt *parsetree,
  * Traffic cop for ALTER TABLE Phase 1 operations, including simple
  * recursion and permission checks.
  *
- * Caller must have acquired appropriate lock type on relation already.
+ * *wqueue: resulting AlteredTableInfo structs are added to this list
+ * rel: the relation we are currently considering
+ * cmd: the ALTER TABLE subcommand we are currently considering
+ * recurse: true to recurse to child tables of rel (including partitions)
+ * recursing: true if already recursing (rel is a descendant of original table)
+ * lockmode: lock level held on rel, and to be acquired on children
+ * context: context passed down from ProcessUtility, or NULL if none
+ *
+ * At top level, recurse is true unless the command specified ONLY.
+ * Internally, it can be true if we are descending the inheritance tree
+ * on-the-fly, or false if we already expanded the tree at an outer level
+ * (see ATSimpleRecursion).
+ *
+ * Caller must have acquired lockmode on rel already.
  * This lock should be held until commit.
+ * If we recurse, the same lockmode will be acquired on child tables.
  */
 static void
 ATPrepCmd(List **wqueue, Relation rel, AlterTableCmd *cmd,