Improve comments about AfterTriggerBeginQuery() query level usage.

diff --git a/src/backend/commands/trigger.c b/src/backend/commands/trigger.c
index fa74bd2cf10579f8fa790ac0bda5f257e0643a53..3e92a7c29e519cdf04abd5c6fc424ba924e88bde 100644 (file)
--- a/src/backend/commands/trigger.c
+++ b/src/backend/commands/trigger.c
@@ -3805,10 +3805,14 @@ AfterTriggerEndQuery(EState *estate)
         * IMMEDIATE: all events we have decided to defer will be available for it
         * to fire.
         *
-        * We loop in case a trigger queues more events at the same query level
-        * (is that even possible?).  Be careful here: firing a trigger could
-        * result in query_stack being repalloc'd, so we can't save its address
-        * across afterTriggerInvokeEvents calls.
+        * We loop in case a trigger queues more events at the same query level.
+        * Ordinary trigger functions, including all PL/pgSQL trigger functions,
+        * will instead fire any triggers in a dedicated query level.  Foreign key
+        * enforcement triggers do add to the current query level, thanks to their
+        * passing fire_triggers = false to SPI_execute_snapshot().  Other
+        * C-language triggers might do likewise.  Be careful here: firing a
+        * trigger could result in query_stack being repalloc'd, so we can't save
+        * its address across afterTriggerInvokeEvents calls.
         *
         * If we find no firable events, we don't have to increment
         * firing_counter.
@@ -4046,7 +4050,9 @@ AfterTriggerEndSubXact(bool isCommit)
 
                /*
                 * Release any event lists from queries being aborted, and restore
-                * query_depth to its pre-subxact value.
+                * query_depth to its pre-subxact value.  This assumes that a
+                * subtransaction will not add events to query levels started in a
+                * earlier transaction state.
                 */
                while (afterTriggers->query_depth > afterTriggers->depth_stack[my_level])
                {