Fix psql \d's query for identifying parent triggers.

The original coding (from c33869cc3) failed with "more than one row
returned by a subquery used as an expression" if there were unrelated
triggers of the same tgname on parent partitioned tables.  (That's
possible because statement-level triggers don't get inherited.)  Fix
by applying LIMIT 1 after sorting the candidates by inheritance level.

Also, wrap the subquery in a CASE so that we don't have to execute it at
all when the trigger is visibly non-inherited.  Aside from saving some
cycles, this avoids the need for a confusing and undocumented NULLIF().

While here, tweak the format of the emitted query to look a bit
nicer for "psql -E", and add some explanation of this subquery,
because it badly needs it.

Report and patch by Justin Pryzby (with some editing by me).
Back-patch to v13 where the faulty code came in.

Discussion: https://postgr.es/m/20211217154356.GJ17618@telsasoft.com

src/bin/psql/describe.c

@@ -2990,16 +2990,38 @@ describeOneTableDetails(const char *schemaname,
 		printfPQExpBuffer(&buf,
 						  "SELECT t.tgname, "
 						  "pg_catalog.pg_get_triggerdef(t.oid, true), "
-						  "t.tgenabled, t.tgisinternal, %s\n"
+						  "t.tgenabled, t.tgisinternal,\n");
+
+		/*
+		 * Detect whether each trigger is inherited, and if so, get the name
+		 * of the topmost table it's inherited from.  We have no easy way to
+		 * do that pre-v13, for lack of the tgparentid column.  Even with
+		 * tgparentid, a straightforward search for the topmost parent would
+		 * require a recursive CTE, which seems unduly expensive.  We cheat a
+		 * bit by assuming parent triggers will match by tgname; then, joining
+		 * with pg_partition_ancestors() allows the planner to make use of
+		 * pg_trigger_tgrelid_tgname_index if it wishes.  We ensure we find
+		 * the correct topmost parent by stopping at the first-in-partition-
+		 * ancestry-order trigger that has tgparentid = 0.  (There might be
+		 * unrelated, non-inherited triggers with the same name further up the
+		 * stack, so this is important.)
+		 */
+		if (pset.sversion >= 130000)
+			appendPQExpBufferStr(&buf,
+								 "  CASE WHEN t.tgparentid != 0 THEN\n"
+								 "    (SELECT u.tgrelid::pg_catalog.regclass\n"
+								 "     FROM pg_catalog.pg_trigger AS u,\n"
+								 "          pg_catalog.pg_partition_ancestors(t.tgrelid) WITH ORDINALITY AS a(relid, depth)\n"
+								 "     WHERE u.tgname = t.tgname AND u.tgrelid = a.relid\n"
+								 "           AND u.tgparentid = 0\n"
+								 "     ORDER BY a.depth LIMIT 1)\n"
+								 "  END AS parent\n");
+		else
+			appendPQExpBufferStr(&buf, "  NULL AS parent\n");
+
+		appendPQExpBuffer(&buf,
 						  "FROM pg_catalog.pg_trigger t\n"
 						  "WHERE t.tgrelid = '%s' AND ",
-						  (pset.sversion >= 130000 ?
-						   "(SELECT (NULLIF(a.relid, t.tgrelid))::pg_catalog.regclass"
-						   " FROM pg_catalog.pg_trigger AS u, "
-						   "      pg_catalog.pg_partition_ancestors(t.tgrelid) AS a"
-						   " WHERE u.tgname = t.tgname AND u.tgrelid = a.relid"
-						   "       AND u.tgparentid = 0) AS parent" :
-						   "NULL AS parent"),
 						  oid);
 
 		/*

src/test/regress/expected/triggers.out

@@ -2122,6 +2122,20 @@ Triggers:
 alter table trigpart attach partition trigpart3 FOR VALUES FROM (2000) to (3000); -- fail
 ERROR:  trigger "trg1" for relation "trigpart3" already exists
 drop table trigpart3;
+-- check display of unrelated triggers
+create trigger samename after delete on trigpart execute function trigger_nothing();
+create trigger samename after delete on trigpart1 execute function trigger_nothing();
+\d trigpart1
+             Table "public.trigpart1"
+ Column |  Type   | Collation | Nullable | Default 
+--------+---------+-----------+----------+---------
+ a      | integer |           |          | 
+ b      | integer |           |          | 
+Partition of: trigpart FOR VALUES FROM (0) TO (1000)
+Triggers:
+    samename AFTER DELETE ON trigpart1 FOR EACH STATEMENT EXECUTE FUNCTION trigger_nothing()
+    trg1 AFTER INSERT ON trigpart1 FOR EACH ROW EXECUTE FUNCTION trigger_nothing(), ON TABLE trigpart
+
 drop table trigpart;
 drop function trigger_nothing();
 --

src/test/regress/sql/triggers.sql

@@ -1428,6 +1428,11 @@ create trigger trg1 after insert on trigpart3 for each row execute procedure tri
 alter table trigpart attach partition trigpart3 FOR VALUES FROM (2000) to (3000); -- fail
 drop table trigpart3;
 
+-- check display of unrelated triggers
+create trigger samename after delete on trigpart execute function trigger_nothing();
+create trigger samename after delete on trigpart1 execute function trigger_nothing();
+\d trigpart1
+
 drop table trigpart;
 drop function trigger_nothing();