MDEV-32475: Skip sorting if we will read one row

test_if_skip_sort_order() should catch the join types JT_EQ_REF,
JT_CONST and JT_SYSTEM and skip sort order for these.

Such join types imply retrieving of a single row of data, and sorting
of a single row can always be skipped.

mysql-test/main/order_by.result

@@ -3446,7 +3446,7 @@ CREATE TABLE t2 SELECT * FROM t1;
 EXPLAIN SELECT (SELECT 1 FROM t1 WHERE t1.a=t2.b ORDER BY t1.b LIMIT 1) AS c FROM t2;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	t2	ALL	NULL	NULL	NULL	NULL	2	
-2	DEPENDENT SUBQUERY	t1	index	PRIMARY	b	5	NULL	1	Using where
+2	DEPENDENT SUBQUERY	t1	eq_ref	PRIMARY	PRIMARY	4	test.t2.b	1	Using where
 SELECT (SELECT 1 FROM t1 WHERE t1.a=t2.b ORDER BY t1.b LIMIT 1) AS c FROM t2;
 c
 1
@@ -3706,4 +3706,23 @@ drop table t1,t2,t3,t4;
 SELECT 1.000000 two UNION SELECT 1 ORDER BY ( SELECT two LIMIT 1 OFFSET 1 ) ;
 two
 1.000000
+# MDEV-32475: test_if_skip_sort_order() should catch the join types
+#             JT_EQ_REF, JT_CONST and JT_SYSTEM and skip sort order for these
+#
+CREATE TABLE t1 (a INT PRIMARY KEY, b INT, KEY(b));
+CREATE TABLE t2 (a INT, b INT);
+INSERT INTO t1 SELECT seq, seq+1 FROM seq_1_to_100;
+INSERT INTO t2 VALUES (0, 1),(1, 2);
+ANALYZE TABLE t1, t2 PERSISTENT FOR ALL;
+Table	Op	Msg_type	Msg_text
+test.t1	analyze	status	Engine-independent statistics collected
+test.t1	analyze	status	Table is already up to date
+test.t2	analyze	status	Engine-independent statistics collected
+test.t2	analyze	status	OK
+# Table t1 must use eq_ref, not index below:
+EXPLAIN SELECT (SELECT 1 FROM t1 WHERE t1.a=t2.b ORDER BY t1.b LIMIT 1) AS c FROM t2;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t2	ALL	NULL	NULL	NULL	NULL	2	
+2	DEPENDENT SUBQUERY	t1	eq_ref	PRIMARY	PRIMARY	4	test.t2.b	1	Using where
+DROP TABLE t1,t2;
 # End of 10.4 tests

mysql-test/main/order_by.test

@@ -2452,4 +2452,20 @@ drop table t1,t2,t3,t4;
 --echo #
 SELECT 1.000000 two UNION SELECT 1 ORDER BY ( SELECT two LIMIT 1 OFFSET 1 ) ;
 
+--echo # MDEV-32475: test_if_skip_sort_order() should catch the join types
+--echo #             JT_EQ_REF, JT_CONST and JT_SYSTEM and skip sort order for these
+--echo #
+
+CREATE TABLE t1 (a INT PRIMARY KEY, b INT, KEY(b));
+CREATE TABLE t2 (a INT, b INT);
+INSERT INTO t1 SELECT seq, seq+1 FROM seq_1_to_100;
+INSERT INTO t2 VALUES (0, 1),(1, 2);
+
+ANALYZE TABLE t1, t2 PERSISTENT FOR ALL;
+
+--echo # Table t1 must use eq_ref, not index below:
+EXPLAIN SELECT (SELECT 1 FROM t1 WHERE t1.a=t2.b ORDER BY t1.b LIMIT 1) AS c FROM t2;
+
+DROP TABLE t1,t2;
+
 --echo # End of 10.4 tests

sql/sql_select.cc

@@ -23613,6 +23613,14 @@ test_if_skip_sort_order(JOIN_TAB *tab,ORDER *order,ha_rows select_limit,
   /* Check that we are always called with first non-const table */
   DBUG_ASSERT(tab == tab->join->join_tab + tab->join->const_tables);
 
+  /* Sorting a single row can always be skipped */
+  if (tab->type == JT_EQ_REF ||
+      tab->type == JT_CONST  ||
+      tab->type == JT_SYSTEM)
+  {
+    DBUG_RETURN(1);
+  }
+
   /*
     Keys disabled by ALTER TABLE ... DISABLE KEYS should have already
     been taken into account.