MDEV-19134: EXISTS() slower if ORDER BY is defined

Step #2: "[ORDER BY ...] LIMIT n" should not prevent EXISTS-to-IN
conversion, as long as
- the LIMIT clause doesn't have OFFSET
- the LIMIT is not "LIMIT 0".

mysql-test/main/subselect4.result

@@ -2543,12 +2543,38 @@ create table t1(a int, b int);
 insert into t1 select
 A.a + B.a*10, A.a + B.a*10 from t0 A, t0 B;
 create table t2 as select * from t1;
-# This will not be able to convert to semi-join but will not require filesort:
+# This will be converted to semi-join:
 explain
-select * from t1 where exists (select * from t2 where t2.a=t1.a order by t2.b);
+select * from t1
+where exists (select * from t2 where t2.a=t1.a order by t2.b);
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	PRIMARY	t1	ALL	NULL	NULL	NULL	NULL	#	Using where
+1	PRIMARY	t1	ALL	NULL	NULL	NULL	NULL	100	
-2	DEPENDENT SUBQUERY	t2	ALL	NULL	NULL	NULL	NULL	#	Using where
+1	PRIMARY	<subquery2>	eq_ref	distinct_key	distinct_key	4	func	1	
+2	MATERIALIZED	t2	ALL	NULL	NULL	NULL	NULL	100	
+# query with a non-zero constant LIMIT is converted to semi-join, too:
+explain
+select * from t1
+where exists (select * from t2 where t2.a=t1.a order by t2.b limit 2);
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t1	ALL	NULL	NULL	NULL	NULL	100	
+1	PRIMARY	<subquery2>	eq_ref	distinct_key	distinct_key	4	func	1	
+2	MATERIALIZED	t2	ALL	NULL	NULL	NULL	NULL	100	
+# Zero LIMIT should prevent the conversion (but it is not visible atm
+#   due to MDEV-19429)
+explain
+select * from t1
+where exists (select * from t2 where t2.a=t1.a order by t2.b limit 0);
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t1	ALL	NULL	NULL	NULL	NULL	100	
+1	PRIMARY	<subquery2>	eq_ref	distinct_key	distinct_key	4	func	1	
+2	MATERIALIZED	t2	ALL	NULL	NULL	NULL	NULL	100	
+# LIMIT+OFFSET prevents the conversion:
+explain
+select * from t1
+where exists (select * from t2 where t2.a=t1.a order by t2.b limit 2,3);
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t1	ALL	NULL	NULL	NULL	NULL	100	Using where
+2	DEPENDENT SUBQUERY	t2	ALL	NULL	NULL	NULL	NULL	100	Using where; Using filesort
 # This will be merged and converted into a semi-join:
 explain
 select * from t1 where t1.a in (select t2.a from t2 order by t2.b);

mysql-test/main/subselect4.test

@@ -2088,14 +2088,31 @@ insert into t1 select
 
 create table t2 as select * from t1;
 
---echo # This will not be able to convert to semi-join but will not require filesort:
+--echo # This will be converted to semi-join:
---replace_column 9 #
 explain
-select * from t1 where exists (select * from t2 where t2.a=t1.a order by t2.b);
+select * from t1
+where exists (select * from t2 where t2.a=t1.a order by t2.b);
+
+--echo # query with a non-zero constant LIMIT is converted to semi-join, too:
+explain
+select * from t1
+where exists (select * from t2 where t2.a=t1.a order by t2.b limit 2);
+
+--echo # Zero LIMIT should prevent the conversion (but it is not visible atm
+--echo #   due to MDEV-19429)
+explain
+select * from t1
+where exists (select * from t2 where t2.a=t1.a order by t2.b limit 0);
+
+--echo # LIMIT+OFFSET prevents the conversion:
+explain
+select * from t1
+where exists (select * from t2 where t2.a=t1.a order by t2.b limit 2,3);
 
 --echo # This will be merged and converted into a semi-join:
 explain
 select * from t1 where t1.a in (select t2.a from t2 order by t2.b);
 
+
 drop table t0, t1, t2;
 

mysql-test/main/subselect_innodb.result

@@ -311,7 +311,7 @@ EXPLAIN SELECT 1 FROM t1 WHERE NOT EXISTS
 (SELECT 1 FROM t2 WHERE d = (SELECT d FROM t2 WHERE a >= 1) ORDER BY d);
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	t1	ALL	NULL	NULL	NULL	NULL	1	Using where
-2	DEPENDENT SUBQUERY	t2	eq_ref	PRIMARY,d	PRIMARY	1	func	1	Using where
+2	DEPENDENT SUBQUERY	t2	unique_subquery	PRIMARY,d	PRIMARY	1	func	1	Using where
 3	DEPENDENT SUBQUERY	t2	index	NULL	d	2	NULL	1	Using index
 DROP TABLE t2;
 CREATE TABLE t2 (b INT, c INT, UNIQUE KEY (b), UNIQUE KEY (b, c )) ENGINE=INNODB;

sql/item_subselect.cc

@@ -2906,7 +2906,6 @@ bool Item_exists_subselect::exists2in_processor(void *opt_arg)
                                 !upper_not->is_top_level_item())) ||
       first_select->is_part_of_union() ||
       first_select->group_list.elements ||
-      first_select->order_list.elements ||
       join->having ||
       first_select->with_sum_func ||
       !first_select->leaf_tables.elements||
@@ -2914,8 +2913,31 @@ bool Item_exists_subselect::exists2in_processor(void *opt_arg)
       with_recursive_reference)
     DBUG_RETURN(FALSE);
 
-  DBUG_ASSERT(first_select->order_list.elements == 0 &&
+  /*
-              first_select->group_list.elements == 0 &&
+    EXISTS-to-IN coversion and ORDER BY ... LIMIT clause:
+
+    - "[ORDER BY ...] LIMIT n" clause with a non-zero n does not affect
+      the result of the EXISTS(...) predicate, and so we can discard
+      it during the conversion.
+    - "[ORDER BY ...] LIMIT m, n" can turn a non-empty resultset into empty
+      one, so it affects tthe EXISTS(...) result and cannot be discarded.
+
+    Disallow exists-to-in conversion if
+    (1). three is a LIMIT which is not a basic constant
+    (1a)  or is a "LIMIT 0" (see MDEV-19429)
+    (2).  there is an OFFSET clause
+  */
+  if ((first_select->select_limit &&                        // (1)
+       (!first_select->select_limit->basic_const_item() ||  // (1)
+        first_select->select_limit->val_uint() == 0)) ||    // (1a)
+      first_select->offset_limit)                           // (2)
+  {
+    DBUG_RETURN(FALSE);
+  }
+
+  /* Disallow the conversion if offset + limit exists */
+
+  DBUG_ASSERT(first_select->group_list.elements == 0 &&
               first_select->having == NULL);
 
   if (find_inner_outer_equalities(&join->conds, eqs))
@@ -2946,9 +2968,6 @@ bool Item_exists_subselect::exists2in_processor(void *opt_arg)
   if ((uint)eqs.elements() > (first_select->item_list.elements +
                               first_select->select_n_reserved))
     goto out;
-  /* It is simple query */
-  DBUG_ASSERT(first_select->join->all_fields.elements ==
-              first_select->item_list.elements);
 
   arena= thd->activate_stmt_arena_if_needed(&backup);