MDEV-33747: Optimization of (SELECT) IN (SELECT ...) executes subquery at prepare stage

Make IN->EXISTS rewrite not to compute constant left expression if it
has a subquery in it.

mysql-test/main/subselect4.result

@@ -3292,4 +3292,33 @@ a
 2
 DEALLOCATE PREPARE stmt;
 DROP TABLE t1,t2,t3;
+#
+# MDEV-33747: Optimization of (SELECT) IN (SELECT ...) executes subquery at prepare stage
+#
+create table t1 (a int, b int);
+insert into t1 select seq, seq from seq_1_to_200;
+create table t2 as select * from t1;
+create table t3 as select * from t1;
+analyze table t1,t2,t3;
+Table	Op	Msg_type	Msg_text
+test.t1	analyze	status	Engine-independent statistics collected
+test.t1	analyze	status	OK
+test.t2	analyze	status	Engine-independent statistics collected
+test.t2	analyze	status	OK
+test.t3	analyze	status	Engine-independent statistics collected
+test.t3	analyze	status	OK
+select @@expensive_subquery_limit < 200 as DEFAULTS_ARE_SUITABLE;
+DEFAULTS_ARE_SUITABLE
+1
+flush status;
+explain select * from t1 where a<3 or (select max(a) from t2) in (select b from t3);
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t1	ALL	NULL	NULL	NULL	NULL	200	Using where
+3	SUBQUERY	t3	ALL	NULL	NULL	NULL	NULL	200	Using where
+2	SUBQUERY	t2	ALL	NULL	NULL	NULL	NULL	200	
+# Must show 0. If this shows 200, this means subquery was executed and you have a bug:
+show status like 'Handler_read_rnd_next%';
+Variable_name	Value
+Handler_read_rnd_next	0
+drop table t1,t2,t3;
 # End of 10.4 tests

mysql-test/main/subselect4.test

@@ -2666,4 +2666,19 @@ DEALLOCATE PREPARE stmt;
 
 DROP TABLE t1,t2,t3;
 
+--echo #
+--echo # MDEV-33747: Optimization of (SELECT) IN (SELECT ...) executes subquery at prepare stage
+--echo #
+create table t1 (a int, b int);
+insert into t1 select seq, seq from seq_1_to_200;
+create table t2 as select * from t1;
+create table t3 as select * from t1;
+analyze table t1,t2,t3;
+select @@expensive_subquery_limit < 200 as DEFAULTS_ARE_SUITABLE;
+flush status;
+explain select * from t1 where a<3 or (select max(a) from t2) in (select b from t3);
+--echo # Must show 0. If this shows 200, this means subquery was executed and you have a bug:
+show status like 'Handler_read_rnd_next%';
+drop table t1,t2,t3;
+
 --echo # End of 10.4 tests

sql/item_cmpfunc.cc

@@ -1363,7 +1363,13 @@ bool Item_in_optimizer::fix_left(THD *thd)
   copy_with_sum_func(args[0]);
   with_param= args[0]->with_param || args[1]->with_param;
   with_field= args[0]->with_field;
-  if ((const_item_cache= args[0]->const_item()))
+
+  /*
+    If left expression is a constant, cache its value.
+    But don't do that if that involves computing a subquery, as we are in a
+    prepare-phase rewrite.
+  */
+  if ((const_item_cache= args[0]->const_item()) && !args[0]->with_subquery())
   {
     cache->store(args[0]);
     cache->cache_value();