MDEV-27238: Assertion `got_name == named_item_expected()' failed in Json_writer

make_join_select() calls const_cond->val_int(). There are edge cases
where const_cond may have a not-yet optimized subquery.

(The subquery will have used_tables() covered by join->const_tables. It
will still have const_item()==false, so other parts of the optimizer
will not try to evaluate it.  We should probably mark such subqueries
as constant but that is outside the scope of this MDEV)

Author: spetrunia

mysql-test/main/opt_trace.result

@@ -2393,7 +2393,8 @@ select t1.a from t1 left join t2 on t1.a=t2.a	{
             "best_join_order": ["t2", "t1"]
           },
           {
-            "condition_on_constant_tables": "1"
+            "condition_on_constant_tables": "1",
+            "computing_condition": []
           },
           {
             "attaching_conditions_to_tables": {
@@ -2550,7 +2551,8 @@ explain select * from t1 left join t2 on t2.a=t1.a	{
             "best_join_order": ["t1", "t2"]
           },
           {
-            "condition_on_constant_tables": "1"
+            "condition_on_constant_tables": "1",
+            "computing_condition": []
           },
           {
             "attaching_conditions_to_tables": {
@@ -2708,7 +2710,8 @@ explain select t1.a from t1 left join (t2 join t3 on t2.b=t3.b) on t2.a=t1.a and
             "best_join_order": ["t3", "t2", "t1"]
           },
           {
-            "condition_on_constant_tables": "1"
+            "condition_on_constant_tables": "1",
+            "computing_condition": []
           },
           {
             "attaching_conditions_to_tables": {
@@ -3021,7 +3024,8 @@ explain extended select * from t1 where a in (select pk from t10)	{
             "best_join_order": ["t1", "<subquery2>"]
           },
           {
-            "condition_on_constant_tables": "1"
+            "condition_on_constant_tables": "1",
+            "computing_condition": []
           },
           {
             "attaching_conditions_to_tables": {
@@ -4719,7 +4723,8 @@ explain select * from t1 where a in (select t_inner_1.a from t1 t_inner_1, t1 t_
             "best_join_order": ["t1", "<subquery2>"]
           },
           {
-            "condition_on_constant_tables": "1"
+            "condition_on_constant_tables": "1",
+            "computing_condition": []
           },
           {
             "attaching_conditions_to_tables": {
@@ -7365,7 +7370,8 @@ t_outer_2.a in (select t_inner_3.a from t2 t_inner_3, t1 t_inner_4)	{
             ]
           },
           {
-            "condition_on_constant_tables": "1"
+            "condition_on_constant_tables": "1",
+            "computing_condition": []
           },
           {
             "attaching_conditions_to_tables": {
@@ -8632,5 +8638,14 @@ SELECT 'a\0' LIMIT 0	{
 }
 SET optimizer_trace=DEFAULT;
 #
+# MDEV-27238: Assertion `got_name == named_item_expected()' failed in Json_writer::on_start_object
+#
+CREATE TABLE t1 (a INT KEY,b INT,KEY(b)) ENGINE=MEMORY;
+SET optimizer_trace=1;
+INSERT INTO t1 VALUES (0,0);
+SELECT a FROM t1 WHERE (a,b) in (SELECT @c,@d);
+a
+DROP TABLE t1;
+#
 # End of 10.4 tests
 #

mysql-test/main/opt_trace.test

@@ -643,6 +643,16 @@ SELECT 'a\0' LIMIT 0;
 SELECT query, trace FROM INFORMATION_SCHEMA.OPTIMIZER_TRACE;
 SET optimizer_trace=DEFAULT;
 
+--echo #
+--echo # MDEV-27238: Assertion `got_name == named_item_expected()' failed in Json_writer::on_start_object
+--echo #
+
+CREATE TABLE t1 (a INT KEY,b INT,KEY(b)) ENGINE=MEMORY;
+SET optimizer_trace=1;
+INSERT INTO t1 VALUES (0,0);
+SELECT a FROM t1 WHERE (a,b) in (SELECT @c,@d);
+DROP TABLE t1;
+
 --echo #
 --echo # End of 10.4 tests
 --echo #

sql/sql_select.cc

@@ -11409,7 +11409,11 @@ make_join_select(JOIN *join,SQL_SELECT *select,COND *cond)
           }
           else
           {
-            const bool const_cond_result = const_cond->val_int() != 0;
+            bool const_cond_result;
+            {
+              Json_writer_array a(thd, "computing_condition");
+              const_cond_result= const_cond->val_int() != 0;
+            }
             if (!const_cond_result)
             {
               DBUG_PRINT("info",("Found impossible WHERE condition"));