MDEV-25380: JSON_TABLE: Assertion `join->best_read < double(1.797...) fails

The query used a subquery of this form:

SELECT ...
WHERE
   EXISTS( SELECT ...
           FROM JSON_TABLE(outer_ref, ..) as JT
           WHERE trivial_correlation_cond)

EXISTS-to-IN conversion code was unable to see that the subquery will
still be correlated after the trivial_correlation is removed, which
eventually caused a crash due to inability to construct a query plan.

Fixed by making Item_subselect::walk() also walk arguments of Table
Functions.

mysql-test/suite/json/r/json_table.result

@@ -734,5 +734,21 @@ Shirt	blue	20
 Shirt	white	20
 drop table t1;
 #
+# MDEV-25380: JSON_TABLE: Assertion `join->best_read < double(1.797...) fails
+#
+CREATE TABLE t1 (a INT, b TEXT);
+INSERT INTO t1 VALUES (1,'{}'),(2,'[]');
+explain
+SELECT *
+FROM t1
+WHERE
+EXISTS(SELECT *
+FROM JSON_TABLE(b, '$' COLUMNS(o FOR ORDINALITY)) AS jt
+WHERE jt.o = t1.a);
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t1	ALL	NULL	NULL	NULL	NULL	2	
+1	PRIMARY	jt	ALL	NULL	NULL	NULL	NULL	40	Table function: json_table; Using where; FirstMatch(t1)
+drop table t1;
+#
 # End of 10.6 tests
 #

mysql-test/suite/json/t/json_table.test

@@ -627,6 +627,23 @@ select t.item_name, jt.* from (select t1.item_name, concat(concat(concat("{\"col
 
 drop table t1;
 
+--echo #
+--echo # MDEV-25380: JSON_TABLE: Assertion `join->best_read < double(1.797...) fails
+--echo #
+
+CREATE TABLE t1 (a INT, b TEXT);
+INSERT INTO t1 VALUES (1,'{}'),(2,'[]');
+
+explain
+SELECT *
+FROM t1
+WHERE
+  EXISTS(SELECT *
+         FROM JSON_TABLE(b, '$' COLUMNS(o FOR ORDINALITY)) AS jt
+         WHERE jt.o = t1.a);
+
+drop table t1;
+
 --echo #
 --echo # End of 10.6 tests
 --echo #

sql/item_subselect.cc

@@ -719,6 +719,34 @@ bool Item_subselect::unknown_splocal_processor(void *argument)
 }
 
 
+static
+int walk_table_functions_for_list(Item_processor processor,
+                                  bool walk_subquery, void *argument,
+                                  List<TABLE_LIST>& join_list)
+{
+  List_iterator<TABLE_LIST> li(join_list);
+  int res;
+  while (TABLE_LIST *table= li++)
+  {
+    if (Table_function_json_table *tf= table->table_function)
+    {
+      if ((res= tf->walk_items(processor, walk_subquery, argument)))
+      {
+        return res;
+      }
+    }
+    if (table->nested_join)
+    {
+      if ((res= walk_table_functions_for_list(processor, walk_subquery,
+                                              argument,
+                                              table->nested_join->join_list)))
+        return res;
+    }
+  }
+  return 0;
+}
+
+
 bool Item_subselect::walk(Item_processor processor, bool walk_subquery,
                           void *argument)
 {
@@ -751,6 +779,11 @@ bool Item_subselect::walk(Item_processor processor, bool walk_subquery,
                                              argument))
         return 1;
       /* TODO: why does this walk WHERE/HAVING but not ON expressions of outer joins? */
+      /*       Consider walking ON epxression in walk_table_functions_for_list */
+
+      if (walk_table_functions_for_list(processor, walk_subquery, argument,
+                                        *lex->join_list))
+        return 1;
 
       while ((item=li++))
       {

sql/json_table.cc

@@ -1341,6 +1341,12 @@ int Table_function_json_table::setup(THD *thd, TABLE_LIST *sql_table,
   return FALSE;
 }
 
+int Table_function_json_table::walk_items(Item_processor processor,
+                                          bool walk_subquery, void *argument)
+{
+  return m_json->walk(processor, walk_subquery, argument);
+}
+
 void Table_function_json_table::get_estimates(ha_rows *out_rows,
                                               double *scan_time,
                                               double *startup_cost)

sql/json_table.h

@@ -200,6 +200,9 @@ class Table_function_json_table : public Sql_alloc
   /*** Name resolution functions ***/
   int setup(THD *thd, TABLE_LIST *sql_table, SELECT_LEX *s_lex);
 
+  int walk_items(Item_processor processor, bool walk_subquery,
+                 void *argument);
+
   /*** Functions for interaction with the Query Optimizer ***/
   void fix_after_pullout(TABLE_LIST *sql_table,
                          st_select_lex *new_parent, bool merge);