fix JSON_ARRAYAGG not to over-quote json in joins

This replaces 8711adb786

if a temptable field is created for some json expression (is_json_type()
returns true), make this temptable field a proper json field.

A field is a json field (see Item_field::is_json_type()) if it
has a CHECK constraint of JSON_VALID(field).

Note that it will never be actually checked for temptable fields,
so it won't cause a run-time slowdown.

sql/sql_select.cc

@@ -18412,6 +18412,25 @@ Field *Item_func_sp::create_tmp_field_ex(MEM_ROOT *root, TABLE *table,
   return result;
 }
 
+
+static bool make_json_valid_expr(TABLE *table, Field *field)
+{
+  THD *thd= table->in_use;
+  Query_arena backup_arena;
+  Item *expr, *item_field;
+
+  if (!table->expr_arena && table->init_expr_arena(thd->mem_root))
+    return NULL;
+
+  thd->set_n_backup_active_arena(table->expr_arena, &backup_arena);
+  if ((item_field= new (thd->mem_root) Item_field(thd, field)) &&
+      (expr= new (thd->mem_root) Item_func_json_valid(thd, item_field)))
+    field->check_constraint= add_virtual_expression(thd, expr);
+  thd->restore_active_arena(table->expr_arena, &backup_arena);
+  return field->check_constraint == NULL;
+}
+
+
 /**
   Create field for temporary table.
 
@@ -18457,6 +18476,9 @@ Field *create_tmp_field(TABLE *table, Item *item,
                       make_copy_field);
   Field *result= item->create_tmp_field_ex(table->in_use->mem_root,
                                            table, &src, &prm);
+  if (item->is_json_type() && make_json_valid_expr(table, result))
+    result= NULL;
+
   *from_field= src.field();
   *default_field= src.default_field();
   if (src.item_result_field())

sql/table.cc

@@ -61,6 +61,17 @@ public:
   }
 };
 
+bool TABLE::init_expr_arena(MEM_ROOT *mem_root)
+{
+  /*
+    We need to use CONVENTIONAL_EXECUTION here to ensure that
+    any new items created by fix_fields() are not reverted.
+  */
+  expr_arena= new (alloc_root(mem_root, sizeof(Table_arena)))
+                Table_arena(mem_root, Query_arena::STMT_CONVENTIONAL_EXECUTION);
+  return expr_arena == NULL;
+}
+
 struct extra2_fields
 {
   LEX_CUSTRING version;
@@ -1155,14 +1166,8 @@ bool parse_vcol_defs(THD *thd, MEM_ROOT *mem_root, TABLE *table,
            table->s->table_check_constraints * sizeof(Virtual_column_info*));
 
   DBUG_ASSERT(table->expr_arena == NULL);
-  /*
+
-    We need to use CONVENTIONAL_EXECUTION here to ensure that
+  if (table->init_expr_arena(mem_root))
-    any new items created by fix_fields() are not reverted.
-  */
-  table->expr_arena= new (alloc_root(mem_root, sizeof(Table_arena)))
-                        Table_arena(mem_root, 
-                                    Query_arena::STMT_CONVENTIONAL_EXECUTION);
-  if (!table->expr_arena)
     DBUG_RETURN(1);
 
   thd->set_n_backup_active_arena(table->expr_arena, &backup_arena);

sql/table.h

@@ -1612,6 +1612,8 @@ public:
     m_needs_reopen= value;
   }
 
+  bool init_expr_arena(MEM_ROOT *mem_root);
+
   bool alloc_keys(uint key_count);
   bool check_tmp_key(uint key, uint key_parts,
                      uint (*next_field_no) (uchar *), uchar *arg);