Bug#35201901: Server failure of MySQL #3

Two problems were identified for this bug. The first is seen by looking
at the reduced query:

select subq_1.c1 as c1
from (select subq_0.c0 as c0,
             subq_0.c0 as c1,
             90 as c2,
             subq_0.c1 as c3
      from (select (select v2 from table1) as c0,
                   ref_0.v4 as c1
            from table1 as ref_0
           ) as subq_0
      ) as subq_1
where EXISTS (select subq_1.c0 as c2,
                     case
                     when EXISTS (select (select v0 from table1) as c1
                                          from table1 as ref_8
                                          where EXISTS (select subq_1.c2 as c7
                                                        from table1 as ref_9
                                                       )
                                         )
                     then subq_1.c3
                     end as c5
              from table1 as ref_7);

In the innermost EXISTS predicate, a column subq_1.c2 is looked up.
It is erroneously found as the column subq_1.c0 with alias c2 in the
query block of the outermost EXISTS predicate. But this resolving is not
according to SQL standard: A table name cannot be part of a column alias,
it has to be a simple identifier, and any referencing column must also
be a simple identifier. By changing item_ref->item_name to
item_ref->field_name in a test in find_item_in_list, we ensure that the
match is against a table (view) name and column name and not an alias.

But there is also another problem. The EXISTS predicate contains a few
selected columns that are resolved and then immediately deleted since
they are redundant in EXISTS processing. But if these columns are
outer references and defined in a derived table, we may actually
de-reference them before the initial reference increment. Thus, those
columns are removed before they are possibly used later. This happens
to subq_1.c2 which is resolved in the outer-most query block and
coming from a derived table. We prevent this problem by incrementing
the reference count of selected expressions from derived tables earlier,
and we try to prevent this problem from re-occuring by adding an
"m_abandoned" field in class Item, which is set to true when the
reference count is decremented to zero and prevents the reference count
from ever be incremented after that.

Change-Id: Idda48ae726a580c1abdc000371b49a753e197bc6

Author: roylyseng

sql/item.h

@@ -3210,12 +3210,16 @@ class Item : public Parse_tree_node {
   bool is_blob_field() const;
 
   /// Increment reference count
-  void increment_ref_count() { ++m_ref_count; }
+  void increment_ref_count() {
+    assert(!m_abandoned);
+    ++m_ref_count;
+  }
 
   /// Decrement reference count
   uint decrement_ref_count() {
     assert(m_ref_count > 0);
-    return --m_ref_count;
+    if (--m_ref_count == 0) m_abandoned = true;
+    return m_ref_count;
   }
 
  protected:
@@ -3435,6 +3439,7 @@ class Item : public Parse_tree_node {
        be unused and can be removed.
   */
   uint m_ref_count{0};
+  bool m_abandoned{false};    ///< true if item has been fully de-referenced
   const bool is_parser_item;  ///< true if allocated directly by parser
   int8 is_expensive_cache;    ///< Cache of result of is_expensive()
   uint8 m_data_type;          ///< Data type assigned to Item

sql/sql_base.cc

@@ -8268,7 +8268,8 @@ bool find_item_in_list(THD *thd, Item *find, mem_root_deque<Item *> *items,
         Item_field for tables.
       */
       Item_ident *item_ref = down_cast<Item_ident *>(item);
-      if (item_ref->item_name.eq_safe(find_ident->field_name) &&
+      if (!my_strcasecmp(system_charset_info, item_ref->field_name,
+                         find_ident->field_name) &&
           item_ref->table_name != nullptr &&
           !my_strcasecmp(table_alias_charset, item_ref->table_name,
                          find_ident->table_name) &&
@@ -9061,6 +9062,11 @@ bool setup_fields(THD *thd, ulong want_privilege, bool allow_sum_func,
     if (!ref.is_null()) {
       ref[0] = item;
       ref.pop_front();
+      /*
+        Items present in ref_array have a positive reference count since
+        removal of unused columns from derived tables depends on this.
+      */
+      item->increment_ref_count();
     }
     Item *typed_item = nullptr;
     if (typed_items != nullptr && typed_it != typed_items->end()) {

sql/sql_resolver.cc

@@ -281,11 +281,6 @@ bool Query_block::prepare(THD *thd, mem_root_deque<Item *> *insert_field_list) {
                    insert_field_list, &fields, base_ref_items))
     return true;
 
-  // Ensure that all selected expressions have a positive reference count
-  for (auto it : fields) {
-    it->increment_ref_count();
-  }
-
   resolve_place = RESOLVE_NONE;
 
   const nesting_map save_allow_sum_func = thd->lex->allow_sum_func;