MDEV-36295 INTERVAL function returns inconsistent results when used in a WHERE clause

A query like

  select * from (select a, b from t1 group by a, b) d
    where interval(d.a, 3, 7) = 1

returned no rows when the condition was pushed into the derived table,
although the row (5,20) matches, as condition_pushdown_for_derived=off
shows.

Item_func_interval kept its first argument, the Item_row, in a member
of its own as well as in args[0]. Condition pushdown copies the condition
with deep_copy(). The copy of the node got new arguments but kept the
member, so it went on reading the original row: the INTERVAL() pushed into
the derived table compared the original outer column, and the pushed
condition rejected every row.

Item_func_interval now reads the row through args[0] alone, so a copy
cannot point anywhere else.

This commit was prepared with Claude Code (Opus 5).

mysql-test/main/derived_cond_pushdown.result

@@ -21808,3 +21808,13 @@ id	id	id
 0	3	3
 drop table t1,t2,t3;
 # End of 10.11 tests
+#
+# MDEV-36295 INTERVAL() pushed into a derived table
+#
+create table t1 (a int, b int);
+insert into t1 values (1,10),(5,20),(9,30);
+select * from (select a, b from t1 group by a, b) d where interval(d.a, 3, 7) = 1;
+a	b
+5	20
+drop table t1;
+# End of 13.2 tests

mysql-test/main/derived_cond_pushdown.test

@@ -4372,3 +4372,14 @@ select * from (select id  from t1 union select id from t2) dt1,
 drop table t1,t2,t3;
 
 --echo # End of 10.11 tests
+
+--echo #
+--echo # MDEV-36295 INTERVAL() pushed into a derived table
+--echo #
+
+create table t1 (a int, b int);
+insert into t1 values (1,10),(5,20),(9,30);
+select * from (select a, b from t1 group by a, b) d where interval(d.a, 3, 7) = 1;
+drop table t1;
+
+--echo # End of 13.2 tests

sql/item_cmpfunc.cc

@@ -1947,17 +1947,17 @@ bool Item_func_opt_neg::eq(const Item *item, const Eq_config &config) const
 
 bool Item_func_interval::fix_length_and_dec(THD *thd)
 {
-  uint rows= row->cols();
+  uint rows= row()->cols();
 
   for (uint i= 0 ; i < rows; i++)
   {
-    if (row->element_index(i)->check_cols(1))
+    if (row()->element_index(i)->check_cols(1))
       return true;
   }
 
-  use_decimal_comparison= ((row->element_index(0)->result_type() ==
+  use_decimal_comparison= ((row()->element_index(0)->result_type() ==
                             DECIMAL_RESULT) ||
-                           (row->element_index(0)->result_type() ==
+                           (row()->element_index(0)->result_type() ==
                             INT_RESULT));
   if (rows > 8)
   {
@@ -1965,7 +1965,7 @@ bool Item_func_interval::fix_length_and_dec(THD *thd)
 
     for (uint i= 1; not_null_consts && i < rows; i++)
     {
-      Item *el= row->element_index(i);
+      Item *el= row()->element_index(i);
       not_null_consts&= el->const_item() && !el->is_null();
     }
 
@@ -1979,7 +1979,7 @@ bool Item_func_interval::fix_length_and_dec(THD *thd)
       {
         for (uint i= 1; i < rows; i++)
         {
-          Item *el= row->element_index(i);
+          Item *el= row()->element_index(i);
           interval_range *range= intervals + (i-1);
           if ((el->result_type() == DECIMAL_RESULT) ||
               (el->result_type() == INT_RESULT))
@@ -2003,16 +2003,16 @@ bool Item_func_interval::fix_length_and_dec(THD *thd)
       {
         for (uint i= 1; i < rows; i++)
         {
-          intervals[i-1].dbl= row->element_index(i)->val_real();
+          intervals[i-1].dbl= row()->element_index(i)->val_real();
         }
       }
     }
   }
   base_flags&= ~item_base_t::MAYBE_NULL;
   max_length= 2;
-  used_tables_and_const_cache_join(row);
-  not_null_tables_cache= row->not_null_tables();
-  with_flags|= row->with_flags;
+  used_tables_and_const_cache_join(row());
+  not_null_tables_cache= row()->not_null_tables();
+  with_flags|= row()->with_flags;
   return false;
 }
 
@@ -2040,15 +2040,15 @@ longlong Item_func_interval::val_int()
 
   if (use_decimal_comparison)
   {
-    dec= row->element_index(0)->val_decimal(&dec_buf);
-    if (row->element_index(0)->null_value)
+    dec= row()->element_index(0)->val_decimal(&dec_buf);
+    if (row()->element_index(0)->null_value)
       return -1;
     my_decimal2double(E_DEC_FATAL_ERROR, dec, &value);
   }
   else
   {
-    value= row->element_index(0)->val_real();
-    if (row->element_index(0)->null_value)
+    value= row()->element_index(0)->val_real();
+    if (row()->element_index(0)->null_value)
       return -1;
   }
 
@@ -2056,7 +2056,7 @@ longlong Item_func_interval::val_int()
   {					// Use binary search to find interval
     uint start,end;
     start= 0;
-    end=   row->cols()-2;
+    end=   row()->cols()-2;
     while (start != end)
     {
       uint mid= (start + end + 1) / 2;
@@ -2082,9 +2082,9 @@ longlong Item_func_interval::val_int()
             value < range->dbl) ? 0 : start + 1;
   }
 
-  for (i=1 ; i < row->cols() ; i++)
+  for (i=1 ; i < row()->cols() ; i++)
   {
-    Item *el= row->element_index(i);
+    Item *el= row()->element_index(i);
     if (use_decimal_comparison &&
         ((el->result_type() == DECIMAL_RESULT) ||
          (el->result_type() == INT_RESULT)))

sql/item_cmpfunc.h

@@ -1159,7 +1159,6 @@ struct interval_range
 
 class Item_func_interval :public Item_long_func
 {
-  Item_row *row;
   bool use_decimal_comparison;
   interval_range *intervals;
   bool check_arguments() const override
@@ -1168,8 +1167,9 @@ class Item_func_interval :public Item_long_func
   }
 public:
   Item_func_interval(THD *thd, Item_row *a):
-    Item_long_func(thd, a), row(a), intervals(0)
+    Item_long_func(thd, a), intervals(0)
   { }
+  Item_row *row() const { return static_cast<Item_row*>(args[0]); }
   longlong val_int() override;
   bool fix_length_and_dec(THD *thd) override;
   LEX_CSTRING func_name_cstring() const override