Fixed bug #26963: invalid optimization of the pushdown conditions

after single-row table substitution could lead to a wrong result set.
The bug happened because the function Item_field::replace_equal_field
erroniously assumed that any field included in a multiple equality
with a constant has been already substituted for this constant.
This not true for fields becoming constant after row substitutions
for constant tables.
 


mysql-test/r/select.result:
  Added a test case for bug #26963.
mysql-test/t/select.test:
  Added a test case for bug #26963.
sql/item.cc:
  Fixed bug #26963: invalid optimization of the pushdown conditions
  after single-row table substitution could lead to a wrong result set.
  The bug happened because the function Item_field::replace_equal_field
  erroneously assumed that any field included in a multiple equality
  with a constant has been already substituted for this constant.
  This not true for fields becoming constant after row substitutions
  for constant tables.

1 files changed, 13 insertions, 2 deletions

diff --git a/sql/item.cc b/sql/item.cc
index ec33bf1ddc7..11a5039ca19 100644
--- a/sql/item.cc
+++ b/sql/item.cc
@@ -4075,7 +4075,9 @@ bool Item_field::set_no_const_sub(byte *arg)
   DESCRIPTION
     The function returns a pointer to an item that is taken from
     the very beginning of the item_equal list which the Item_field
-    object refers to (belongs to).  
+    object refers to (belongs to) unless item_equal contains  a constant
+    item. In this case the function returns this constant item, 
+    (if the substitution does not require conversion).   
     If the Item_field object does not refer any Item_equal object
     'this' is returned 
 
@@ -4084,7 +4086,8 @@ bool Item_field::set_no_const_sub(byte *arg)
     of the thransformer method.  
 
   RETURN VALUES
-    pointer to a replacement Item_field if there is a better equal item;
+    pointer to a replacement Item_field if there is a better equal item or
+    a pointer to a constant equal item;
     this - otherwise.
 */
 
@@ -4092,6 +4095,14 @@ Item *Item_field::replace_equal_field(byte *arg)
 {
   if (item_equal)
   {
+    Item *const_item= item_equal->get_const();
+    if (const_item)
+    {
+      if (cmp_context != (Item_result)-1 &&
+          const_item->cmp_context != cmp_context)
+        return this;
+      return const_item;
+    }
     Item_field *subst= item_equal->get_first();
     if (subst && !field->eq(subst->field))
       return subst;