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.