This bug in the legacy code could manifest itself in queries with
semi-join materialized subqueries.
When a subquery is materialized all conditions that are imposed
only on the columns belonging to the tables from the subquery 
are taken into account.The code responsible for subquery optimizations
that employes subquery materialization  makes sure to remove these
conditions from the WHERE conditions of the query obtained after
it has transformed the original query into a query with a semi-join.
If the condition to be removed is an equality condition it could
be added to ON expressions and/or conditions from disjunctive branches
(parts of OR conditions) in an attempt to generate better access keys
to the tables of the query. Such equalities are supposed to be removed
later from all the formulas where they have been added to.
However, erroneously, this was not done in some cases when an ON
expression and/or a disjunctive part of the OR condition could
be converted into one multiple equality. As a result some equality
predicates over columns belonging to the tables of the materialized
subquery remained in the ON condition and/or the a disjunctive 
part of the OR condition, and the excuter later, when trying to
evaluate them, returned wrong answers as the values of the fields
from these equalities were not valid.  
This happened because any standalone multiple equality (a multiple
equality that are not ANDed with any other predicates) lacked
the information about equality predicates inherited from upper
levels (in particular, inherited from the WHERE condition).
The fix adds a reference to such information to any standalone
multiple equality.