a worse execution plan than in 4.1 for some queries.
It happened due the fact that at some conditions the 
optimizer always preferred range or full index scan access
methods to lookup access methods even when the latter were much
cheaper. 
The problem was not observed in 4.1 for the reported query
because the WHERE condition was not of a form that could
cause the problem.
Equality propagation introduced on 5.0 added an extra 
predicate and changed the WHERE condition. The new condition
provoked the optimizer to make a bad choice.

The problem was fixed by the patch for bug 17379.


mysql-test/r/select.result:
  Added a test case for bug #18940.
  The problem was fixed by the patch for bug 17379.
mysql-test/t/select.test:
  Added a test case for bug #18940.
  The problem was fixed by the patch for bug 17379.