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.

into  zippy.(none):/home/cmiller/work/mysql/mysql-5.0-maint

into  mysql.com:/home/mysql-5.0-19263

  Compile in all character sets in Max (bug#14918)

into  production.mysql.com:/usersnfs/rkalimullin/mysql-5.0

into  production.mysql.com:/usersnfs/rkalimullin/mysql-5.0

into  zippy.(none):/home/cmiller/mysql-5.0__bug19564

Re-work best_access_path() and find_best() to reuse E(#rows(range access)) as
E(#rows(ref[_or_null](const) access) only when it is appropriate.
[This is the final cumulative patch]

into  mysql.com:/usr/home/ram/work/mysql-5.0

into  zippy.(none):/home/cmiller/mysql-5.0__bug19564

into  mysql.com:/usr/home/ram/work/4.1.b18501

into  mysql.com:/home/kgeorge/mysql/5.0/B18068

Correct a bug (that I introduced, after using Oracle's database software for 
too many years) where the length of the database-sent data is incorrectly 
used to infer NULLness.

into  mysql.com:/home/psergey/mysql-5.0-best_access_path_j-push

into  mysql.com:/home/psergey/mysql-5.0-best_access_path_j-push

into  xiphis.org:/home/antony/work2/mysql-5.0-engines-merge

into  xiphis.org:/home/antony/work2/p1-bug10952.1

  "alter table from MyISAM to MERGE lost data without errors and warnings"
  Add new handlerton flag which prevent user from altering table storage
  engine to storage engines which would lose data. Both 'blackhole' and 
  'merge' are marked with the new flag.
  Tests included.

When converting DISTINCT to GROUP BY where the columns are from the covering
index and they are quoted twice in the SELECT list the optimizer is creating
improper processing sequence. This is because of the fact that the columns
of the covering index are not recognized as such and treated as non-index
columns.

Generally speaking duplicate columns can safely be removed from the GROUP
BY/DISTINCT list because this will not add or remove new rows in the
resulting set. Duplicates can be removed even if they are not consecutive
(as is the case for ORDER BY, where the duplicate columns can be removed
only if they are consecutive).

So we can safely transform "SELECT DISTINCT a,a FROM ... ORDER BY a" to
"SELECT a,a FROM ... GROUP BY a ORDER BY a" instead of 
"SELECT a,a FROM .. GROUP BY a,a ORDER BY a". We can even transform 
"SELECT DISTINCT a,b,a FROM ... ORDER BY a,b" to
"SELECT a,b,a FROM ... GROUP BY a,b ORDER BY a,b".

The fix to this bug consists of checking for duplicate columns in the SELECT
list when constructing the GROUP BY list in transforming DISTINCT to GROUP
BY and skipping the ones that are already in.

into  neptunus.(none):/home/msvensson/mysql/mysql-5.0-maint

into  neptunus.(none):/home/msvensson/mysql/mysql-5.0-maint

into  mysql.com:/home/marty/MySQL/mysql-5.0

into  mysql.com:/home/marty/MySQL/mysql-5.0

 - Add a check to see if the .mysql_history file a symlink to /dev/null 
   and in such case, skip reading and writing to it. 
     

into  neptunus.(none):/home/msvensson/mysql/mysql-5.0

into  neptunus.(none):/home/msvensson/mysql/mysql-5.0-maint

No commit message

into mysql.com:/Users/kent/mysql/bk/mysql-5.0-new

Add function 'vio_end' that will cleanup resources allocated by vio and the components it uses.

into  neptunus.(none):/home/msvensson/mysql/mysql-5.0-maint

into  neptunus.(none):/home/msvensson/mysql/mysql-5.0-maint

save global server variables before fiddling with them and restore them later

into  rurik.mysql.com:/home/igor/mysql-5.0

into  rurik.mysql.com:/home/igor/dev/mysql-5.0-0

into  rurik.mysql.com:/home/igor/mysql-4.1

into  mysql.com:/usr_rh9/home/elkin.rh9/MySQL/Merge/5.0-bug19136

recalculating results