refs #6090 clustering grammar

git-svn-id: file:///svn/mysql.com/mariadb-5.5.30@54869 c7de825b-a66e-492c-adef-691d508d4ae1
refs #6090 query optimizer changes for clustering keys

git-svn-id: file:///svn/mysql.com/mariadb-5.5.30@54891 c7de825b-a66e-492c-adef-691d508d4ae1
#32 query plan diff due to find shortest clustering key patch