Originally Posted by PAVB
Let me quote from the Metalink note, may be if pre-digested you will be able to see it a little clearly.
"The GROUP BY clause still performs sort operations but in version 10G, new internal Sort algorithms have been introduced which do not guarantee the order of data retrieval and may change the order for returned rows"
"Sort operation used in 9.2.0 optimizer is Sort by group while 10g optimizer is using HASH GROUP BY operation. Each sort algorithm orders data in a different way"