Re: How does group by algorithm works?
In the original case, the data will be scanned in some order, probably the order it happens to exist in the data file. Or it might be scanned in the order of some other index it happens to use. After DELETE/UPDATE/REPLACE, the order in the data file becomes even more jumbled, hence less predictable.
The above discussion is for MyISAM tables. For InnoDB tables, the order will probably be the PRIMARY KEY order.
Bottom line -- don't trust the order unless you have an explicit ORDER BY.
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Re: How does group by algorithm works?
June 12, 2009 09:12AM
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