From: Roy Lyseng
Date: October 7 2010 6:48am
Subject: Re: bzr commit into mysql-5.5-bugteam branch (epotemkin:3213) Bug#57095
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On 07.10.10 08.29, Evgeny Potemkin wrote:

>> Do we have the same issue with other functions? E.g. CASE?
> The difference between coalesce and case is that case just passes val_xxx call
> to the appropriate item from the internal items array while coalesce is a
> descendant of Item_func_numhybrid and it operates in a bit different way.
> Item_func_numhybrid::val_xxx first calls xxx_op function of appropriate result
> type (str_op in case of datetime values) and then converts what it got to asked
> type. So there is no easy and safe way to fix it this way, IMO.
> With this fix even if there are similar problems they'll be hidden. Prior to it
> Item_cache_datetime expected a datetime returning item to also return correct
> int datetime representation. Now this is explicitly checked.
> However it's worth adding better implementation of result_as_longlong for
> IF/CASE functions. Will do in the next patch.

Just to chime in here, COALESCE is just a special case of a CASE, and could 
easily have been transformed directly to a CASE expression. Simpler 
optimization, simpler execution and less errors would be the result.

The same applies to NULLIF.

Thanks,
Roy