From: Jorgen Loland
Date: February 9 2011 9:12am
Subject: Re: bzr commit into mysql-trunk branch (jorgen.loland:3605) Bug#59793
List-Archive: http://lists.mysql.com/commits/130807
Message-Id: <4D525A8B.7040401@oracle.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 8bit

On 02/08/2011 01:08 PM, Roy Lyseng wrote:
> Hi Jørgen,
>
> On 07.02.11 15.07, Jorgen Loland wrote:
>> #At file:///export/home/jl208045/mysql/mysql-trunk-59793/ based on
>> revid:tor.didriksen@stripped
>>
>> 3605 Jorgen Loland 2011-02-07
>> Bug#59793: crash in Item_field::register_field_in_read_map
>> with view
>>
>> Prior to the refactoring in this patch, Item_cond_xor behaved
>> partially as an Item_cond and partially as an Item_func. The
>> reasoning behind this was that XOR is currently not optimized
>> (thus should be Item_func instead of Item_cond), but it was
>> planned optimize it in the future (thus, made Item_cond anyway
>> to ease optimization later).
>
> I dislike this solution, because conceptually an XOR is an Item_cond,
> regardless of whether it is being optimized or not, and regardless of
> whether an optimization is planned for it.
>
> Conceptually, XOR belongs together with AND and OR and should be treated
> as such. I am afraid that making XOR an Item_func will make some future
> changes difficult, even though I have no concrete suspicion of this.

Since Item_xor_cond has mimiced the behavior of Item_func, I think (based on 
what I've seen when working on the XOR item) that it is

  * much more work and risk involved in making it a proper Item_cond instead
    of Item_func. The reason for both is that a lot of our code use these
    in an implicit manner [1]
  * almost the same amount of work to make Item_xor_func a proper Item_cond
    as it is to make the current Item_func-behaving Item_xor_cond a proper
    Item_cond.

I have created WL#5800 for this task, but I think it should be done as part of 
item tree refactoring. I volunteer to do this WL at the point we decide to start 
that activity.

[1] See WL#5800

-- 
Jørgen Løland | Senior Software Engineer | +47 73842138
Oracle MySQL
Trondheim, Norway