From: Jorgen Loland
Date: February 9 2011 1:06pm
Subject: Re: bzr commit into mysql-trunk branch (jorgen.loland:3605) Bug#59793
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On 02/08/2011 12:53 PM, Tor Didriksen wrote:
>> +Item *Item_func_xor::neg_transformer(THD *thd)
>> +{
>> +  Item *neg_argument;
>> +  Item_func_xor *new_item;
>> +  if ((neg_argument= args[0]->neg_transformer(thd)))
>> +    // args[0] has neg_tranformer
>> +    new_item= new Item_func_xor(neg_argument, args[1]);
>> +  else if ((neg_argument= args[1]->neg_transformer(thd)))
>> +    // args[0] has neg_tranformer
>> +    new_item= new Item_func_xor(args[0], neg_argument);
>> +  else
>> +  {
>> +    neg_argument= new Item_func_not(args[0]);
>> +    new_item= new Item_func_xor(neg_argument, args[1]);
>
> If you use placement new here, you will save my_pthread_getspecific_ptr,
> and since you have the thd here anyways:
>
> new_item= new (thd->mem_root) Item_func_xor(neg_argument, args[1]);

Good suggestion. Thanks.

>> +  const char *func_name() const { return "xor"; }
>> +  longlong val_int();
>> +  void top_level_item() {}
>
> Could you explain why you override this virtual function?

IIUC, top_level_item() is used to tell an item that it doesn't matter if it 
returns FALSE or NULL. Consider

SELECT ...
WHERE cond1 AND cond2;

Here Item_cond_and has two conditions, and because

WHERE FALSE
and
WHERE NULL

are treated equally, Item_cond_and::val_int() can stop processing and return 0 
for this item if any of the operands evaluates to FALSE. That's because this AND 
is top-level, as in

      AND
      / \
cond1   cond2

If the AND is not top level, as in

WHERE cond0 OR (cond1 AND cond2)

the OR needs to know if AND evaluates to NULL or FALSE: If NULL, the OR is also 
NULL. If FALSE, OR evaluates to TRUE if cond0 is TRUE.

Note that even if it's not a top-level item, AND can still short circuit if 
either of it's operands turns out to be NULL. Top-level is about whether or not 
processing can stop when an operand is FALSE.

For XOR, a FALSE operand does not mean that processing can be short circuited, 
hence top_level_item() does nothing.

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