Vladislav Vaintroub wrote:
>
>> -----Original Message-----
>> From: Jim Starkey [mailto:jstarkey@stripped]
>> Sent: Monday, February 02, 2009 3:33 PM
>> To: Lars-Erik Bjørk; FalconDev
>> Subject: Re: NULLs and stuff
>>
>> Lars-Erik Bjørk wrote:
>>
>>> On Sat, 2009-01-31 at 18:31 -0500, Jim Starkey wrote:
>>>
>>>
>>>> Lars-Erik Bjørk wrote:
>>>>
>>>>
>>>>> Hi all!
>>>>>
>>>>> This week I have been looking at
>>>>> http://bugs.mysql.com/bug.php?id=42208 (Falcon's ORDER BY .. LIMIT
>>>>> gives wrong/inconsistent results on NULL values).
>>>>>
>>>>> The problem is that NULL values sort together with numeric zero. I
>>>>> have talked this issue over with Vlad, and have tried several
>>>>> different solutions to make it work for both single-field and
>>>>> multi-field indexes. What seems to work, is for
>>>>>
>> Index::makeKey(Field
>>
>>>>> *field, Value *value, int segment, IndexKey *indexKey) to return a
>>>>> zero length key for NULL values, and to prepend 0x00 to all keys
>>>>> starting with 0x00 (to make NULL sort before the empty (string),
>>>>>
>> and
>>
>>>>> to make the empty string, now 0x00, sort before the previous 0x00,
>>>>>
>> now
>>
>>>>> 0x0000, etc, etc)
>>>>>
>>>>>
>>>>>
>>>> I think it's a lot simpler than this. Remember that we do
>>>>
>> significant
>>
>>>> violence to numbers while turning them into keys -- turn them into
>>>> double precision and then zapping the sign bit (if positive) or
>>>> complimenting the whole thing (if negative) then truncating trailing
>>>> zeros. The upshot of this all is that a binary zero is actually
>>>>
>> stored
>>
>>>> as 0x80.
>>>>
>>>> The solution is to represent nulls (string and numeric) as one byte
>>>>
>> of
>>
>>>> 0x00. This will collate below everything (though equal to some 56
>>>>
>> bit
>>
>>>> negative integer).
>>>>
>>>>
>>>>
>>> Hmmm, won't NULL still sort strange with strings that are empty or
>>>
>> 0x00,
>>
>>> that are not subject to the same well deserved violence?:)
>>>
>>>
>>>
>> Yes, null strings and empty strings will be intermingled. I think we
>> can live with that.
>>
>
> Hmm, in this case order by (limit?) will probably still return wrong result with
> respect to NULLs, if empty string are involved.
>
>
Vlad, can you construct a scenario where someone needs to do a limit
with an order on a field with both nulls and empty strings, and cares
about the actual ordering of null strings and empty strings? Even
constructing a scenario where someone cares about the difference between
null string and empty strings is hard...
--
Jim Starkey
President, NimbusDB, Inc.
978 526-1376
