List:Falcon Storage Engine« Previous MessageNext Message »
From:Jim Starkey Date:February 2 2009 2:32pm
Subject:Re: NULLs and stuff
View as plain text  
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.
Thread
NULLs and stuffLars-Erik Bjørk30 Jan
  • RE: NULLs and stuffVladislav Vaintroub30 Jan
    • Re: NULLs and stuffAnn W. Harrison30 Jan
  • Re: NULLs and stuffPhilip Stoev30 Jan
    • RE: NULLs and stuffVladislav Vaintroub30 Jan
      • RE: NULLs and stuffVladislav Vaintroub30 Jan
        • RE: NULLs and stuffHakan Kuecuekyilmaz30 Jan
          • RE: NULLs and stuffVladislav Vaintroub30 Jan
            • Re: NULLs and stuffJim Starkey1 Feb
      • Re: NULLs and stuffJim Starkey1 Feb
  • Re: NULLs and stuffKevin Lewis30 Jan
  • Re: NULLs and stuffAnn W. Harrison30 Jan
  • Re: NULLs and stuffJim Starkey1 Feb
    • Re: NULLs and stuffLars-Erik Bjørk2 Feb
      • Re: NULLs and stuffJim Starkey2 Feb
        • RE: NULLs and stuffVladislav Vaintroub2 Feb
          • Re: NULLs and stuffJim Starkey2 Feb
            • RE: NULLs and stuffVladislav Vaintroub2 Feb