Lars, I'm pretty comfortable with the first transformation since we have
discussed this one.
After original value to key conversion with makeKey, transform
NULL => zero length key
Zero length key (e.g empty string) => 0x00
Keys in form of 0x00<something> => 0x00 0x00 <something>
This seems to retain the original sorting when keys for memcmp based
comparison and making NULL less then everything else.
I think this fixed lot of issues. But, if we go for it, we need to issue a
big red "warning, incompatible change" in the release notes
Luckily we're still alpha and I think we can allow us to make such changes
without writing a conversion utility.
Adding blanks was Kevin's idea I have not understood.. yet.
> -----Original Message-----
> From: Lars-Erik.Bjork@stripped [mailto:Lars-Erik.Bjork@stripped]
> Sent: Friday, January 30, 2009 2:23 PM
> To: FalconDev
> Subject: NULLs and stuff
> 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)
> Being in this area, I picked up http://bugs.mysql.com/bug.php?id=23692
> again, which we have discussed on a previous meeting. The idea Kevin
> proposed here (if I remember correctly) was to append 0x20 to all
> upper-bound search keys ending in >= 0x20 (Kevin, please correct me if
> my memory is corrupt). This means passing an extra argument all the
> way from StorageTable::setIndexBounds down to Index::makeKey, because
> we don't want to store the extra byte in the regular index keys.
> How does the rest of you feel about these solutions? I get this hacky
> feeling:) We have to make sure there is always room for a 0x00 and a
> 0x20 in the key, meaning that the actual key can no longer be
> MAX_PHYSICAL_KEY_LENGTH long, but two less, as well as
> [prep,app]ending 0x00 and 0x20 being pretty magic.
> Is this something that you are comfortable with? I don't have any
> alternative solutions:)