A while ago I wrote about problem with NULLs and IN/=ANY subqueries MySQL had. I was completely correct when I wrote that the fix won’t be simple. It took 3 bug entries (BUG#8804, BUG#24085, BUG#24127), several review iterations by Igor, and the final patch is around 2,300 lines long.
The good news is that this patch solves the problem completely, and it is already in the 5.0.36 tree. The documentation is not yet updated, doing that is now on my todo. There is quite a lot to document: we’ve had to introduce “switchable access methods”, where a table accessed using ref is sometimes accessed using full table scan. (for the impatient: no, the new access method is not not called ref_or_null_or_all :-), it is still ref[_or_null] but with “Full scan on NULL key” in the “Extra” column).
I’ll post here when the docs become available.
Sergei Petrunia's blog 