maria-developers team mailing list archive

[Sergei Petrunia <psergey@xxxxxxxxxxxx>]

Hi Elena,

On Thu, Jul 19, 2012 at 03:58:21PM +0400, Sergei Petrunia wrote:
> I've made a patch that fixes both BUG#992942 and MDEV-325.
...
> The code is at:
> bzr+ssh://bazaar.launchpad.net/~maria-captains/maria/5.5-show-explain-test1
> 
> it is based on 5.5-show-explain, so SHOW EXPLAIN part can be tested, too.
>

I've now also pushed a patch for MDEV-298 there. This means, now there are 
patches for all existing SHOW EXPLAIN issues. I need a round of testing for all
this. 

It is ok to combine testing of MDEV-325 and MDEV-298, however:

Fix for MDEV-298 is not a regular fix. Usually, fixes are expected to be a full
solution of the problem, i.e. after the fix everything should work.

However, ORDER/GROUP BY code is structured so that it makes a decision about
whether to do "Using filesort" at a very late stage. This means, there is no
way SHOW EXPLAIN could always be true about "Using filesort". Actually, even 
EXPLAIN is known to lie for certain queries).

The "right" solution would be to move the decision-making code, but I'm afraid
this could open a big can of worms, and it will go outside of the scope of SHOW
EXPLAIN patch, so I didn't do it.

Instead, I've coded what I think is a best approximation: SHOW EXPLAIN should 
produce correct results in all but the most complicated cases.

I'm still interested in testcases which show that EXPLAIN doesn't match SHOW
EXPLAIN but I will need to examine them to see whether they will be considered
SHOW EXPLAIN's (or EXPLAIN's) bugs.

BR
 Sergei
-- 
Sergei Petrunia, Software Developer
Monty Program AB, http://askmonty.org
Blog: http://s.petrunia.net/blog