Thursday, May 26, 2011

Annoyances, annoyances. Or Yet another HBF (Half Baked Feature)

About any product, be it computer hardware, software or any other product, has features that are annoying to some of us. But few products has so many features that are annoying to just about everyone as computer software. And among computer software, database software in particular seems to to have these features, which some people seems to like, and some just find annoying. And then there are features, or lack of them or implementation specific details that seems to annoy just about everyone. Things that work in a partuicular way because someone, somewhere, in some distant universe, had the notion that this was a good thing. Often features relating back to ancient times. And sometimes features that you just know work in a weirdo way because the person, if it was a person, figuring out the feature of the implementation of it, really must have been smoking something that is illegal in many parts of the world. Which is not to say that these features sometimes have a use or that the lack of a feature doesn't have a reason (like "It seemed like a good idea at the time").

And for some reason, MySQL seems to be hit with these things more than most software and database software products. And although I like working with MySQL, having done so for many years by now, there are certain aspects of it that just annoys the h*ll out of me. Things such as:
  • We have feature A, yes. And we we also support feature B. Great. But you cannot use feature A and B at the same time.
  • Oh, yeah, we support feature C, but really it is so slow that we recommend against using it.
  • Sure we support feature D, but the implementation really sucks.
  • Oh, storage engine E is sure supported, but the quality is so bad that we have disabled it completely.
  • On platform F, we don't really support feature G, as there is a limitation H on that platform. A limitation that was removed from that platform sometime during the Nixon administration, but all the same.
  • Feature I has been replaced by feature J, so using I will not be supported in the near future, but J still lacks the full functionality of I.
  • Oh, yes, you can sure do that! You can use either feature K or feature L, which have overlapping functionality and none is really feature complete in and of itself.
  • Our development team is hard at work at A) NOT getting rid of these annoyances and B) adding more of them.
So now I have another annoyance (I guess you could see that coming). Again, a new cool useful feature bogged down by someone, somewhere, in some universe wasn't thinking. InnoDB (a great Storage Engine BTW) has from version 5,5 of MySQL (and for longer than that in the plugin) been able to drop an index without rebuilding the whole table. Or rather, this is not a cool new feature unless you look at nothing else than MySQL, this feature was way WAAAY overdue. InnoDB also supports another neat feature, which is FOREIGN KEYS. You may say that FKs are no good for performance, and maybe that is so, but they are useful for, at a declarative level, ensure data consistency and they also add a level of "documentation". The implementation of FKs assumes that you have an index on the referencing column (why I do not know. There are cases when you don't want this.), but the rest is just in the declaration of the table and in the checks for FK consistency when issuing DML on the FK referencing and referenced tables or cource. So, if we now assume that in terms of data base data related to the table in question, the only thing that is added when we add an FK is an index, a fully normal index, then you should be able to drop the FK without rebuilding the table, as we can do that with the index? Right?

Nope. No cigar. Dropping an index is nearly instantaneous and doesn't require rebuilding the table, dropping the FK does. (I just tried this in 5.6, just to make a point. Same thing there). But have I not forgotten something, should I not be able to issue a:
SET FOREIGN_KEY_CHECKS=0;
And then drop the FK and then the index? Nope, that actually seems to make things worse and seems to confuse MySQL a lot. If you do this, you may drop the index, without the FK, and in a sense things now works, as although the FK is there, it isn't checked! Even if you set FOREIGN_KEY_CHECKS back to 1! This really makes no sense at all to me!

Playing with FOREIGN_KEY_CHECKS and droping FKs and indexes in MySQL 5.6 seems to confuse the h*ll out of MySQL also. I might one day get down to create a reproducible case here.

Anyway, why can't you drop an FK without rewriting the table? When the index can be dropped without rewriting the table? And don't tell me not to use FKs, if a feature is implemented it really should work, else remove it. And if I am not to use FKs with MySQL (in almost any other RDBMS they work just fine), then why were they implemented? No, FKs are useful, not always and sometimes they are too limiting for performance, but in many cases, they are all OK and really should work.

As Thomas Watson of IBM fame put it: THINK!

/Karlsson

2 comments:

Peter said...

This "SET FOREIGN_KEY_CHECKS=0;"
And then drop the FK and then the index? Nope, that actually seems to make things worse and seems to confuse MySQL a lot. If you do this, you may drop the index, without the FK" looks like a plain bug. You should report it as such, I think.

Maybe CREATE and DROP of FK's should be disallowed in "SET FOREIGN_KEY_CHECKS=0;" -mode?

dabest1 said...

Interesting article. Did you file a bug or feature request with MySQL by any chance to ask that foreign keys could be dropped instantly as well? And did you ever create a reproducible case?