While it's not always good design to use triggers for this, sometimes, it's a valid reason to use them for integrity checking or enforcing. Having `on delete` triggers not run for some delete's is violating the principle of least surprise.
When feature break this way, people start to distrust them and best practices get adopted that discourage using them, killing the features all together.
Using a database that does not have this misfeature, both triggers and foreign keys are perfectly safe to use, but because the one database that is the most widely used has issues like the one described here, you often hear the recommendation to not use triggers or even foreign keys.
Yes, you can potentially move the logic normally contained in either of them into the application, but as you deal with more concurrency and as you start accumulating bugs, sooner or later your data will be messy and you will need to add cleanup-scripts, or, heaven forbid, work around messy data in your application logic (don't. do. that. it's the path to the dark side of unmaintainability).
Or, of course, use a database system whose features work correctly and the principle of least surprise is in effect.
PostgreSQL is one of these, by the way.
It makes apple's application simple stupid and efficient.
And they have triggers on foreign keys... with sqlite.
...
It is not postgres in one of it, it is much more mysql is not a correct rdbms. None of the other databases have this flaw.
Mysql is to RDBMS what mongo is to NoSQL, nowhere short of achieving anything that matters correctly.
EDIT: or better: Mysql is a realistic in the Hollywood way implementation of a RDBMS
postgres usually outperforms mysql for more complex operations, but is not as fast as mysql for simple ones.
sqlite is really only appropriate for things like mobile or desktop applications or things with generally low concurrency requirements.
i would be careful throwing the baby out with the bath water in your dismissal of mysql, but thats just my opinion:)
can you shed some more detail on that apple uses triggers with sqlite? that is interesting to me, and would like to learn more.
I agree with most of what you said, but in many many use cases data integrity can be maintained in application code (a statically typed system helps), it simply can't be true that all those applications running on NoSql database will have messy data and/or cleanup-scripts.
nosql or sql large platforms almost always have some amount of work always ongoing to clean, shuttle, and maintain data integrity.
that doesn't make me not a fan of them; for lower transaction rate applications ( which most probably are ), they are a small time investment to have some sanity checking. but it's a right tool for the job thing.
this is yet another reason why people dislike FKs in best practice stuff.
Edit: Hi down-voters. Curious to hear why I'm being down voted and possibly be offer a rebuttal?
https://bugs.mysql.com/search.php?search_for=&status=Active&...
That's all the active bugs against MySQL. Note that this 10 year old bug isn't the oldest.
This is the oldest, from March 2003: https://bugs.mysql.com/bug.php?id=199 Note someone submitted a fix for that over a year ago (and verified under the OCA in November) but it still hasn't shipped. This is a bug that should never have taken 11 years to fix.
https://bugs.mysql.com/bug.php?id=3052 This one is great. ROLLBACK in a stored procedure doesn't close the cursor.
MySQL, for all its strengths, often feels like there is a bunch of typical geeks running the show. Lots of focus on the 'sexy' new features, not so much focus on keeping the lights on.
We are currently looking into bug 199. The patch itself is probably too large to backport into a GA release.
- Triggers are at the SQL Layer
- Foreign keys are implemented natively by InnoDB at the storage engine layer
It is not as easy to fix as it sounds.
I guess every project as a few "we'll fix it some day" tickets.
MariaDB documents "foreign keys do not activate triggers" as the standard behavior:
https://mariadb.com/kb/en/mariadb/trigger-limitations/
Personally, I am grateful for software that changes as seldom as possible. I don't want to spend time on "updating" my application because something down in the stack changed.
Application can just not update, so there is no need for windows level of compatibility management
Personally never jumped on the mysql train and can say I'm really glad for that.
[30 Jun 2005 19:04] Dmitry Lenev We will fix this in 5.1
(For those interested, it's the ability to create a new, empty file on the server.)
One of the major reasons to pick open-source software over a proprietary software is you can fix bugs that the vendor or developers won't.
Well it is true mysql is fast ... when it is not a problem.
They sacrificed correctness (stuff about big O notation in worst case) for benchmarks... and it works ... amazingly well as long as you are not under heavy load or give up on integrity, or relationship, or correctness.
Amazing. As long as you store data that need not to be transactional, or relational, or needing integrity mysql is fast. When data matters, or load is heavy mysql is just not there. It is chaotic, inconsistent, unicode retarded... full of pits... and resource greedy.
Sure, if you are a masochist, or if you fear to loose your job and need to elevate pain to the rank of art, then mysql is alright.
I have been sysadmin as long as developer and on both sides of the track mysql is insanely not consistent. (sysadmin: 400 config parameters, replication that fear any butterfly flapping its wings near a network cable, dev: collation behaviour, (clumsy) LDAP integration for auth, inconsistencies...).
I even prefer MSsql to mysql. I have been working almost only in linux environment since 2000, and I still think MySQL is crap. 10 years of horror stories with mysql made me hate it as much as taking LSD before trying to fill your taxes.
For a short vademecum I was not going to add noise in the post.
I think what you mean here is low concurrent write requirements.
(Edit in response to below: Isn't sqlite file oriented and read-only opens eschew locking? Thus replication via network filesystems should be adequate, no?)
Other things like replication, are also not there, so it's not what I would choose for a website database, but if you can serve all your reads from one server, and don't care about high availability, then go for it.
Yes, foreign keys were essential to data correctness. No, they were not being enforced in production. Stew on that just a bit. Not enforced in production.
During client application development, development databases enforced the constraint. Any errors resulting in foreign key mismatches disqualified the client app for release. The applications were required to be aware of the constraint and to learn to work within it. Production gained the speed benefits of not having to enforce the foreign keys.
You can indeed eat your cake and have it, too, iff you are disciplined.
It is very common to have many applications, from different vendors and from different time periods interacting with a database.
It often also happens that data can be modified directly in the database, not via the application. I agree this is not best practice but in larger enterprises it happens.
Once you start implementing foreign keys in the client you are adding a lot of complexity to a problem that most mature relational database system do well enough to be trusted by a lot of traditional financial institutions.
Facebooks mysql architecture (at least used to, likely still is) based on this: https://backchannel.org/blog/friendfeed-schemaless-mysql
Note the simple schema, and lack of FKs.
FKs are rarely present in extremely high transactions per second systems that operate on the same few tables. This is due to index contention and locking.
I say this with lots of experience working on different production systems that see hundreds of thousands of transactions per second. I have yet to see one arrive at these kind of numbers using foreign keys, unless it's something like a giant shared hosting platform, that is operating on hundreds and hundreds of different tables. (therefore, less index contention)
Often, in these shops, data correctness is not validated in realtime in this way, but often in a way that is not in the critical path of answering queries. (more like, eventually consistent). Some places have entire teams for this.
To your point on correctness, my experience ranges a lot in areas where correctness can be eventually consistent. For something like healthcare or banking, you would rather spend the money on way more hardware, because you can't afford correctness to be off. However, with things like globally scaled social apps, this is just not the case.
Most people tend to still use Oracle in those situations. Which is legions slower , and legions more expensive than mysql or pgsql.
"Eventually consistent"? Explain. (Do you mean in an ACID transaction?)
And lest you think there is some hidden constant factor here that destroys real performance at the cost of "scalability," the techniques from that paper were used to completely destroy the TPC-C benchmark: http://arxiv.org/pdf/1402.2237.pdf
Admittedly this stuff is not yet implemented in traditional RDBMSes, but there is absolutely no reason why it could not be and I fully expect it to in the future. Certainly, if you are having performance problems now, this knowledge doesn't help. But I'd venture to guess (from personal experience) that most people removing foreign keys are doing it preemptively, rather than evaluating whether it is actually causing performance problems.
Yup, 3rd level support "winging it" in production.
And also migration scripts.
In things like a large social media app where the reality is no FKs, and 100k queries-per-second, for instance, you would write into your application logic how to deal with child rows and parent rows, and not rely on FKs to raise an exception, or perform cascading deletes and things.
So if ( when ) there is either a bug, hiccup, or a variety of other reasons that can as you mention make things question correctness (or specifically: referential integrity, if we are going to nitpick), these are usually cleaned up out of band.
So you may have a table you have to slowly iterate through later and remove rows in which their parent rows no longer exist anymore, as an example.
In something like the friend feed schema used in the previous example, read up on how that works, as you will see they sort of turn SQL on its head a bit, to make it more flexible and deal with these shortcomings.
Having said that, I have worked on some credit card infrastructure that does get up there, and it too, does not have FKs. Having or not having FKs does not magically mean your data will be accurate or in-accurate. ACID, as well as traditional database transactions, still apply, and keep your data integrity high. In conjunction with well designed application logic, and proper planning, FKs are not needed to maintain accurate data whatsoever.
Also, there is commerce related things (Amazon) that work on truly eventually consistent data stores, like DynamoDB. Don’t see amazon getting laughed at much. (they use it for their shopping cart, source: http://www.allthingsdistributed.com/2007/10/amazons_dynamo.h... , this paper is also a great read fwiw)
Anyways, I think we are getting a bit into the weeds here, as the OPs link is about a mysql bug. Cheers.
the fact is, Amazon does use Dynamo for a mission critical financial purpose. If Dynamo lost data Amazon would not use it for arguably one of the key pieces of their success.
the article I linked covers how they do this, and how it stays accurate, scalable, and robust.
anyways cheers, I hope we have not hijacked this thread about MySQL bugs too much.
First of all, any single shopping cart is typically accessed by one single process at a time: the user's browser. I can of course open two or more browser sessions, but this is not a typical use case, it definitely has an upper limit (how many browser tabs can I use concurrently?) and most importantly, lost or duplicated transactions are easily spotted and corrected (you do get plenty of chances to revise your order before paying).
A bank account can be "hit" by different sources at the same time (just think of a company bank account, not just a personal one) and most importantly, duplicated or lost transactions are much harder to prove and rectify.
But, I can assure you, you could open 1000 tabs, and dynamo will be fine, and not lose your data.
You should really read the article I linked, especially the part about vector clocks, it discusses a common approach for dealing with duplicate data, conflicts, and resolution, at scale.