22 ms·

We do not use foreign keys (2016)

• hesk 7y ago
  The main criticism seems to be that the FK relationship makes migrating the referenced table difficult. But why not remove the FK with ALTER TABLE before the migration, migrate, and add the FK back again (which will catch any missing primary keys), preferably inside a transaction?
  • deleted 7y ago
    [deleted]
  • brobinson 7y ago
    >preferably inside a transaction? MySQL does not support transactional DDL, unfortunately.
    • zerg2k 7y ago
      That’s no longer true since MySQL8 https://dev.mysql.com/doc/refman/8.0/en/atomic-ddl.html https://dev.mysql.com/doc/refman/8.0/en/atomic-ddl.html
      • brlewis 7y ago
        MySQL 8 was extremely new at the time this comment was posted. It's likely that mysql limitations were a big part of the thinking. The mention of pt-online-schema-change at the bottom makes me think so.
      • etaioinshrdlu 7y ago
        Sadly I don't think this is actually true. From that page: DDL statements, atomic or otherwise, implicitly end any transaction that is active in the current session, as if you had done a COMMIT before executing the statement. This means that DDL statements cannot be performed within another transaction, within transaction control statements such as START TRANSACTION ... COMMIT, or combined with other statements within the same transaction. Whereas with real transactional DDL, the DDL can be done within large transactions, multiple ALTER's could be rolled back, no changes are visible outside the transaction...
      • evanelias 7y ago
        MySQL 8 DDL is atomic at the statement level, but cannot be combined with other SQL DDL or DML statements in transactions yet.
    • shawnz 7y ago
      So then don't do it inside a transaction. It still wouldn't be any less safe than their current approach of just not using FKs at all
      • evanelias 7y ago
        Adding a foreign key to a massive table can take a long time, since all existing row data must be validated against the constraint. With logical replication, long operations like this can be quite disruptive, even with a multi-threaded logical replication scheme.
  • mumblemumble 7y ago
    I think the emphasis there is the word "online". The method you propose might work, but not necessarily well. Catching missing primary keys at the end is a problem because then you have a bunch of data integrity issues to sort out, preventing you from re-applying the constraint, in a live system where apps are relying on the database to do integrity checks, meaning that there's a decent chance that the running system is creating integrity issues faster than you can sort them out. Possibly orders of magnitude faster. Doing everything inside a transaction may invite concurrency issues around locking, maybe even deadlocks. Which is again a problem in a live system, because you might be causing services to fail. I'm not sold on FKs being something you should never do, ever, but I will say that I used to work at a place that pushed their database servers very hard on real-time tasks, and also tended to avoid foreign keys, and my biggest complaint was not data integrity, it was just that there was extra effort that needed to go into documenting the logical foreign keys (the keys still exist, they're just not enforced by a database constraint), since you couldn't just read them from the database schema. I'd like to say that this can be something where you default to having FKs, and remove them as you hit performance problems, except that the poster is right: Developers have a habit of leaning on database constraints instead of doing their own sanity checks, and they simply don't know all the places they're doing it - they're not necessarily even aware they're doing it when they're doing it - meaning that removing FK's after the fact doesn't get you to "constraints are enforced by the apps", it gets you to "constraints aren't enforced". I think you probably want to try and anticipate ahead of time if you'll have a problem, so that everyone can know from the get-go whether they'll be working without a safety net.
    • jayd16 7y ago
      I'm not sure "you can't add checks because they might fail!" is sound reasoning...
      • mumblemumble 7y ago
        It's not "you can't add checks because they might fail"; it's "you can't temporarily remove checks in a live system where the database's users are used to leaning on them, because when you try to re-apply them you just might find yourself in a world of fail".
  • cryptonector 7y ago
    I didn't quite understand the migration issue TFA talks about, but at least with PG, there's more than enough clever ways to make schema changes incrementally.
• deleted 7y ago
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• conroy 7y ago
  When posts like these come up, I'd like to remind people that context matters when making technical decisions. What works for large companies with huge scale (GitHub, Google, Facebook) may not work for you. As a counter point to the linked issue, I operate a few small applications. Foreign-keys (and constraints in general) are great at ensuring that invalid data doesn't find its way into your database. Yes, they have a performance cost. Yes, they make sharding more difficult. In my experience, at smaller scale the trade-offs are worth it. YMMV
  • DaiPlusPlus 7y ago
    As someone currently fighting a battle with an LoB application written without foreign-key constraints with hundreds of thousands of rows of corrupted data because of bugs in sprocs that assigned the wrong value to the wrong foreign key column because they were similarly named - THIS! The reply in the GitHub thread we’re talking about makes it clear that they still perform FK validation - it’s just performed in the application code rather than the DBMS. I note there is another alternative: deferred constraints - or just run a query to check for invalid rows at 3am every morning.
    • spookthesunset 7y ago
      The application code isn’t enforcing FK constraints because that is impossible for the application to do. Their database is almost 100% guaranteed to be corrupt as a result. Application code has bugs. Application code can fail in ways that result in corruption. A primary job of the database is to keep itself from getting corrupted. Enforcing foreign key violations is only something the database can do correctly. Punting the responsibility to a higher layer will result in corruption.
      • GauntletWizard 7y ago
        There are 100% foreign key violations in their database. That is not the same as their database being corrupt. They have engineered for, and understand the implications of, foreign key violations. Typically, it's as simple as "This row can be deleted", and that can cascade - at a totally different rate than you'd find in a database and with totally different performance characteristics.
• deleted 7y ago
  [deleted]
• amingilani 7y ago
  How would a Rails app be impacted by this? How would you create one-to-many, or many-to-many relationships? I'm sorry if this question sounds silly, but I've never heard this line of reasoning before and I'm fascinated by it.
  • steveklabnik 7y ago
    GitHub is a Rails app. [1] Rails does not create foreign keys by default, so most Rails apps are doing exactly this. I think you may be missing the distinction between the database feature "foreign keys", and the general concept of an entry that refers to another entry. Rails does the latter, but not the former, unless you explicitly do it yourself. (Maybe Rails has changed dramatically in the last few years, but up until at least Rails 4, this was the case) 1: Obviously it's also way more, but the main codebase is a Rails app, and has been forever. This thread is from 2016, so that was probably even more true then than now.
    • rexpop 7y ago
      Thank you, this was helpful for me. I still don't, frankly, recall (did I ever know) how Rails implements the latter without the former.
      • steveklabnik 7y ago
        No problem! The summary is basically this: if you say that a User has_many foos, then the foos table will have a user_id in it. When you ask for the foos a user has, the SQL will be emitted that has the "user_id=1" or whatever clause on it. If you add a foreign key constraint, the database itself will verify that the foos have valid user_ids. If you don't, then it won't. You're basically giving up the ability for the database to verify the relationship for you. Like any kind of checks, this is more restrictive, but you get some value out of it. The question is, is the validation worth the restrictiveness. The OP comment argues no. Many would argue yes. It really just depends.
        • rexpop 7y ago
• fdr 7y ago
  There seems to be a certain mysql tinge to this value calculation. I use foreign keys and like it (I use Postgres). I do not encounter the problems the author does. I omit them when they are a problem for some reason. This is rare in routine work.
  • Hamuko 7y ago
    I'm gonna wager a guess and say that most MySQL users do use foreign keys without a problem. Most MySQL users are not running Github scale.
    • fdr 7y ago
      That's probably true. The only tell for that, really, was the complexity around migrations/schema changes, which seems a lot more painful on MySQL.
      • cryptonector 7y ago
        I don't understand why anyone would choose MySQL over PG, honestly.
    • jeltz 7y ago
      True, but the people I know who run PostgreSQL at that scale do not complain about foreign keys. Not since PostgreSQL added separate lock levels to improve concurrency with foreign keys.
• jagged-chisel 7y ago
  Worked at a bank years ago. One of their DBAs eschewed FKs in production. Databases were designed with FKs which were enforced in Dev and QA. If your app survived testing (automated, QA team ... the entire gamut) without producing FK violation exceptions, your app could be promoted to production, where FKs were not enforced, making things pretty fast. I feel like this was a stopgap on the way to eliminating FKs. I have no idea what his roadmap was because I wasn't in that group. But his process made for good thought fodder.
  • OJFord 7y ago
    Hm, that is 'good thought fodder'. My feeling on it though is that it just shifts all the faith to your test data being true to not only does but conceivably could happen in prod. All very well testing with a bunch of constraints, but if it turns out one actually isn't exercised and then happens in production? It also means you can't use them for validation or whatever. Which, I don't know, maybe you shouldn't anyway, but if you turn them off in prod you can't.
  • munk-a 7y ago
    This approach is quite scary to me and I would have argued very vocally against their rejections of FKs. There are ways to rely on replication for read serving, periodic disabling of FKs during batch inserts, FK integrity checks on replications, ... these can all address the performance issues inherent in FKs, it's also (generally) quite possible to attempt to architecturally disentangle too large networks of interdependent data in an effort to reduce how large any single data store you are relying on will get. But disabling FKs in production is... eh. I'd much rather see the opposite, declare the FKs on a testing environment NOT VALID, run application code, VALIDATE CONSTRAINT those you'd marked and see if anything is in violation - ensuring that application code won't (in the normal course of operation) generally hit FK constraints can greatly reduce your performance hit from having them... then in prod you can turn them on and be confident in your comparably lighter performance loss to data integrity.
    • barrkel 7y ago
      FKs add read locks to referenced rows. It limits concurrency and it is observable. FKs constrain your ability to incrementally widen 32-bit FKs once you go over 2 billion rows, if you start out with 32-bit PKs. Two concrete reasons to avoid FKs in production, or at least disable them for longer running transactions. I think this perspective is something you only get once you've run bigger databases in production.
• davidw 7y ago
  How about this: if your service gets as big as github, then maybe consider doing odd things to eke out more performance or shard or whatever. Otherwise: use FK's to maintain stronger data integrity. As hesk mentions below, in Postgres, you can do all kinds of table ALTERing if needs be.
  • spookthesunset 7y ago
    FK’s don’t just maintain “stronger” data integrity, they are the only way to maintain relational data integrity. Application code cannot maintain that relational integrity, period. Any developer who thinks application code can enforce relational integrity is naive and does not understand relational database systems. It is impossible for any layer above the database itself to keep things from getting corrupt.
    • munk-a 7y ago
      I agree with your general gist, but it's not impossible, you just, essentially, are writing the engine yourself. I've seen a few applications that keep some data hot-loaded into a shared slice of memory and have tight controls over the altering and saving of that data - and that's sort of one of the easier ways to partially avoid a full reliance on FKs, if you want to use write-through caching then you're essentially accepting the cost of implementing data integrity checks in the application layer which can be done - if you're very very careful.
      • mdavidn 7y ago
        Unless you're working with immutable primary keys (i.e. the values can neither be updated nor deleted), it is actually not possible to reliably enforce a foreign key constraint outside the database.
        • munk-a 7y ago
          Speaking in terms of theory, it absolutely is. RDBMSs aren't magic boxes, they stage data for insertion, validate it, execute that insertion - all while littering the WAL (or equivalent) with the steps necessary for the guarantee of that operation. You absolutely can write this logic into your application, at that point your application may basically be Postgres but it's possible. If you attempt this and don't exhaust a small rainforest's worth of notepad paper and a few truckloads of dry erase markers then you're probably doing it wrong - but the problem is solvable. There are some unsolvable problems in this realm (like holding to ACID while also ensuring that all valid communications from a client are properly processed) but you can accomplish anything your RDBMS does, you just need to be really really absurdly careful and know what you're doing at a theoretical level.
• andreimatei1 7y ago
  At GitHub they should be using CockroachDB (:P), a scalable relational database that also supports schema changes. Then their FKs would work just fine (albeit with some performance implications).
  • hw 7y ago
    Or, just not use FKs at all. There are many factors to consider when deciding on a DB, and being able to use FKs is probably not high on the list.
    • brlewis 7y ago
      >being able to use FKs is probably not high on the list. No, that's not at all probable. For many, "database" is synonymous with "ACID database", for which foreign keys are important. https://en.wikipedia.org/wiki/ACID#Consistency https://en.wikipedia.org/wiki/ACID#Consistency
• bregma 7y ago
  Build a firewall to keep them foreign keys out. Make the database great again.
• PopeDotNinja 7y ago
  Because I've never thought about this, I'll ask the dumb question... A shopping cart has many items. An item belongs to a shopping cart. In a relational database, without foreign keys, how do you associate the shopping cart with the items?
  • greggyb 7y ago
    The suggestion is to not use foreign key constraints in the database. The suggestion is not to eschew columns which might be JOINed on in a query. I.e. "Don't enforce FK relationships with a constraint in the DB. Make sure the values in both tables which may be joined are consistent by using application code to enforce this."
    • spookthesunset 7y ago
      > Don't enforce FK relationships with a constraint in the DB. Make sure the values in both tables which may be joined are consistent by using application code to enforce this. For those reading at home who aren't good with databases. This is exactly the same statement as: "Don't enforce valid inputs on the backend. The javascript front-end will enforce it for us". Always validate your inputs at the appropriate layer. Your backend can never trust input from a web front-end (even if there is "only one web front-end"). Likewise, your database should never trust input from its clients (even if there is "only one client"). Ignore this rule at your own peril.
      • Aeolun 7y ago
        Except hopefully your customers do not have direct access to your backend.
        • spookthesunset 7y ago
          Are your engineers omnipotent beings who never fail, never make mistakes and understand the system entirely (even all the parts they don't know about)? Is your QA process perfect? Your servers never crash in weird states? Lucky for you!
• shayanjavadi 7y ago
  Genuine question, what are the alternatives?
  • Litmus2336 7y ago
    In this case, they use "foreign keys" in the sense that there are keys which correspond to keys in other tables. There are no foreign key constraints, the keyword is never used.
  • Hamuko 7y ago
    You have columns that refer to the PKs of other tables, just without constraints. So your invoice table would have a column `user_id`, which tells you which user the invoice is for. But if you enter an invoice for user ID 999,999 when you have no users, the database eats it happily, since it has no opinion as to what that data should be.
• SamuelAdams 7y ago
  That's fine when one application is using the database. I work in the enterprise space, and we have at least 5 different applications all working with the same database. Each of these applications has their own team. Expecting each team to handle constraint checking uniformly in their applications isn't always feasible, so we use foreign keys. GitHub's approach might work fine for small, focused teams who have exclusive control over a product. However, that's not often the reality of most organizations.
  • spookthesunset 7y ago
    > That's fine when one application is using the database. It's not even fine for a single application. Said application has bugs in it. Said application can (and will) crash in unexpected ways that leave the database in an invalid state. And if you think that "oh, that will never happen to me". You just haven't been around long enough. It will happen, every single time. Any system without foreign keys will have corrupt data in it. Said system will have user-visible issues as a result. Just like only the backend can truly validate user input from a webform, only the database can validate its input.
    • Mahn 7y ago
      There are ways to prevent invalid states in a database without foreign keys, for example enforcing "all or nothing" operations via transactions.
      • Volundr 7y ago
        Until a programmer forgets to wrap the new feature in a transaction, or just has a bug in their logic that breaks the integrity. Or a db admin working directly in the database breaks it. Or a schema migration. It's not impossible, sure, but it requires you and everyone working with the data two be very, very disciplined, and is only as effective as the weakest link.
• spookthesunset 7y ago
  The number of times I’ve seen serious data corruption because “foreign keys are bad and we can just enforce it in code” is amazing. There is zero excuse to not use FK’s Any database that doesn’t use FK’s is almost guaranteed to have crap in it that didn’t get cleaned up, resulting in data corruption (and yes, dangling stuff in tables count as data corruption). Developers aren’t perfect. Shit will slip through even with the most rigorous process. The database should always provide tools to keep its self from getting corrupted. This is why good database systems have constraints like FK’s. If your database software makes using those tools painful (cough MySQL), get a better database system. Not using FK’s is just plain old ignorance.
  • Litmus2336 7y ago
    Seriously. "As a C developer, I never check exit codes of child processes. We can just enforce it by ensuring child processes don't have bugs"
    • benburleson 7y ago
      `malloc` won't fail, right?
      • spookthesunset 7y ago
        the javascript will enforce the user input. no need to have the backend check it again!
        • hnarn 7y ago
          This should be a joke, but considering how often it's actually the case it's unfortunately not that funny.
      • cryptonector 7y ago
        It won't on Linux! But hey, random processes will get OOM-killed.
• brlewis 7y ago
  > It may even rely on FK to cascade deletes (shudder) Is this just the author's personal taste, or is there something about mysql that makes ON DELETE CASCADE a bad idea? In postgresql it's a useful tool for maintaining database consistency.
  • Hamuko 7y ago
    I'm guessing it's the possibility for data loss. So someone accidentally deletes a user and it cascade deletes all invoices referenced to that user (deleting invoices is a big no-no in accounting).
    • brlewis 7y ago
      If the author is thinking of that kind of situation, he's right that ON DELETE CASCADE would be inappropriate, but he's also incredibly wrong to not want a foreign key enforcing that relationship. Deleting a user associated with invoices that should not be deleted is a big no-no too. A foreign key would not let the user be deleted until after the associated invoices are deleted. There are plenty of other situations where ON DELETE CASCADE is the best option.
      • papln 7y ago
        the problem with ON DELETE CASCADE isn't the CASCADE, it's the DELETE. Almost always, you want to to mark an entity disbled, not really DELETE it. If you want to DELETE it (for GDPR?) you should have something in place to fail your delete unless you've properly defined how to clean up danglig keys. (Perhaps what you need to do is delete the non-primary-key fields containing user data, but keep the row for relational integrity) ON CASCADE DELETE is used for deleting logical sub-components (a row owned by another row) in a well normalized database -- it's for deleting your dadress book when you delete your account. It's not for deleting all your friends when your account is deleted.
        • brlewis 7y ago
          > It's not for deleting all your friends when your account is deleted. Very nice summary up until that point. But in a well normalized db the friends relationship (many-to-many) would be its own table. Friend relationships with the deleted user would be deleted, as they should be. Friends would not. (You still have time to edit for a better example.)
• geophile 7y ago
  Wow. No discussion at all regarding the correctness of their databases. Is all the data correct, or are there FKs referencing missing PKs? Actually, even discovering this would be difficult, since any FK violation could be just viewing the middle of what would be a consistent update in flight. I get that cross-shard FKs are especially difficult. But any discussion of this topic without addressing correctness concerns is woefully incomplete.
  • Hamuko 7y ago
    It's not like you can avoid incorrect data just with foreign keys. Say you have an invoice model. There's a boolean value indicating that the invoice is final and a numerical value for invoice number. Final invoices must have invoice numbers. But a bug in your system manages to update an invoice so that it's final, but missing an invoice number. That row is incorrect and it's gonna cause an issue somewhere. I can only imagine how many production databases contain incorrect data.
    • zabzonk 7y ago
      How would the system update the invoice at all without an invoice number?
      • Hamuko 7y ago
        I'm not sure if I understand.
        • yellowapple 7y ago
          If you're finalizing the invoice, you're hopefully doing something like this, right? UPDATE invoices SET final = TRUE, invoice_number = @InvoiceNumber WHERE id = @InvoiceId; (Where @InvoiceNumber is some variable the application's substituting into the query) If so, then the problem you present should never happen (unless the DB doesn't do atomic updates by default, but wrapping the update in a transaction should provide the necessary guarantees to prevent the problem from happening; if your DB doesn't support transactions, then you really should be switching to a different DB yesterday). If that somehow could happen, though (i.e. you don't trust the application to be bug-free, which is a reasonable attitude), a CHECK constraint (as others have pointed out) would make the database enforce that: ALTER TABLE invoices ADD CONSTRAINT final_invoices_have_invoice_number CHECK (final = FALSE OR invoice_number IS NOT NULL); Of course, I'd also be wondering why an invoice would ever exist without an invoice number (it's easy enough to just make the invoice number NOT NULL - or, better yet, make it the primary key), but hey, if that's the business requirement, then that's the business requirement.
• deleted 7y ago
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• jackcviers3 7y ago
  I thought foreign keys were necessary for the consistency part of ACID compliance. Once you remove the integrity constraint, the database cannot guarantee consistency, even with isolation and atomicity. Sure, removing the constraint will get you a huge burst in performance, but now you are in charge of guaranteeing consistency outside of transactions in a distributed system involving at least one client and one server. That's a tall order. As Runar Bjarnson put it: "Constraints liberate, liberties constrain,"[1]. If you give something the freedom to be inconsistent at any extreme scale, Murphy's law requires that anything that can go wrong, will go wrong. It is hubris to think that the average developer who hasn't spent years becoming a consistency expert is going to be able to guarantee that data won't become corrupted better than the engineers that build and maintain and acacademics that study rdbms systems full-time.Even experts get this wrong all the time [2]. To all junior devs out there, until you know you need to abandon a guarantee available to you in a given system, exhaust every other possible option. 1. https://youtu.be/GqmsQeSzMdw https://youtu.be/GqmsQeSzMdw 2. https://aphyr.com/posts/282-jepsen-postgres https://aphyr.com/posts/282-jepsen-postgres
  • mumblemumble 7y ago
    Consistency, in the context of ACID, just means that the integrity constraints that exist are enforced. It does not mean that constraints that don't exist should be enforced.
  • nwallin 7y ago
    My understanding is that it's the other way around. Without ACID you can't guarantee FK correctness. You can't be sure another transaction might add a key to a row you're about to delete. It's an easy mistake to make though.
• deleted 7y ago
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• muglug 7y ago
  I work at Vimeo. We don't have any foreign key constraints in our database either.
  • spookthesunset 7y ago
    Sorry to hear it!
  • Mahn 7y ago
    This thread makes it sound like it's impossible to make a medium scale application work without FKs constrains. I work on a commercial application that depends on relational relationships without FKs and the number of data corruption cases that we run into on regular basis is zero. Data integrity is handled at the application side, unexpected crashes are accounted for by heavy use of transactions, and everything works just fine. To be clear, I'm not advocating against the use of foreign keys. But not using them is perfectly doable and not at all what this thread would have you believe.
• dekhn 7y ago
  I have been perpetually annoyed at the SQL/RDBMS/relational calculus model. It always feels like a huge context shift from imperative programming. after many years of writing SQL, I noticed that many other people end up writing SQL statements that look more or less like computer programs (CASE statements, subselects, etc). It all came to a head when I naively asked an experienced SQL developer how to represent a tree in SQL and learned one way to do it was to have CHILD nodes with FK references to PARENT nodes. So any time you want to get all the CHILDREN of a PARENT you have to query all children to see if they have a FK to the appropriate PARENT. This always seemed strange and backwards to me and when I did a deeper dive I learned that tree representations in SQL are a rabbit hole of insanity all arranged around referential integrity.
  • topmonk 7y ago
    > So any time you want to get all the CHILDREN of a PARENT you have to query all children to see if they have a FK to the appropriate PARENT. It's just an index lookup, so what? You'd rather mash all the data into a single parent field, is that it?
    • dekhn 7y ago
      i will admit that once I saw postgres has arrays and composite types, it seemed more natural to model this as NODE with array of CHILDREN. I think it's more about the expressivity of pointers and arrays compared to the "backwardness-feeling" of index lookups.
      • topmonk 7y ago
        So now you have just created a many-to-many relationship. Due to a bug in software, a child could now be linked to multiple parents, or even no parents. Part of good database design is preventing corruption of these sorts of relationships. > I think it's more about the expressivity of pointers and arrays compared to the "backwardness-feeling" of index lookups. Greater expressivity in ways to store bad data is never a good thing.
      • papln 7y ago
• edlebert 7y ago
  Also, to be noted, Github was a Rails app since long before there was native support for adding foreign keys. I've worked on a lot of large monorails that were created in Rails' early days and none of them have foreign keys.
• chase-seibert 7y ago
  This decision makes sense from the perspective of the person who owns doing database migrations, or who owns the performance of the database. However, the other 99% of the engineers in the company end up dealing with the fallout of not having FKs -- more bugs, and slower development velocity. One alternative is to have FKs in smaller, sharded databases, where you don't have to migrate between shards. For example, all the database rows related to one repo could be co-located in a shared with strong FKs between them.
• SkyBelow 7y ago
  I fixed one too many data corruptions to do away with FKs. If we are large enough that FKs are causing performance issues, then we are large enough to invest in and deploy another performance boosting solution while still protecting the integrity of our database from contractors/employees running around with direct SQL access (another nightmare unto itself, but one that I as a developer have less control over than if there are FKs or not).
• redact207 7y ago
  One great thing about not enforcing FKs is that it makes integration testing a lot easier. You can load just the data you need to test with, and none of the FKs need to point to rows that exist that aren't within the testing scope. This approach isn't for everyone. It works well with DDD where aggregates form contextual table boundaries and is eventually consistent by default.
  • yellowapple 7y ago
    If you ain't testing against the full system state, then I'd be hard-pressed to call that "integration testing". If you're generating the test data from scratch, then it shouldn't be hard to generate the dependent data while you're at it. If you're testing against (anonymized) production data, then it shouldn't be hard to pull the dependent data while you're at it. In either case, you should be validating the integrity of those data relationships as part of the test criteria.
    • Ma8ee 7y ago
      It isn’t hard, but often very cumbersome. Oh, so you want to test invoicing, for this Customer, which must have a Delivery Address and an Invoicing Adress which both needs valid Postal Codes. And the Customer must have a Contact person. Then we need the Product, that must consist of at least one Article, and each Article must be connected to the Company that we bought it from, with Addresses and Contact Persons and half a dozen other entities. But we have forgotten that the article also need to be in a Category so we know which Sales tax or VAT that needs to applied, for the State or Country of Sale. Then of course, which Sales Person, belonging to a Sales office, with contact person, addresses etc should get credited for the sale. And we still haven’t been able to actually create the order yet, because we haven’t created the delivery options. I think you get my point. I’ve easily used more than a full day just to get enough data in a naked system to make just the simplest test. I’m very grateful for tools like tsqlt that make unit testing possible (by temporarily turning of FKs)
      • yellowapple 7y ago
        That all seems pretty reasonable to me, and darn well should be included when "test[ing] invoicing": - You surely want to make sure invoice creation depends on a valid billing address at the very least, right? If that breaks, you're gonna have a lot of rather irate AR clerks. - You surely want to make sure the Contact is aware of the new invoice on the order, right? In fact, that might very well be part of the Contact's performance metrics, so if that breaks, you're gonna have a lot of rather irate sales/support reps (whatever "Contact" means in this context). - You surely want to make sure your invoices are against valid items, right? And you surely want to make sure that expected revenue correctly ties back to an inventory movement, which in turn ties back to an inventory receipt, which in turn ties back to a paid invoice to a vendor, right? If that chain breaks, you're gonna have a lot of rather irate accountants and financial auditors. - You surely want to make sure you're getting the right sales tax calculations, right? If that breaks, you're gonna have a lot of rather irate accountants, financial auditors, and tax collectors. - You surely want to make sure the Sales Person and Sales Office both get credit for the invoice, right? If that breaks, you're gonna have a lot of rather irate sales reps and managers thereof. So... no, I ain't exactly getting your point, lol. If you're changing invoicing, then all of the dependencies and dependents of invoicing ought to be tested. But more to my point: "very grateful for tools like tsqlt that make unit testing possible" Unit testing != integration testing. If you're unit testing, then sure, turn off foreign keys and practice your quick draw while you cowboy it up. If you're integration testing, then that inherently means testing the whole system as a whole; what you call "cumbersome" I call "the bare minimum of comprehensiveness". "I’ve easily used more than a full day just to get enough data in a naked system to make just the simplest test." That's usually pretty easy to script, even with foreign key constraints. It might take you a day, but future days should be able to call upon that same script, saving you quite a bit of time :)
• malvosenior 7y ago
  So basically they don't use foreign keys because they're into premature optimization. If you need to shard your database, you're big enough that you'll have the resources to engineer the new solution. The vast, vast majority of use cases will benefit from the database ensuring the integrity of your data.
  • tomlu 7y ago
    Github is probably past the stage of premature optimization. Presume (for argument's sake) that we accept the premise that foreign keys do not scale. Then the question is whether there exists some sort of realistic migration strategy once usage grows past that. If it does, then using FK makes total sense until we hit that point, doesn't it? And if true this should apply to 99% of users. But if there exists no such migration strategy, then maybe the guy has a point?
    • malvosenior 7y ago
      In all of my years of development, I’ve never seen data that can’t be migrated to a new system. I’m not even sure what that would mean tbh.
• devit 7y ago
  All of these are mostly issues with the database engine implementation: 1. "FKs are in your way to shard your database." => not a problem for distributed databases that can query and duplicate data across shards/servers; alternatively, if the referenced data just isn't there in the database, a foreign key is not usable by definition 2. "FKs are a performance impact" => the app either just got the foreign key value from the database, so it should be cached in memory in a properly implemented database engine, or otherwise, the application relies on the database checking so you need the foreign key for correctness 3. "FKs don't work well with online schema migrations." => not a problem with database engines that properly support online schema changes without locking, downtime or table renaming hacks
  • evanelias 7y ago
    The performance impact is at write time, and having the data cached in memory is irrelevant. It's the locking overhead that's non-trivial, especially if each table has several FKs, when operating in a high-volume OLTP environment. As for "database engines that properly support online schema changes without locking, downtime or table renaming hacks", please name some. In my experience, every major DBMS has cases where certain ALTERs block concurrent DML, which is extremely problematic on very large tables. Or even cases where there's no locking, but the operation still takes a very long time, which is conceptually problematic for logical replication.
• anshul008 7y ago
  The performance of my team's application at Myntra (Bangalore) was badly affected by foreign keys on MySQL and we decided to drop them. The trick we used was to drop them only in production, not in test environments. In test environments they acted as guardrails to ensure that our application did not break the constraint. So in a way it gave us best of both worlds
• baby 7y ago
  Never used them in my personal projects either eventhough I learned about them in school. It just didn’t make sense to me, why would I constrain myself in the database when I could manage all of this in the application logic? And if you want to experiment with different schema it’s just a nightmare.
• wwarner 7y ago
  To me the issue is that the data model should be defined in a single place. If you're using foreign keys, then the development process needs to take that into account and provide a sensible way to modify the data model in a way that the FKs keep up with the evolution of the product. It's going to mean a lot more migrations for sure. I personally have never seen foreign keys implemented correctly -- they've captured an outdated version of the data model and are just in the way. For me, they're similar to stored procedures, usually to be avoided.
• gigatexal 7y ago
  Ahh to fkey or not to fkey. The problem with doing referential integrity in the app is what happens if a change is made to the database outside of the app? I guess it depends. If you’re at the scale where you can’t refactor your database to keep up and you, after profiling or some other method, have proven that moving integrity enforcement into the model of your app is faster then by all means do that. That being said: foreign keys help the query optimizer make better decisions. If you’re using and ORM though that doesn’t matter as you’ll get generic queries and you’re likely not profiling your queries anyway. They also are a way of self documenting the relationship between things. If you’re going to get rid of foreign keys then why not denormalize so that the data is easier to query and less likely to get corrupted? Reduce the number of tables say into a single table a-la the DynamoDB approach
• uses 7y ago
  I'm confused, how do you design a database where things in A can't reference things in B? It's not relational at that point right? Or is this just about the problems of DBs where you enforce referential integrity? In which case, if that's such a problem, like just don't enforce referential integrity...right?
• dylanz 7y ago
  Application vs Integration databases: https://martinfowler.com/bliki/IntegrationDatabase.html https://martinfowler.com/bliki/IntegrationDatabase.html. If you're going to have one application writing to your database, having your constraints live in your application is definitely a viable option.
  • tiglionabbit 7y ago
    Not if you have multiple processes accessing the database simultaneously without some additional coordination layer.
  • randomidiot123 7y ago
    It's definitely asking for trouble. Murphy's Law.
• sarah180 7y ago
  "We do not use foreign workers" (2019) (gitlab.com) (Yes, I know it's more nuanced than that.)
• postit 7y ago
  I'm working with a company that also refuses to use FK and indexes. We have a huge DB cluster instead.
  • nwallin 7y ago
    > I'm working with a company that also refuses to use [...] indexes. ... ?
• MadWombat 7y ago
  > FKs are a performance impact. The fact they require indexes is likely fine, since those indexes are needed anyhow. But the lookup made for each insert/delete is an overhead. You have to check referential integrity somewhere. You can do it in the application or you can let the RDBMS do it for you, but it will have to happen somewhere. So the overhead argument is kind of bullshit. Yes, managing the integrity in the app might provide more flexibility, but it comes with a price of, well, having to do your own integrity checking and introducing more bug space into your code. > FKs don't work well with online schema migrations. ... > Doing schema migration of P is just not going to work. Recall that gh-ost renames the table at the end. So basically, the problem is not foreign keys, the problem is gh-ost implementation. Since they don't use FKs, they implemented their migrations in such a way as to make it impossible to support foreign keys. Yes, in the parent-child example, to migrate the parent you would have to migrate the child. But it is not an impossible thing to do. Create the new parent, import the data, map the record ids from the old parent to the new parent, create new child, drop the foreign key, import the data, update the FK field using the map and create a new FK. Rename the tables and you are done. > When eventually you want to shard or extract data out, you need to change & test the app to an unknown extent. Sharding your data is not something you do on a whim. And you will need to test your app. But you were going to test your app anyway, weren't you? Or did you think that you could split your database into multiple databases without any testing? As for unknown extent, that is kinda bullshit. Look for places that catch integrity exceptions. Thats what you need to look at. Look at your database schema and if any of your tables are "ON CASCADE DELETE", go and punch your DBA in the face. If you are the DBA you might want to start running before someone else finds out.
  • cryptonector 7y ago
    > > FKs are a performance impact. The fact they require indexes is likely fine, since those indexes are needed anyhow. But the lookup made for each insert/delete is an overhead. > > You have to check referential integrity somewhere. You can do it in the application or you can let the RDBMS do it for you, but it will have to happen somewhere. So the overhead argument is kind of bullshit. Yes, managing the integrity in the app might provide more flexibility, but it comes with a price of, well, having to do your own integrity checking and introducing more bug space into your code. Perhaps they meant "latency" rather than "performance". With eventual consistency you can have lower latency, and spend more cycles fixing referential integrity later. Of course, sometimes things fail in eventually-consistent systems (e.g., you order some item from a vendor who claims to have stock but oops! they don't, so your order will take a few more days to ship than they claimed and now you're a bit sad but hey, it all works out in the end, right?).
• bfrog 7y ago
  Well when you use MySQL, you end up with MySQL hacks like this
  • spookthesunset 7y ago
    Dunno why you are getting downvoted. MySQL is such a poor database that is always leads to hacks. The people who use MySQL almost always get burnt by something and figure all DBs are like that. Then people who have never worked with non-toy databases assume all databases are like MySQL and dismiss the whole field, which is a complete shame. The fact is, a real database system like PostgreSQL is one of the coolest, most underutilized tools developers have in your toolbox. Good databases can do so much cool shit to help you. It is amazing how well a good database system can take your weird crazy query and return something in mere milliseconds.
• daveisfera 7y ago
  FKs are a tool. Sometimes it's great to use a hammer for a nail, but sometimes a screwdriver just seems to work better when working with screws.
• desc 7y ago
  Er, what? Foreign keys are not necessarily a net performance cost. Depending on the scale of the database, they can be essential for fast queries.
• ctataryn 7y ago
  While we’re at it: should databases handle transactions or should we let the application deal with it all? IMHO The answer is the same for this as with foreign keys.
  • cryptonector 7y ago
    Ahh, but what is your answer to both of those then?
• kenshaw 7y ago
  Foreign keys should definitely be used in most schema designs. While I can agree that they are problematic when doing a schema migration, a schema migration would only happen "very rarely" whereas inserts/deletes happen regularly. I believe this comment is more of a reflection of the specific database (MySQL) which has relatively poor FK performance and adjunct issues when compared to other SQL databases. Also, from a process perspective of migrating a schema, the ideal mechanism would be to incrementally copy from the existing schema to the new schema (meaning there is no breakage in the interim). Granted (and somewhat obviously) for databases as large as what GitHub is likely using, this is not possible -- and so a "full migration" at once is their only option. However, part of that "full migration", to me, would be to _remove_ the original foreign keys from the schema, rename/delete/alter tables/fields/indices, and then after all of that is complete, as a final step, re-add referential integrity (ie, foreign keys). I'd specifically like to point out the following: 1) This comment gives advice about all SQL databases based on the conclusion / operation of only one database (MySQL), whereas the same base facts do not apply to others (eg, PostgreSQL, Microsoft SQL Server, ...), and should not be used to evaluate whether using FKs makes sense for other databases or not. 2) Schema migrations should be done rarely, if ever. A better approach would have been to start with a schema design that was less prone to _needing_ change. Regardless, even if schema migrations are done with _regularity_, that frequency would still be orders of magnitude less regular than, eg, code changes or CRUD operations. As such, throwing away FKs simply to accommodate either a poor or non-thoroughly thought-out schema is tossing the baby with the bathwater, especially since the purpose of FKs is to reduce the ability of other tiers in the application from losing or otherwise corrupting the structured data storage. 3) FKs are useful when doing CRUD operations. They are not useful during schema changes. As such, the proper process in this case would be to temporarily remove FKs, perform the schema migration, and then re-institute the FKs. This may not always be possible, due to, eg, active business logic components or some such that can't allow the database to be taken offline to perform a migration (see point #2). In that instance, there are a number of other mitigations that can be put in place in the logic tiers, such as being able to recognize different schema revisions, and deploying that _prior_ to doing a schema change. As such, one would first migrate an active "smart" logic tier that can recognize different schema revisions, and allow for incremental migration of schemas without enacting a cluster-wide lock/disabling of reads/writes.
• barrkel 7y ago
  Enable FKs for test and development, disable them in production, for tables that see billions of rows. FKs have other downsides. They add locks on the referenced rows, to ensure they don't disappear before the transaction commits. If you start out with 32-bit primary keys, you have a major pain when you go over 2 billion records - incrementally upgrading all your FKs before your PK breaks your referential integrity, since FKs generally need to have the same type on each side.
• ken 7y ago
  GitHub is a special case, perhaps in all of computerdom, in that most of their public data is stored not in (SQL) databases, but in git repositories. That's rather unique! So we know they must already have a great indexing and search system in place, and they can't use foreign key constraints on most of their data, anyway. Git simply doesn't have that. It's not that kind of database. Given this environment, it's not surprising they don't use FKs. It wouldn't surprise me if they don't use JOINs much! Throw it all in a repo, and let the indexer sort it out. That's great for them, but my data isn't shaped like that. Data types and referential integrity are more important to me than history and bisection.
• nickpeterson 7y ago
  My advice, put constraints that make sense as you're developing an application, and marvel at how often you accidentally violate them while writing trivial code. You'll realize pretty quickly that even when you know the whole area of what you're working on you still screw up. FKs might not add a lot of value on bet the company, mission critical, heavily tested code. But on less rigorous codebases they will save your ass.
• jerzyt 7y ago
  Granted, I have never worked with a database the size of Github, so I've never had to deal with sharding issues. But, even though foreign keys can be a PITA in some situations, I prefer the DBMS to do it for me than rely on the application. Foreign keys have always kept my back. I've been burned a few times when they weren't used.
• wisnesky 7y ago
  Many people on this thread have asked if there is a rigorous way to migrate data between relational schemas that contain non-trivial foreign keys, and the answer is yes: In the open-source categorical query language CQL, data migrations necessarily respect foreign keys - a property guaranteed by the mathematics of category theory and the automated theorem prover that ships with CQL. http://categoricaldata.net http://categoricaldata.net
• rukuu001 7y ago
  Let's just consider some of the 'non-technical' issues surrounding the use of foreign keys. I've encountered resistance because - Using them would require up-skilling existing devs - They'd highlight crap data coming in from other parts of the org, which would be politically uncomfortable - DB would need to be re-configured - We're agile now and we can't be locked into those kind of constraints edit: formatting
• jayd16 7y ago
  I'm curious what these comments think they're saying. What effort went into GitHub such that they feel they can make the claim that FKs are an anti pattern. Do they think they're just better than everyone else or is there some other practice that makes this claim more interesting than simple hubris? Me a
• CGamesPlay 7y ago
  Oh man, that migrations point. I remember at my last company needing to handle this. We had some (huge) read-only tables that were referenced by other tables and we needed up update the read-only tables in batches from raw mysql dumps. The keys in the dumps were stable, so we could swap out the table and the references would still be valid. Since the referenced tables were huge, they took a long time to upload the batches, and we needed a migration that was ACID and fast. The process ended up looking like this: 1. Create a `foo_load` and a `foo_unload` database, dropping any that already exist. 2. Populate `foo_load` with all of my new data. 4. Rename `foo.table` to `foo_unload.table` and `foo_load.table` to `foo.table`. This is atomic, fast, and updates references to point to `foo_unload.table`. At this point the application reads from the new data, but referential integrity checks still verify against the old data. 5. Update each reference to `foo_unload.table` to point to `foo.table` using ALTER TABLE DROP FOREIGN KEY, ADD FOREIGN KEY. Again this is atomic and runs quickly. 6. Drop the `foo_unload` database. Now this process is slow, but the switchover is 2 atomic steps and requires locking the tables for only a few milliseconds. The process is safe as well, if it crashes at any step, running the script again will cause it to safely recover. Well, with one more step zero: 0. If `foo_unload` exists, do the same rename step as step 5. This solution worked for us for a few years. Eventually we swapped out the whole architecture around these tables and ended up with a more traditional ETL flow that didn't involve moving round huge mysql dumps.
• doctor_eval 7y ago
  I had a database with no RI and it was a nightmare, filled with bugs. So we added RI and it was awesome, and the bugs did go away with time. But then we became dogmatic and had to have RI for everything, including things like maintaining local RI lookup tables for non local data. This just bogged us down so much that we couldn’t move. Today, we use RI strictly within a single database. For foreign keys to remote databases, we assume the remote database knows what it’s talking about and that the incoming data has already been checked. This works really well for us and in particular sharding is not a problem (sharding being a design decision not an implementation decision IMO) Someone said elsewhere that Rails does the RI so the DB doesn’t need to. I don’t use Rails, and I don’t bet, but I’d wager that it’s faster to do RI inside Postgres than inside Rails. The idea that we should throw out ALL RI is just dogmatic nonsense. I should know :)
  • mceachen 7y ago
    Rails in 2010 was just coming around to adopting foreign keys, and the rubygem Foreigner made it happen before it was a proper rails feature. The idea of rails enforcing unique constraints and FKs back then was... Optimistic. The design constraints assumed exactly one rails instance would access the database at any given time to ensure consistency. Not a great assumption.
• megapatch 7y ago
  Confusing. Foreign keys are a vital part in RDBMS and very useful. He is talking about foreign key constraints, which can be problematic as described.
• arpa 7y ago
  Last time a DBA suggested me dropping foreigb keys for performance reasons, i suggested him to write code ensuring data consistency with distributed parallel writes. Haven't heard anything about FKs from him since.
• hootbootscoot 7y ago
  while most rdbms-based projects will continue using foreign key constraints (and all will be referentially cast in stone), and postgressives will enjoy 100000x query speedups by remembering to include your hornsnaggle_tree_index_lookup_speeder_upper index, there is a certain fanatical mindset from the rdbms world that sounds a bit like: never forget always include jquery with every web app you make. it's made by real profeshunalz who know more than you and lots of people already included it and you should simply just learn to accept that all front-end web problems have already all been solved by jqueremy and you should simply refactor it all in terms of jqueremy. anything else is korrupshun. In other words, always only ever consider the same tools and techniques. LOL A mere search for other database types produces pages upon pages of "why sql is beating nosql" and all stack overflow answers always doom you to the 7th level of hell for anything but 3rd normalized form. hey let's all use lookup tables for many to many. it's a cult. it's nuts. anyone who has spent any amount of time with database-backed applications is surely aware of the tradeoffs they are making when choosing non-relational databases of any kind. The obsession with forcing every all all use cases into the rdbms mindset makes me say "well then, let's just flatten everything even further into mapped key-value a la ohm." in which one encodes the key with path info and the value with the value, etc. if the act of suffering for the sake of suffering and breaking things into iddy bidd y pieces is virtuous in and of itself, surely this is a more true pure and rigorous religion to bend every single thing to. (before smarty pants says "um, querying" may I remind you of adding another entire secondary index of any kind via adding it with the id of the first key as it's value.) I'm not advocating everyone do this. no freakouts pls. ideally we would just persist our objects, which is coming with persistent memory, but i digress... This Lance fellow seems to have taken plenty of "you have no idea what you are talking about moron!" abuse, despite having a phd in computation theory and writing erp software since the dinosaurs roamed the earth. (I'm also not advocating his solutions for any and all problems, but not taking his ideas seriously is a mistake IMO) https://codeburst.io/databases-the-future-buggy-whips-of-software-31a716b0ed2b https://codeburst.io/databases-the-future-buggy-whips-of-sof... https://codeburst.io/doing-without-databases-in-the-21st-century-6e25cf495373 https://codeburst.io/doing-without-databases-in-the-21st-cen... mongodb is an incredibly powerful tool for when you want the whole json, all the json, and packed as bson. When you want to store/dish out blobs and not join nor have no use for the parts separately, then you can do that nicely. (besides this usual story about "for when you don't know your schema or have lots of irregular semi-structured data, stripe, your mailgun webhooks, whatever. It's a great DB, for some things, despite the people saying "since it didn't do this one thing i really obsess over and think all dbs should always do that therefore its' crap" (well, copying the same master-slave with single-write master failover or vertically scale story as the rdbms people was probably a weak point...) pg as a jsonb store with secondary indices on actual columns LOL... I guess we just basically need to accept that the same tools can be used as primitives/components in different contexts in terms of the actual application. Some people use Redis as a primary datastore. zealotry is best avoided, I guess.. look, this silo-ing stuff is also silly, and a product of the corporate dysfunctional mentality: split everyone up and get DBA's warring with Devs...