Design better databases(web.archive.org) |
Design better databases(web.archive.org) |
Simple things like dialogs getting stacked on top of each other, using `prompt("...")` sometimes and modals other times, foreign key relationship arrows not indicating cardinality, etc --
Looks good but please don't consider it "finished" yet.
> jnichols created new pattern
> Penis
> 24 seconds ago
Ok... maybe they need to start with some patterns about filtering spam and noise.
If you give all your ID fields unique names, e.g. by calling your field "reservation_id" instead of "id", even in the reservation table, you can do stuff like:
SELECT * FROM reservation JOIN guest USING (reservation_id);
By doing "USING (reservation_id)" instead of "ON reservation.id = guest.reservation_id", the field will be automatically deduplicated, so you don't have to qualify it elsewhere in the query, and "SELECT *" will return only one copy.
But I can see arguments in both directions. I think the big thing is consistency. If you consistently use <table name>_id as the PK and your tooling doesn't fight it, then cool.
I do feel like this benefit breaks down rather quickly though. For example, if you wanted to track who created a guest record, you probably wouldn't name the column user_id as it's ambiguous. So you might use creator_id to signify it points to the user who created the guest, not to the user invited to the reservation – thus negating your ability to join to a user table via USING.
That's not to say it's a bad tip, just because there are cases where it doesn't fit. Definitely worth knowing. Thanks for sharing.
All just to shorten join clauses with a natural join or USING? You already get shortened joins - less so, but still a gain - by using table aliases so as not to repeat the long table names in each query multiple times. IMO it also takes an extra moment to parse the ambiguous "id" columns to resolve them mentally to which id is being referred to.
Apparently, it made joins more clear, I seriously wonder how often people fucked that up for them to think it was a good idea to prefix every fucking column with its table name.
It's like when I see unit tests for setters and think, gee, setters seem pretty straightforward to me, how often to people fuck them up?
SELECT * FROM long_table_name l LEFT JOIN another_table_here a ON l.id = a.rel_id
Regardless of the design choices made it's more important to be consistent in naming conventions and to document why a particular design/schema was chosen.
Nobody ever uses the class "Reservation" with a property named "reservation_id" in their code. They will name their column "reservation_id" in the database, but then work to undo that at the application level by telling the ORM to map "reservation_id" to an object property named "id". It creates the situation where developers fight against the database schema in their code, and adds an unnecessary level of complexity when comparing code to database. "Why is it 'id' in code but 'reservation_id' in the database?".
A common counterargument from pure DBAs, who are not themselves writing code that integrates with the schemas they create, is that databases should be approached as a completely standalone component that should not be designed with applications' structures in mind. That applications are free to bastardize the database schema in any way they please, so it really doesn't matter anyway. I personally prefer to see it as a reality that applications will be the primary consumer of the database resource, and deserve to have their intended integrations analyzed. Once the database is finalized, it is the application developers who will be studying the schema on a weekly basis. Anything to ease that constant inspection process, the better.
Of course, this is all personal preference. I have learned over the years to finally stop arguing so much over these little details. In real life with coworkers, anyway. It's still fun to battle for your point of view with forum strangers! :D
The database already knows that guest.reservation_id is a foreign key referencing reservation.id. Why should you have to repeat yourself?
Problem is that it becomes problematic as soon as you keep such queries around (in stored procedures, source code, or batch files), and then modify your schema by adding or removing a foreign key.
Added:
Also SQL language itself has nothing to do with keys, relationships and other details of database design. It's different level of abstraction.
Various query builders/SQL generators do what you want.
This is almost exactly what "NATURAL JOIN" does. It is like "JOIN USING" with a pre-filled column list.
However, this isn't decided based on the schema, but based on equal column names. So you need to apply the dicipline as noted by the parent comment.
See also the sibling discussion thread at: https://news.ycombinator.com/item?id=11450187
You still have in outer joins. Also, SELECT * is a bad practice (with some exceptions) when used in code, suitable only for ad-hoc queries.
SELECT
table1.*,
table2.field
FROM table1
JOIN table2
ON table2.rel_id = table1.idSELECT * FROM reservation NATURAL INNER JOIN guest;
Imagine two two-column tables, A with id and name, and B with id and value. (Doesn't matter what the columns are; this is just for concreteness.)
select * from A JOIN B using (id) will give you a three column result set, id, name, value.
Now, suppose you want to do a schema modification to add "last _login" to A. Suddenly, your query is returning a 4 column result set, id, name, last_login, value and code using ordinal positions is now reading last_login into the "value" slot.
Maybe it's a corner-case for our use case and zero-downtime release processes, but it's something to be aware of anyway.
left.id = 'Something' COLLATE case insensitive
right.id = 'SOMETHING' COLLATE case insensitive
SELECT id FROM left JOIN right USING (id);
This way, my db fetch is always on id and not a longer name ( reservation_id), which on the end of an webapp, makes me save some reading / writing :)
SELECT * FROM reservation r JOIN guest USING (r.id);As someone in the healthcare space, looking at some of these models gives me a better idea of how various aspects of the healthcare industry work, and the things they interact with. Ex) http://www.databaseanswers.org/data_models/patient_data_ware...
https://www.hl7.org/implement/standards/product_brief.cfm?pr...
the ones in these books are better:
https://dba.stackexchange.com/questions/12991/ready-to-use-d...
http://dbpatterns.com/documents/5091f74289cbad03bc958bc0/
It has the "let's put a UUID on every row" disease common to designers who have never really learned anything other than object oriented design. Price is a string (I guess so you can put "market price" on the fish?), and there's a currency symbol on every "delivery". The whole thing just makes no sense. 18 stars. Unbelievable.
http://www.wiley.com/WileyCDA/WileyTitle/productCd-047138023...
http://www.wiley.com/WileyCDA/WileyTitle/productCd-047135348...
By far one of the most useful things a resource can do.
It doesn't just broaden your understanding of a domain, but it can teach you how to think about design problems.
I can't imagine using it for anything real but it was fun to play with.
If the creator is reading this, here's a bug report: If the user hasn't hit "Save" when they export the schema, they get an empty text file and they'll think the app is broken. It should either auto-save or prompt the user "You have unsaved content, do you want to save before exporting?"
UPDATE: One more bug report, the export doesn't escape the columns names at all. It just naively adds double quotes around them. This breaks with double quotes in field names (yeah yeah which is stupid but still..):
CREATE TABLE "foo" (
"id" int,
"baz" varchar(255),
"bar" varchar(255),
""test"" varchar(255)
);There's something very therapeutic about organizing data using a system of rules, and the relational data model is a powerful one.
It hurts my pragmatism to solve a distributed systems problem before we even had a distributed system! Don't be a sheep and use UUIDs, rather than a simple integer primary key or composite key that is natural to the table. People say it's just an id, who cares? But I say people making these kinds of decisions are probably making other poor choices backed by something they read and don't understand.
Here's a great article on locality and the need for uniqueness and why UUIDs should only be introduced when needed and to solve very specific distributed systems problems. The reality is almost all applications will never need to have multiple concurrent writers.
The only time I use UUIDs is when I need to pass that id out, but I want to hidr how many rows there are or the rate that they are being created. However, I usually use COMBs to prevent fragmentation.
Most of the patterns I've seen are obvious simple things but they are missing a lot of content like : - What was the design's intended properties? - Expected volumes, access patterns? - Good places for indices? Maybe the comments could be of some help but really haven't seen much. I've actually found more interesting content in the HN comments than on the site itself.
This should be called something like database designs.
If you apply some recipe to structure code, that's a "design pattern"; if we apply some recipe for structuring data, it's a "data(base) design"?
But code is data. But sometimes it's not clear whether something is code or data, or whether it's closer to being code than to data.
Design patterns incorporate data. The Observer pattern requires a list of observers that are notified; that is a data structure which we can have in a database: an observer table joined to an event source by event source ID.
If I make such a database (say for a large number of users to set up persistent notifications about some interesting interesting things), am I doing "database design" or implementing the Observer pattern? :)
- it could allow sql exports for various platforms - it could feature an API so people can write their own framework drivers (for example creating migrations, importing existing schemas etc)
Lets hope that it gains users, so that the voting system (the star) can become more useful to filter out the garbage.
Edit: please remove the login wall to see the starred items, it raises the entry barrier quite a bit. Do you really want new visitors to see low rated schemas as the first thing?
Most egregious example for me is probably the prevalence of a lot of "type" tables when a simple enum column would do. Or maybe the sheer number of UUIDs that are being thrown around.
I even saw a circular ID chain in one. Would be fun setting up foreign key constraints for that!
How do you see the forks?
Example: t0: 7458e3a9-716b-4352-b2e4-b5b67d0c089b t1: 4d8d753c-1777-439d-8725-b093b1bd8430
Using this as a PK in any relational database will mean the rows are stored in the clustered index order, which causes extreme fragmentation because the db engine constantly has to find "holes" in the data pages to insert more recent records instead of adding those at the bottom of the table as would happen with any continuously increasing key.
Others embrace surrogate keys because: They make joins between tables easier (usually one field rather than a compound key where one might accidentally not include all the join predicates). They save space since an integer is almost always smaller than a compound key.
So if you are in the camp that favors surrogate keys, then consider these points:
Some quick downsides for UUIDs: If your table has a clustered index, UUIDs are generally not created in a regular order, and so you'll be constantly inserting into a part of your index rather than the end. Some databases allow sequential UUID generation to help mitigate this. UUIDs are usually 16 bytes, but an int is usually 1-8 bytes. Also keep in mind you pay this price multiple times since the primary key (generally) becomes the foreign key in other tables.
Some upsides for UUIDs: If you have a process that has no master, being able to tie disparate things together with a shared UUID is very useful sometimes.
Often developers use UUIDs/GUIDs because: Their app wants to perform a 'Create' operation and they want to generate the key in advance.
This can usually be mitigated by creating a routine that generates a range of reserved surrogate integer keys, and gives a starting value to the application layer. Then the application layer can use those values with knowledge they won't already exist. For instance, the app layer say, "I'm going to make 100 customers", and the response comes back "10232". Meaning that 10132-10232 is reserved for that process to use for customer primary keys. This allows larger (chunkier) requests which can be interrelated without constantly making requests to the Database layer. One could extent this to be a true cache for entity reserved numbers that gets dolled out as needed by the app layer.
Some popular ways to do PK: natural key, sequentially generated key, and UUID. Personally I would prefer natural key if I can find an immutable natural key for the table. However, natural keys are hard to find. The food example in this case doesn't have natural key. Also if the natural key requires a compound key, it's just not worth the pain.
Sequential auto generated key is good when you need to hand it off to users, like order id or ticket number. It's short and simple and it's auto generated. The downsize is when you migrate databases, you need to seed the new database carefully or it would start from beginning again. Also in record creation, you need to do extra read to get back the newly generated key.
UUID is mostly worry free. It can be generated anywhere and doesn't need to be in the database. For a setup with distributed databases, I would use UUID just to have global uniqueness. For an offline app, I would use UUID to create the data records locally and later sync them with the main database. UUID is good when it's used internally and not exposed to the users.
Using random UUIDs as an invoice number wouldn't cut it, you'd have to have a sequential ID in some form. You could of course have both but that's adding unnecessary complexity.
Another example, if you are designing a website and lets say give each blog post a UUID, a URL of:
website.com/blog/my-post-34
Looks better than
website.com/blog/my-post-d173affb-6bb8-4435-bef1-4e29409dff4c
Unique sequential Id's also give some clues about the relative age of rows (eg ID 100 is older than ID 50). This doesn't come up often but can sometimes be useful.
Regardless, I always find using a unique int ID for pretty much every record makes your job 10x easier and I fail to see why you would ever want to do anything different. If you want to 'hide' the ID in special cases (eg order numbers) better to generate a unique random int ID.
The only price is on the 'delivery' table, which seems to imply that the price is only set when someone orders something. I guess this is a delivery-only place? But because the 'delivery' table can only have one food_uuid, you'd have to put in for a bunch of deliveries to order more than one thing.
Maybe the 'price' on the menu is in that 'Additional Info' table?
SELECT A.NAME, A.DESCRIPTION, B.VALUE FROM FOOD A LEFT OUTER JOIN "Additional Info" B ON (A.UUID = B.UUID AND B.TYPE = 'Price');
http://dbpatterns.com/documents/5707794a1514b4252236dcbe/
better, but not finished...
Like any open advice community like this, its important there are checks in place to not perpetuate bad ideas...
But the actual modelling functionality itself is really lacking. The lines joining the entities don't even indicate cardinality (does a ninja have many ninja_nelts? Or is it the other way around?). That's pretty much 101 for any visual database modelling.
Also the lines overlap, meaning you can't see whether ninja_weapon is related to ninja, or to ninja_belt.
I can answer these questions myself by perusing the foreign key columns - but that makes the lines redundant. Since several decades ago, Chen, Bachman, Grady Booch and a bunch of others have given ways to denote cardinality etc., and these are widely used. This tool should use one.
(apologies in advance if I have this wrong and the tool does support this but the diagram authors elected not to use them).
How do I browse these models? Is there a taxonomy of organization? How many models are in the system? Are there models relevant to general domains of interest to me? None of this is made apparent.
(We had a prior system that relied on column order and so had already banned select * and only allowed additions to the result set on the right side before we even got to the point of doing codegen or zero-downtime releases. This was 2003 that we created the codegen wrapper and 2005 or so when we switched to zero downtime releases.)
I guess "because we're lazy" is the somewhat overly glib answer. ;-)
There might be an argument about excessive normalization in some cases also. Take some of those layouts too far and try to extend them and you might wind up with tons of little tables. Normalization was I think more important back in those days (not that it's not still important... but some of the downsides of going overboard on normalization have become apparent I think.... at least to me).
SELECT
*
FROM table1
JOIN table2
ON table2.rel_id = table1.id
SELECT
table1.*
FROM table1
JOIN table2
ON table2.rel_id = table1.id
But I agree with your other arguments.
What should the primary key be if not a UUID?
I fail to see how
customer.id = product.id
customer.customerID = product.productID c.id = p.id
Which is why I don't use aliases, so that future me doesn't have to wonder what the hell he wrote a few months ago.
In project management, tasks might have an assigner_id and assignee_id
In transaction management, transactions might have a sender_id and receiver_id
In sports, matches might have a team1_id and team2_id
in normal form, shouldn't you have "transactions", "transaction_senders", and "transaction_receivers" anyway? similarly for the other cases?
i know sometimes the simplicity of a single table is a valid choice, so your point still stands, but it does feel like the examples are cases of "well, if you're were doing the right way(TM), that wouldn't happen"
I suspect that this has to do with the fact that NATURAL JOIN is intended to work for sub-SELECTs and VIEWs, too. On those, it is quite hard and error-prone to determine "foreign key" equivalents.
Also note that restricting NATURAL JOINs to tables wouldn't be a good solution here, either, because that would prevent you from ever JOINing more than 2 tables: When JOINing 3 tables, you effectively do have some kind of sub SELECT after having joined the first two tables, which is then supposed to be joined with the third one.
That is, you lose absolutely nothing - just changes its semantics.
So why not be explicit?
I ask because I have an app where previously I was accessing a particular data point via pk, and the user saw the pk in the url bar. But it could expose user/site data as any user could access that pk, or just guess at the next sequentially generated pk. I switched to uuid and now it's a little ugly in the url bar, but no data is exposed.
If there's no human interaction with the key, UUID is perfectly fine.
Yeah, I know, this could easily fixed by not friggin' showing surrogate keys to those users in the first place, but, well, data integration is ugly business.
Using a UUID instead of a sequentially rolling integer ID isn't solving your problem, you're just doing security through obscurity. The real solution is implementing real authentication & authorization - not making the key harder to guess.
A common sentiment, but not true if your id is cryptographically random. It amounts to capability security which is the right approach to security if used comprehensively.
were doing the right way(TM), that wouldn't happen
Having a separate transaction_senders and transaction_receivers wouldn't be the right way(TM) unless it's possible to have multiple senders/receivers. If a transaction can only have 1 sender/receiver then normalization is complete when the sender/receiver data is removed from 'transactions' table and replaced with the sender_id and receiver_id columns.
When introducing transaction_senders/transaction_receivers tables without further constraints, it would be immediately possible to have multiple transaction senders and multiple transaction receivers for a single transaction, which is likely an error.
Perhaps (?) a less controversial example is a table of flights between airports. Should a single flight have departure_ids and destination_ids? (assuming we treat legs as separate flights)
Or what about package delivery? Do we need sender_ids and receiver_ids there?
That's absolutely terrifying. You've just provided my subconscious with new material with which to populate my nightmares.
How did the code using that database operate? Were you using "reservation_name", or did the application revert the naming scheme by mapping the column to "name"?
Either way, FML.
- The field name implies that the primary key field is a string. This entails numerous issues (eg. How do you generate a new unique ID?) - It is not obvious that the field is the primary key. - A 'name' property on a reservation doesn't make sense. Is it the name of the person making the reservation? How can this be unique? etc.
Enforced globally unique column names - natural joins on everything.
Throw an exception if a query requires a table scan (bonus points if it printed the statement that would create the appropriate index).
Translate all update and delete statements into inserts (enforced immutable data + versioning).
EDIT: I guess that doesn't really address your point of needing to join on a composite natural key though.
It ignores things like integrating with other systems. If I export my data to an OLAP database, I can't just cascade that key change through it without manual effort.
It also ignores managing historical data. Maybe I don't want that natural key update cascaded everywhere. In their example of hotel room number, if that public-facing number changed (which does happen), I wouldn't necessarily want that cascaded everywhere. For example, I might need past reservations/invoices from before the change to keep the old room number, to match all correspondence with the guest, etc. Okay, so maybe you don't automatically cascade everywhere, but then you need some way to link that old room number and the new room number in all of your reporting.
Bleh. That's a lot of headache that can be greatly minimized with a surrogate key, for very little drawback in my experience. Sure, I can imagine scenarios where the performance impact or additional storage could actually be a real negative, but for your average CRUD app, I think surrogate keys add way, way more value than cost.
Which means that the original argument about reserveration_id vs id doesn't matter since it's likely that both table and column aliases would be needed anyways.
That's why ideally natural keys should be immutable. That has many advantages over surrogate keys.
There's a follow up post (posts) to this presentation: http://www.databasesoup.com/2015/03/primary-keyvil-reprised.... where many problems caused by the abuse of surrogate keys are illustrated.
But the main point is simple: surrogate keys (abuse) is an easy way to data corruption. Not in the sense of durability as in ACID, but in the sense of humans (manually or by bugs in the software) duplicating information that is not enforced to be unique thanks to using non-sense surrogate keys as the uniqueness criteria.
Your point about immutable keys is important, I think. A "natural key" can mean different things, as I understand the term. You could call a person's social security number a natural key, which for the record is a bad idea as they can change . But you could also call the combination of user_id and reservation_id a composite natural key, which is immutable and not so bad in my book. I'd still use a surrogate key though.
It might seem like I care about this more than I actually do. I've just found over the years of writing systems that I fairly often regretted not having a surrogate key on a table, but I've never once regretted adding a surrogate key that ended up not being essential.
What I've seen happen is that the data model expands/changes and I need to reference a join table. I might just start out referencing it by the composite natural key. Then the model changes again, and now I need to add another piece of data that changes the natural key. For example, maybe the natural key was (user_id, reservation_id), but now it's (user_id, reservation_id, membership_type_id). Now I need to think about anywhere I referenced that join table by the natural key.
I have seen many cases where concepts that are UNIQUE NOT NULL for a given business case are ignored as natural keys and surrogate keys are used instead. This results in "duplicated" records (from a business logic perspective) that often end up being manually cleaned, resulting in costs in the orders of 10s of thousands. Not funny.
The idea of a primary key is that you are guaranteed 0 or 1 results, but in the absence of UNIQUE and NOT NULL you may get more than one. That's a powerful abstraction. If your business logic determine that a given attribute or combination of attributes satisfies that condition, that deserves to be the row key.
If you rather use a surrogate key:
a) either you also have to enforce UNIQUE and specially NOT NULL on those attributes (apart from the PK) which is definitely not free lunch (extra indexes, more expensive joins, not benefiting from index-only scans)
b) or often someone forgets either the UNIQUE or NOT NULL on the attribute(s) that have to be constrained as such and data integrity is compromised.
Both cases are bad enough. What's the justification to systematically use surrogate keys then?
UUID key is a problem only if your main query is a range query on the PK of a clustered indexed table. If your main query is a range query, you should pick something that can be sorted in the range anyway.
Edit: I'm ~wrong, see below.
MSSQL allows clustered indexed table as an option to order the physical storage of rows. Oracle has index-organized table as an option.
Edit: They don't use clustered indexed table by default because record insertion is very expensive since clustered index forces the table to store the records contiguously in the index's order. Also Innodb is not truely clustered indexed. It only stores records contiguously for one page at the B+ Tree leaf level. Records in different pages are scattered all over even if the index values are sequential.
Primary Key is implemented as a unique index coupled with a not null constraint. It's essentially syntactic sugar.
1. http://www.postgresql.org/docs/9.5/static/sql-cluster.html
A UUID is a 128-bit integer. That people do not store, generate, or interact with them that way is the bug.
I'd kill for a 128-bit architecture so a UUID compare would be a single instruction, and it's too bad consensus is that we don't really need it.
I don't believe that's true. You're right that they start the session over, but it'll still be a fresh sequence that doesn't overlap anything generated in the past.
I generally prefer auto-incremented integers, but UUIDs are very useful for client-generated records (like for an app/website that's built for offline usage).
As I always say, if we don't split hairs now, we'll be splitting heads later.
If you have a clustered index based upon that column (and in systems that support this it is common for the primary key to be the clustered index for the table) then the physical layout of the data will follow this column.
See http://use-the-index-luke.com/blog/2014-01/unreasonable-defa... as a good run down of the whys and wherefores of clustered indexes.
Without that ordering, the index is 'dumb', and has to find all your rows one by one in its structure. Admittedly it's not as much of an issue for SSDs, but it does still impact performance.
Using natural keys may offer better performance in certain cases, but does not protect you from b, as you might select a wrong key, or even forget about putting a PK on the table, for example.
Surrogates help you manage the ever changing and interconnected world: the immutable name of something that has been shared between systems is no longer immutable? This type of scenario is also very common and can cost thousands as well.
Which type of problem is more likely in your application? That's up to you to decide.
That looks basically like what you need, though, yeah. Cool.
This perfectly summarizes the bitwise-innumeracy of the argument against UUIDs. Even without 128 bit word sizes, most UUIDs are going to be non-matches in their lower bits, assuming a random distribution. There's no computational efficiency gained in the vastly wide critical path here.
But you can't assume a random distribution because anyone using UUID's for database keys isn't using random UUID's, they're using sequential ones.
You could actually vectorize the compares in SSE, I believe (but I stopped writing assembly before SSE existed and don't know much about it at a low level). This is why it's so critical to treat a UUID as an integer instead of a string, and I see a lot of code that does the latter.