multiple fixed tables vs flexible abstract tables

Question

I was wondering if you have a website with a dozen different types of listings (Shops, Restaurants, Clubs, Hotels, Events) that require different fields, is there a benefit of creating a table with collumns defined like so
Example Shop:

shop_id | name | X | Y | city | district | area | metro | station | address | phone | email | website | opening_hours

Or a more abstract approach similar to this:

object_id | name        
---------------
1         | Messy Joe's  
2         | Bate's Motel 

type_id | name
---------------
1       | hotel
2       | restaurant


object_id | type_id
---------------
1         | 2
2         | 1

field_id | name           | field_type
---------------
1        | address        | text
2        | opening_hours  | date 
3        | speciality     | text

type_id | field_id
---------------
1       | 1
1       | 2
2       | 1
2       | 3

object_id | field_id | value
1         | 1        | 1st street....
1         | 3        | English Cuisine

Of course it can be more abstract if value's are predefined (Example: specialities could have their own list)

If I take the abstract approach it can be very flexible, but queries will be more complex with a lot of joins. But I don't know if this affects the performance, executing these 'more complex' queries.

I would be interested to know what are the up and downsides of both methods. I can just imagine for myself, but I don't have the experience to confirm this.

Answer 1

The "abstract" approach is better known as "Normalization", looks like 3rd Normal Form (3NF).

The other one is called "Denormalized", and can be a valid performance option... when you've encountered speed issues using the Normalized approach, not before.

Answer 2

How do you have the listings represented in code? I'd guess Listing as a supertype, with Shop, Restuarant, etc. as subtypes?

Assuming so, this is a case of how to map subtypes to a relational database. There are generally three choices:

• Option 1: single table per subtype, with common attributes repeated in each table (name, id, etc).
• Option 2: single table for all objects (your single table approach)
• Option 3: table for the supertype and one for each subtype

There's no universally correct solution. My preference is generally to start with option 3; it provides an intituitive structure to work with, is pretty well normalised and can easily be extended. It means a single join for retrieving each instance - but RDBMS are well optimised for doing joins so it doesn't really cause performance problems in practice.

Option 2 can be more performant for queries (no joins) but causes problems if other tables need to refer to all supertype instances (proliferation of foreign keys).

Option 1 appears at first sight to be the most performant, although 2 caveats: (1) It's not resilient to change. If you add a new subtype (and so different attributes) you'll need to change the table structure and migrate it. (2) It can be less efficient than it seems. Because the table population is sparse, some DBs don't store it particularly efficiently. As a consequence it can be less efficicent than option 1 - since the query engine can do joins faster than it can search bloated sparse table spaces.

Which to choose really comes down to knowing details of your problem. I'd suggest reading up a bit on the options: this article is a good place to start.

hth

Answer 3

Certain issues need to be clarified and resolved before we can enter into a reasonable discussion.

Pre-requisite Resolution

1. Labels
   In a profession that demands precision, it is important that we use precise labels, to avoid confusion, and so that we can communicate without having to use long-winded descriptions and qualifiers.
   .
   What you have posted as FixedTables, is Unnormalised. Fair enough, it may be an attempt at Third Normal form, but in fact it is a flat file, Unnormalised (not "denormalised). What you have posted as AbstractTables is, to be precise, Entity-Attribute-Value, which is almost, but not quite, Sixth Normal form, and is therefore more Normalised than 3NF. Assuming it is done correctly, of course.

   • The Unnormalised flat file is not "denormalised". It is chock full of duplication (nothing has been done to remove repeating groups and duplicate columns or to resolve dependencies) and Nulls, it is a performance hog in many ways, and prevents concurrency.

   • In order to be Denormlaised, it has to first be Normalised, and then the Normalisation backed off a little for some good reason. Since it is not Normalised in the first place, it cannot be Denormalised. It is simply Unnormalised.

   • It cannot be said to be denormalised "for performance", because being a performance hog, it is the very antithesis of performance. Well, they need a justification for the lack of formalised design], and "for performance" is it. Even the smallest formal scrutiny exposed the misrepresentation (but very few people can provide, so it remains hidden, until they get an outsider to address, you guessed it, the massive performance problem).

   • Normalised structures perform far better than Unnormalised structures. More normalised structures (EAV/6NF) perform better than less normalised structures (3NF/5NF).

   • I am agreeing with the thrust of OMG Ponies, but not their labels and definitions

   • rather than saying 'don't "denormalise" unless you have to', I am saying, 'Normalise faithfully, period' and 'if there is a performance problem, you have not Normalised correctly'.
     .

2. Wiki
   The entries re Normal Forms and Normalisation are a complete joke. Specifically, the definitions are incorrect; they confuse the Normal Forms; they are clueless re the process of Normalisation; and they give equal weight to absurd or questionable NFs which have been debunked long ago. The result is, Wiki adds to an already confused and rarely understood subject. So don't waste your time.
   .
   However, in order to progress, without that reference posing a hindrance, let me say this.

   • The definition of 3NF is stable, and has not changed.
   • There is a lot of confusion of the NFs between 3NF and 5NF. The truth is that this is an area that progressed over the last 15 years; and many orgs, academics as well as vendors with their products with limitations, jumped to create a new "Normal Form" to validate their offerings. All serving commercial interests and academically unsound. 3NF in its original untampered state intended and guaranteed certain attributes.
   • The sum total is, 5NF is today, what 3NF was intended to be 15 years ago, and you can skip the commercial banter and the twelve or so "special" (commercial and pseudo-academic) NFs in-between, some of which are identified in Wiki, and even that in confusing terms.
     .

3. Since you have been able to understand and implement the EAV in your post, you will have no problem understanding the following. Of course a true Relational Model is pre-requisite, strong keys, etc. Fifth Normal Form is, since we are skipping the Fourth:

   • Third Normal Form
     • which in simple definitive terms is, every non-key column in every table has a 1::1 relationship to the Primary Key of the table,
     • and to no other non-key columns
   • Zero data duplication (the result, if Normalisation is progressed diligently; not achieved by intelligence or experience alone, or by working toward it as a goal sans the formal process)
   • no Update Anomalies (when you update a column somewhere, you do not have to update the same column located somewhere else; the column exists in one and only one place).
     .

4. Sixth Normal Form is of course Fifth Normal Form, plus:

   • Elimination of missing data (columns). This is the one true solution to the Null Problem (also called Handling Missing Values), and the result is a database without Nulls. (It can be done at 5NF with standards and Null substitutes but that is not optimal.) How you interpret and display the missing values is another story.
     .
5. EAV vs Sixth Normal Form
   All the databases I have written, except one, are pure 5NF. I have worked with (administered, fixed up, enhanced) a couple of EAV databases, and I have implemented one true 6NF database. EAV is a loose implementation of 6NF, often done by people who do not have a good grasp on Normalisation and the NFs, but who can see the value in, and need the flexibility of, EAV. You are a perfect example. The difference is this: because it is loose, and because implementors do not have a reference (6NF) to be faithful to, they only implement what they need, and they write it all in code; that ends up being an inconsistent model.
   .
   Whereas, a pure 6NF implementation does have a pure academic reference point, and thus it is usually tighter, and consistent. Typically this shows up in two visible elements:
   • 6NF has a catalogue to contain metadata, and everything is defined in metadata, not code. EAV does not have one, everything is in code (implementers keep track of the objects and attributes). Obviously a catalogue eases the addition of columns, navigation, and allows utilities to be formed.
   • 6NF when understood, provides the true solution to The Null Problem. EAV implementers, since they are absent the 6NF context, handle missing data in code, inconsistently, or worse, allow Nulls in the database. 6NF implementers disallow Nulls, and handle missing Data consistently and elegantly, without requiring code constructs (for Null handling; you still have to code for missing data of course).
     .
     Eg. For 6NF databases with a catalogue, I have a set of procs that will [re]generate the SQL required to perform all SELECTs, and I provide Views in 5NF for all users, so they do not need to know or understand the underlying 6NF structure. They are driven off the catalogue. Thus changes are easy and automated. EAV types do that manually, due to the absence of the catalogue.

Now, we can start the

Discussion

"Of course it can be more abstract if value's are predefined (Example: specialities could have their own list)"

Sure. But do not get too "abstract". Maintain consistency and implement such lists in the same EAV (or 6NF) manner as other lists.

"If I take the abstract approach it can be very flexible, but queries will be more complex with a lot of joins. But I don't know if this affects the performance, executing these 'more complex' queries."

1. Joins are pedestrian in Relational databases. The problem is not the database, the problem is that SQL is cumbersome when handling joins, especially compound keys.
2. EAV and 6NF databases have more Joins, which just as pedestrian, no more no less. If you have to code each SELECT manually, sure, the cumbersome gets really cumbersome.
3. The entire problem can be eliminated by (a) going with 6NF over EAV and (b) implementing a catalogue, from which you can (c) generate all the basic SQL. Eliminates an entire class of errors as well.
4. It is a common myth that Joins somehow have a cost. Totally false. The join is implemented at compile time, there is nothing of substance to 'cost' CPU cycles. The issue is the size of tables being joined, not the cost of the Join between those same tables. Joining two tables with millions of rows each, on a correct PK⇢FK relation, each of which have the appropriate indices (Unique on the parent[FK] side; Unique on the Child side) is instantaneous; ; where the Child index is not unique, but at least the leading column is valid, it is slower; where there is no useful index, of course that is very slow. None of it has to do with Join cost. Where many rows are returned, the bottleneck will be the network and the disk layout; not the join processing.
5. Therefore you can get as "complex" as you like, there is no cost, SQL can handle it.

I would be interested to know what are the up and downsides of both methods. I can just imagine for myself, but I don't have the experience to confirm this.

1. 5NF (or 3NF for those who have not made the progression) is the easiest and best, in terms of implementation, ease of use (developers as well as users), maintenance. The drawback is, every time you add a column, you have to change the database structure (table DDL). That is fine is some cases, but not in most cases, due to change control in place, quite onerous. Second, you have to change existing code (code handling the new column does not count, because that is an imperative): where good standards are implemented, that is minimised; where they are absent, the scope is unpredictable.

2. EAV (which is what you have posted), allows columns to be added without DDL changes. That is the single reason people choose it. (code handling the new column does not count, because that is an imperative). If implemented well, it will not affect existing code; if not, it will. But you need EAV-capable developers. When EAV is implemented badly, it is abominable, a worse mess than 5NF done badly, but not any worse than Unnormalised which is what most databases out there are (misrepresented as "Denormalised for performance"). of course, it is even more important (than in 5NF/3NF) to hold a strong Transaction context, because the columns are far more distributed. Likewise, it is essential to retain Declarative Referential Integrity: the messes I have seen were due in large part to the developers removing DRI because it became "too hard to maintain", the result was, as you can imagine, one mother of a data heap with duplicate 3NF/5NF rows and columns all over the place. And inconsistent Null handling.

3. There is no difference in performance, assuming that the server has been reasonably configured for the intended purpose. (Ok, there are specific optimisations that are possible only in 6NF, which are not possible in other NFs, but I think that is outside the scope of this thread.) And again, EAV done badly can cause unnecessary bottlenecks, no more so than Unnormalised.

4. Of course, if you go with EAV, I am recommending more formality; buy the full quid; go with 6NF; implement a catalogue; utilities to produce SQL; Views; handle Missing Data consistently; eliminate Nulls altogether. This reduces your vulnerability to the quality of your developers; they can forget about the EAV/6NF esoteric issuses, use Views, and concentrate on the app logic.

Pardon the long post.

Answer 4

In your question, you have presented at least two major issues at the same time. Those two issues are E-A-V and gen-spec.

First, let's talk about E-A-V. Your last table (object_id, field_id, value) is essentially an E-A-V. There is an upside to E-A-V and a downside to E-A-V. The upside is that the structure is so generic that it can accomodate almost any body of data describing almost any subject matter. That means that you can proceed to design and implementation with no data analysis and no understanding of the subject matter, and not worry about wrong assumptions. The down side is that at retrieval time, you have to do the data analysis that you skipped over before building the data base, in order to come up with queries that mean anything. This is much more serious than just retrieval efficiency. But you are also going to have terrible problems with retrieval efficiency. There are only two ways to learn about this pitfall: live through it or read about it from those who have. I recommend the reading.

Second, you have a gen-spec case. Your table (object_id, type_id) captures a gen-spec (generalization-specialization) pattern, along with the related tables. If I had to generalize between hotels and restaurants, I might call it something like "public accomodations" or "venues". But I'm not sure I understand your case, and you may be driving for something even more general than those two names suggest. After all, you've included "events" in your list, and an event is not a type of venue in my mind.

I've referred other people to readings on gen-spec and the relational model in previous responses.
http://stackoverflow.com/questions/1478112/when-two-tables-are-very-similar-when-should-they-be-combined/1478234#1478234

But I hesitate to send you off in the same direction, because it's not clear to me that you want to come up with a relational model of the data before building your database. A relational model of a body of data and an E-A-V model of the same data are almost totally at odds with each other. It seems to me you have to make that choice before you even explore how to express gen-spec in the relational model of data.