On 6/3/2015 2:50 AM, Adrian Stern wrote:
Hi William, thanks for joining the conversation.
1) We do hope for constraints since a connection to an ERP system is
possible in the future. We want to plan ahead.
2) As for the subclass approach: I would need about 30 subclasses and
it will get really hard to add new products since a change in the
database will be necessary each time. That's why we want a more
generic approach.
At the root of your description it appears to me that you are choosing
essentially an EAV design pattern. Constraints and case specific
validation become difficult (or impossible) to consistently enforce. I
have personal experience in cleaning up a system that used this *exact*
pattern (product -> product attribute). Different developers approached
updating information in different ways. They also chose to store the
attributes as text to avoid the text vs number issue. However, they
failed to force any validation and any hint of attempting to prevent
duplicates. That destroyed hope of supporting an underlying business
need to search for specific values during data analysis. (think of how
many different ways you can misspell "poughkeepsie" - but hey... it's
"just a name")
I inherited the results of poor development controls and poor control
over the weakness of the design - e.g. validation... and the cleanup has
been long, and painful.
I think you should evaluate your unease with having to update the
database on release (potentially many times) carefully for what it is
and why you have it. [I'm not saying it is invalid - just know why you
have it] Because no matter how well you design your system - databases
evolve. Manage that.
Anybody can muck up part of a project and cause garbage - but speaking
from experience... this design pattern really encourages it. If you
choose to use it - then you HAVE to control where and when
inserts/updates are done and be very careful with specifying how
validation is to be done to the entire development team (and potentially
the users)... and then review the code (and/or data) regularly.
wide tables/sparse fill on the columns ... I haven't tried to calculate
the overhead on this... but disk space is generally considered to be
cheap. [that doesn't mean your case wouldn't be criminally wasteful]
Choosing 1 wide table or 30 sub-tables to deal with detail data. I don't
know how that directly effects Postgres' performance.... if you choose
30 tables ... meh... at least you'll know exactly where your data is -
and exactly what is allowed for each and every variant. Remember to
enforce a consistent guessable naming convention.
All that said - there are reasons to use this pattern to gain the power
of the dynamically expandable configuration of allowed values. I just
see the use cases where the gain (flexibility) is worth the pain
(validation control) to be few and far between.
just my $0.01
Roxanne
Maybe I don't understand you right, because of the language barrier.
Can you provide me a link to a subclassing example?
->
https://docs.djangoproject.com/en/1.8/topics/db/models/#multi-table-inheritance
?
ORM is a given in my case. This is not a high performance application.
Freundliche Grüsse
Adrian Stern
unchained - web solutions
adrian.st...@unchained.ch <mailto:adrian.st...@unchained.ch>
+41 79 292 83 47
On Tue, Jun 2, 2015 at 5:35 PM, William Dunn <dunn...@gmail.com
<mailto:dunn...@gmail.com>> wrote:
Hello Adrian,
May I ask why you need a non-standard model? By standard models I
mean the following:
1) When you don't need to have subclass specific database
constraints: All subclasses in the same table, subclasses that do
not have an attribute have that column null. This has the best
performance because no joins are required when querying both
superclass and subclass attributes, and all the data for an object
will be in the same block on disk. The disadvantage of this is
that you cannot enforce constraints, such as not-null, on subclass
specific attributes columns because the constraints would also be
applied to the superclasses. If you can ensure that your
application is the only way data can be inserted those constraints
will naturally be enforced there.
2) When you need subclass specific database constraints: Use an
ORM such as Django's ORM or SQLAlchemy which has one table with
the superclass attributes and a table for each subclass with their
subclass specific attributes. This is slower because joins will be
needed and the data for an object will be in 2 different blocks of
disk but it allows you to enforce constraints within the database
which will be checked whenever any application tries to insert
values. There is a lot of complexity added because there will be
so many small tables and indexes but the ORM takes care of that
for you.
*Will J. Dunn*
_willjdunn.com <http://willjdunn.com>_
On Mon, Jun 1, 2015 at 10:35 AM, Adrian Stern
<adrian.st...@unchained.ch <mailto:adrian.st...@unchained.ch>> wrote:
Hi, I'm new
I've been working as the sole administrator of various
postgresql projects for a while now. All of which where django
projects.
Since a new project is starting and we've found the need for a
more generic approach I would like to ask a few questions.
I would like to implement a pattern similar to the product
feature pattern explained in the silverstone book - the data
model resource book vol 1. It is simply explained. There is a
Table PRODUCT holding the fields all the products share, then
there is the table PRODUCT_FEATURE, both of them in a “many to
many“ relationship.
PRODUCT <--- m -------- n ---> PRODUCT_FEATURE (a table in
between of course)
PRODUCT_FEATURE --> PF
PRODUCT --> P
TABLE IN BETWEEN --> TIB
PF defines the feature Type while P stands for the product the
feature is applied to. Some of these PF can have values of
different types (text, numbers, floating, blob, ...) which
would be applied to TIB.
I don't like the idea of having numerous empty fields prepared
in TIB, just to store occasional values of different types,
therefore I need to specialize those TIB Values.
Now how would I do That?
I could create some tables solely for the means of holding
[NUM], [TEXT], [BLOB], [ETC] and reference them with the TIB
PK. When using them I could create a view TIBV containing all
of [NUM, TEXT, BLOB, ETC] in the same column called Value, and
join it with TIB to get the value of a PF.
But is this a good idea?
Is there a better way?
Also, I would have to create a pivot table in order to list
all the products with all the features. As this is implemented
in C (afaik) I suppose it is rather fast or at least fast
enough, but I do not actually know. What I know is, there are
about 30 Product Types and around 50 possible product
features. One product can have up to approximately 25 PF but
are mostly around 5 to 10.
Do you think a pivot table is a good idea?
What alternative do i have?
There is room for caching since the dataset is not updated too
often.
regards, adrian
--
[At other schools] I think the most common fault in general is to teach
students how to pass exams instead of teaching them the science.
Donald Knuth
