Databases: Good practice to have minimal fields? - django

I'm new to web development and databases (I am currently using Django and PostgreSQL) and I have a general question about databases, because other than the fact that they store data, I pretty much know nothing about them.
I was wondering if it's good practice to have as few fields as possible in my models?
For instance, I have a Model that has a DateTimeField() and was considering also creating a CharField() that corresponds to the month that the instance is related to (I won't delve into details). The consideration of adding this month field sparked me to ask this question.
I clearly don't need the month field because I could parse it out of the DateTimeField(), but it's more convenient to just have a string with the month name rather than parse it. Is it acceptable to add another field for convenience or should I have as few fields as possible?

Really not very pleasant to have to much field, but if a field is month like that, you can use computed field. I don't know if that exists in Django. If not, just use a view to show that month. You will get trouble when updating that and forgot to update one of the field holding same information.

My experience: if database is stored on fast hard disk, best much more fields, because you save time to write code and cpu resources of machine where is the client of database; instead of databse is on old or not fast machine, is better that the client make the job.
Same if the connection from client and server is slow.

What you want to do is called Database Denormalization. By doing that you can optimize the read performance of the table, but you also have to take responsibility that any redundant copies are kept consistent.
So do you really have such performance drawback? (I doub it but..) If you really have, what you can do before adding redundant columns is to try adding column indexes.

Related

Django model internationalization

What is the best way to have model fields translated in Django? I've thought about adding extra fields to the model (one field per language) or creating another model with all texts in every language, is there a recommended way to achieve that?
Thank you very much
NB : I first voted to close this as primarily opinion based but then it struck me that there were actually technical reasons to choose one solution or the other...
Both approach are valid and as a matter of fact you'll find reusable django apps based on either one of the other.
Technically, there are pros and cons to each design.
Using distinct "translation" objects means you'll have an additional join or query (to get both the "master" model and it's translation(s)), but you have no overhead on the master model itself (without translation). Also, it makes create/update operations more complicated.
Using additional "hidden" per-language fields avoids the join / additional query overhead and keeps create/update operation simple, but makes records much bigger so it has some overhead wrt/ the database itself (page cache management etc) and the volume of data going back and forth between your django process and the database.
As a general rule, if you have to support a lot of languages and/or have to translate a lot of text fields for each model, you'll probably want to use a distinct model for translations, while if you have few languages and only a couple "translatable" fields per model the "hidden field" approach will be simpler to implement and will avoid the extra queries / joins.
As far as I'm concerned, I've had experience with both solutions and found the "hidden field" solution (using django-modeltranslations) to work fine for our current needs (four languages supported and we should not get much more, no more than =~ four translatable fields per model, and those models are rarely updated so we can cache aggressively if needed), but you may have totally different needs.
In all cases, don't even try implementing this from scratch, use one of the existing django apps instead, it will save you a lot of time and pain.
Check out their docs django translation
This will help too
Localization: How to Create Language Files - Python Django Tutorials

Correct implementation of the Filter (Criteria) Design Pattern

The design pattern is explained here:
http://www.tutorialspoint.com/design_pattern/filter_pattern.htm
I'm working on a software very similar to Adobe Lightroom or ACDSee but with different purposes. The user (photographer) is able to import thousands of images from his hard drive (it wouldn't be weird to have over 100k/200k images).
We have a side panel where users can create custom "filters" which are expressions like:
Does contain the keyword: "car"
AND
Does not contain the keyword "woods"
AND
(
Camera model is "Nikon D300s"
OR
Camera model is "Canon 7D Mark II"
)
AND
NOT
Directory is "C:\today_pictures"
You can get the idea from the above example.
We have a SQLite database where all image information is stored. The question is, should we load ALL Photo objects into memory from the database the first time the program is loaded and implement the Criteria/Filter design pattern as explained in the website cited above so our Criteria classes filter objects or is better to do the criteria classes actually generate an SQL query that is finally executed in order to retrieve only what's needed from the database?
We are developing the program with C++ (QT).
TL;DR: It's already properly implemented in SQLITE3, and look at how long that took. You'll face the same burden.
It'd be a horrible case of data duplication to read the data from the database and store it again in another data structure. Use database queries to implement the query that the user gave you. Let the database execute the query. That's what databases are for.
By reimplementing a search/query system for ~500k records, you'll be rewriting large chunks of a bog-standard database yourself. It'd be a mostly pointless exercise. SQLITE3 is very well tested and is essentially foolproof. It'll cost you thousands of hours of work to reimplement even a small fraction of its capabilities and reliability/resiliency. If that doesn't scream "reinventing the wheel", I don't know what does.
The database also allows you to very easily implement lookahead/dropdowns to aid the user in writing the query. For example, as you're typing out "camera model is", the user can have an option of autocompletion or a dropdown to select one or more models from.
You paid the "price" of a database, it'd be a shame for it all to go to waste. So, use it. It'll give you lots of leverage, and allow you to implement features two orders of magnitude faster than otherwise.
The pattern you've linked to is just a pattern. It doesn't mean that it's an exact blueprint of how to design your application to perform on real data. You'll be, eventually, fighting things such as concurrency (a file scanning thread running to update the metadata), indexing, resiliency in face of crashes, etc. In the end you'll end up with big chunks of SQLITE reimplemented for your particular application. 500k metadata records are nothing much, if you design your query translator well and support it with proper indexes, it'll work perfectly well.

When to use Haystack/ElasticSearch vs Django's ORM

So I implemented Haystack with ElasticSearch a week ago within our BETA application. One thing I can notice is that getting some data (large amount) back to our users (for example listing all the users within the application) is much faster by going through Haystack then Django's ORM. Now, I will be releasing a REST service (with TastyPie) to serve the possible tablets within the next weeks, as I want to be able to access the information from iPads, Nexus tablets and so on.
One thing I was wondering, is when should I be querying the ORM vs Haystack/ElasticSearch? For example, if the user on the tablet is requesting a specific set of users, should we let TastyPie query the ORM, or go to ElasticSearch?
If we look at this answer Django: Haystack or ORM, we can all agree that a DB is made to retrieve and write data. However, could we say that retrieving faster can be faster with Haystack/ElasticSearch once the search engine was updated?
I am a bit confused as to when, should we not be querying Haystack if it is much faster?!
To make things clear I guess you're talking about querying Elasticsearch via Haystack without later instantiating any objects for your search results with data from you database.
Some points to consider besides the points mentioned in the other post:
A search engine like Elasticsearch is highly optimized when dealing with full-text searches (When doing something with SQL it highly depends on the database/engine you are using)
Queries that are involving a lot of relations/joins will most like be easier to handle with the ORM, but on the other hand you can eg save data from foreign-key relations in a denormalized fashion when using ES which could give you a performance boost. Of course you can denormalize your database tables as well but this is quite often considered as a bad practice as long as you know what you are doing, eg when solving a performance bottleneck.
ES is somehow quite easy to scale while scaling your SQL DB might be more complicated.
Most likely this is a decision that depends very much on your use case, the amount of data to process and the queries you are intending to run. So the best thing of course is - as always - to do some benchmarking yourself and compare this two solutions. But don't do any premature optimisations as one big advantage of the ORM is to keep things simple - you don't have to care much about the integrity of your data and maintain an additional system.

Django/Sqlite Improve Database performance

We are developing an online school diary application using django. The prototype is ready and the project will go live next year with about 500 students.
Initially we used sqlite and hoped that for the initial implementation this would perform well enough.
The data tables are such that to obtain details of a school day (periods, classes, teachers, classrooms, many tables are used and the database access takes 67ms on a reasonably fast PC.
Most of the data is static once the year starts with perhaps minor changes to classrooms. I thought of extracting the timetable for each student for each term day so no table joins would be needed. I put this data into a text file for one student, the file is 100K in size. The time taken to read this data and process it for a days timetable is about 8ms. If I pre-load the data on login and store it in sessions it takes 7ms at login and 2ms for each query.
With 500 students what would be the impact on the web server using this approach and what other options are there (putting the student text files into a sort of memory cache rather than session for example?)
There will not be a great deal of data entry, students adding notes, teachers likewise, so it will mostly be checking the timetable status and looking to see what events exist for that day or week.
What is your expected response time, and what is your expected number of requests per minute? One twentieth of a second for the database access (which is likely to be slow part) for a request doesn't sound like a problem to me. SQLite should perform fine in a read-mostly situation like this. So I'm not convinced you even have a performance problem.
If you want faster response you could consider:
First, ensuring that you have the best response time by checking your indexes and profiling individual retrievals to look for performance bottlenecks.
Pre-computing the static parts of the system and storing the HTML. You can put the HTML right back into the database or store it as disk files.
Using the database as a backing store only (to preserve state of the system when the server is down) and reading the entire thing into in-memory structures at system start-up. This eliminates disk access for the data, although it limits you to one physical server.
This sounds like premature optimization. 67ms is scarcely longer than the ~50ms where we humans can observe that there was a delay.
SQLite's representation of your data is going to be more efficient than a text format, and unlike a text file that you have to parse, the operating system can efficiently cache just the portions of your database that you're actually using in RAM.
You can lock down ~50MB of RAM to cache a parsed representation of the data for all the students, but you'll probably get better performance using that RAM for something else, like the OS disk cache.
I agree with some of other answers which suggest to use MySQL or PostgreSQL instead of SQLite. It is not designed to be used as production db. It is great for storing data for one-user applications such as mobile apps or even a desktop application, but it falls short very quickly in server applications. With Django it is trivial to switch to any other full-pledges database backend.
If you switch to one of those, you should not really have any performance issues, especially if you will do all the necessary joins using select_related and prefetch_related.
If you will still need more performance, considering that "most of the data is static", you actually might want to convert Django site a static site (a collection of html files) and then serve those using nginx or something similar to that. The simplest way I can think of doing that is to just write a cron-job which will loop over all needed url-configs, request the page from Django and then save that as an html file. If you want to go into that direction, you also might want to take a look at Python's static site generators: Hyde and Pelican.
This approach will certainly work much faster then any caching system however you will loose any dynamic components of the site. If you need them, then caching seems like the best and fastest solution.
You should use MySQL or PostgreSQL for your production database. sqlite3 isn't a good idea.
You should also avoid pre-loading data on login. Since your records can be inserted in advance, write django management commands and run the import to your chosen database before hand and design your models such that when a user logs in, the user would already be able to access and view/edit his or her related data (which are pre-inserted before the application even goes live). Hardcoding data operations when log in does not smell right at all from an application design point-of-view.
https://docs.djangoproject.com/en/dev/howto/custom-management-commands/
The benefit of designing your django models and using custom management commands to insert the records right way before your application goes live implies that you can use django orm to make the appropriate relationships between users and their records.
I suspect - based on your description of what you need above - that you need to re-look at the approach you are creating this application.
With 500 students, we shouldn't even be talking about caching. If you want response speed, you should deal with the following issues in priority:-
Use a production quality database
Design your application use case correctly and design your application model right
Pre-load any data you need to the production database
front end optimization comes first (css/js compression etc)
use django debug toolbar to figure out if any of your sql is slow and optimize specifically those
implement caching (memcached etc) as needed
As a general guideline.

Interacting with a simple database in c++?

I'm making a game in c++ that will need to store and retrieve information about players (name, email, high score) etc. I thought of trying to just do it myself with XML but I think a real database (maybe SQL?) would do a better job since over time there may be thousands of users.
Are there libraries to do simple interactions with databases like queries, retrieving information, and storing information?
Thanks
Yes, SQLite will do exactly this. It stores the database as a local file, so if you want an online database server then this is perhaps not the best option.
It sounds like sqlite might be a good fit for you. It doesn't need a constantly running database server, and it does things smarter than hand-rolled xml-serialization.
Sqlite has documentation.