BASIC Object-Relation Mapping question asked by a noob

Question

I understand that, in the interest of efficiency, when you query the database you should only return the columns that are needed and no more.

But given that I like to use objects to store the query result, this leaves me in a dilemma:

If I only retrieve the column values that I need in a particular situation, I can only partially populate the object. This, I feel, leaves my object in a non-ideal state where only some of the properties and methods are available. Later, if a situation arises where I would like to the reuse the object but find that the new situation requires a different but overlapping set of columns to be returned, I am faced with a choice.

Should I reuse the existing SQL and add to the list of selected columns the additional fields that are required by the new situation so that the same query and object mapping logic can be reused for both? Or should I create another method that results in the execution of a slightly different SQL which results in the populating of only those object properties that were returned by the 2nd query?

I strongly suspect that there is no magic answer and that the answer really "depends" on the situation but I guess I am looking for general advice. In general, my approach has been to either return all columns from the queried table or to add to the query the additional columns as they are needed but to reuse the same SQL (and mapping code) that is, until performance becomes a concern. In general, I find that unless you are retrieving a large number of row - and I usually am not - that the cost of adding additional columns to the output does not have a noticable effect on performance and that the savings in development time and the simplified API that result are a good trade off.

But how do you deal with this situation when performance does become a factor? Do you create methods like

Employees.GetPersonalInfo Employees.GetLittleMorePersonlInfoButMinusSalary etc, etc etc

Or do you somehow end up creating an API where the user of your API has to specify which columns/properties he wants populated/returned, thereby adding complexity and making your API less friendly/easy to use?

Let's say you want to get Employee info. How many objects would typically be involved?

1) an Employee object 2) An Employees collection object containing one Employee object for each Employee row returned 3) An object, such as EmployeeQueries that returns contains methods such as "GetHiredThisWeek" which returns an Employees collection of 0 or more records.

I realize all of this is very subjective, but I am looking for suggestions on what you have found works best for you.

Answer 1

...the cost of adding additional columns to the output does not have a noticable effect on performance...

Right. I don't quite understand what "new situation" could arise, but either way, it would be a much better idea (IMO) to get all the columns rather than run multiple queries. There isn't much of a performance penalty at all for getting more columns than you need (although the queries will take more RAM, but that shouldn't be a big issue; besides, hardware is cheap). Also, you'd save yourself quite a bit of development time.

As for the second part of your question, it's really up to you. As an example, Rails takes more of a "usability first, performance last" approach, but that may not be what you want. It just depends on your needs. If you're willing to sacrifice a little usability for performance, by all means, go for it. I would.

Answer 2

I would say make your application correct first, then worry about performance in this case.

You could be optimizing away your queries only to realize you won't use that query anyway. Create the most generalized queries that your entire app can use, and then as you are confident things are working properly, look for problem areas if needed.

It is likely that you won't have a great need for huge performance up front. Some people say the lazy programmers are the best programmers. Don't over-complicate things up front, make a single Employee object.

If you find a need to optimize, you'll create a method/class, or however your ORM library does it. This should be an exception to the rule; only do it if you have reason to do so.

Answer 3

If you are using your Objects in a "row at a time" CRUD type application, then, by all means copy all the columns into your object, the extra overhead is minimal, and you object becomes truly re-usable for any program wanting row access to the table.

However if your SQL is doing a complex join or returning a large set of rows, then request precisely and only what you want. You get two performance penalties here, one handling each column each time will eat up cpu for no benefit, and, two most DBMS systems have a bag of tricks for optimising queries (such as index only access) which can only be used if you specify precisely which columns you want.

There is no reuse issue in most of these cases as scan/search processes tend to very specific to a particular use case.