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Why is ORM considered good but "select *" considered bad?

Asked 2008-11-15T00:21:03.557
36

Doesn't an ORM usually involve doing something like a select *?

If I have a table, MyThing, with column A, B, C, D, etc, then there typically would be an object, MyThing with properties A, B, C, D.

It would be evil if that object were incompletely instantiated by a select statement that looked like this, only fetching the A, B, not the C, D:

select A, B from MyThing /* don't get C and D, because we don't need them */

but it would also be evil to always do this:

select A, B, C, D /* get all the columns so that we can completely instantiate the MyThing object */

Does ORM make an assumption that database access is so fast now you don't have to worry about it and so you can always fetch all the columns?

Or, do you have different MyThing objects, one for each combo of columns that might happen to be in a select statement?

EDIT: Before you answer the question, please read Nicholas Piasecki's and Bill Karwin's answers. I guess I asked my question poorly because many misunderstood it, but Nicholas understood it 100%. Like him, I'm interested in other answers.

EDIT #2: Links that relate to this question:

Why do we need entity objects?

http://blogs.tedneward.com/2006/06/26/The+Vietnam+Of+Computer+Science.aspx, especially the section "The Partial-Object Problem and the Load-Time Paradox"

http://groups.google.com/group/comp.object/browse_thread/thread/853fca22ded31c00/99f41d57f195f48b?

http://www.martinfowler.com/blik

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2 Answers

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The case you describe is a great example of how ORM is not a panacea. Databases offer flexible, needs-based access to their data primarily through SQL. As a developer, I can easily and simply get all the data (SELECT *) or some of the data (SELECT COL1, COL2) as needed. My mechanism for doing this will be easily understood by any other developer taking over the project.

In order to get the same flexibility from ORM, a lot more work has to be done (either by you or the ORM developers) just to get you back to the place under the hood where you're either getting all or some of the columns from the database as needed (see the excellent answers above to get a sense of some of the problems). And all this extra stuff is just more stuff that can fail, making an ORM system intrinsically less reliable than straight SQL calls.

This is not to say that you shouldn't use ORM (my standard disclaimer is that all design choices have costs and benefits, and the choice of one or the other just depends) - knock yourself out if it works for you. I will say that I truly don't understand the popularity of ORM, given the amount of extra un-fun work it seems to create for its users. I'll stick with using SELECT * when (wait for it) I need to get every column from a table.

answered 2008-11-15T01:36:02.967
0

SELECT * is not bad. Did you ask whoever considered it to be bad "why?".

answered 2008-11-15T00:25:42.703

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