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Just for starters, your ORM will model an object for a row of data, how many times do you get the whole row when you're really interested in just a column or two? Does your ORM let you just specify the parts of the row you're interested in or does it hydrate an object and populate all of the columns?

I don't know about (n)hibernate or other ORMs, but Linq for SQL let's you specify exactly what you want, and when you inspect the SQL generated, it is usually quite efficient, although not 100% optimal for complex queries.

There is also the issue with nested objects (relations) and if they should be prefetched or not (to avoid in effect nested-loop type SQL), but that is perfectly controllable.

My biggest issue with ORM layers is transaction-handling when you are doing some things in the DB, some things with data from this query and some data from this other query. This can quickly promote to what would in DB be a simple transaction to a distributed transaction.

When used correctly ORMs are very nice tools indeed, but to use them efficiently, you have to know how they work and how the underlying DB they interface with works as well.

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