Yes, I agree with the fact that there is information online, but it's scattered all around and can be inconsistent between sources. That's what I want when I buy a book dedicated to a particular topic. I'm not looking into finding information that is impossible to find online.

Knowing how the query optimizer works will help shaping strategies regarding indexing etc. It's not necessarily deep internals but in my humble opinion:

1) it sure does not hurt (I always end up learning something new everytime I try something, even though I thought I knew how that worked)

2)I'm not sure how far you can get before hitting the "trade secret" wall in this topic

3) the definition of something "deep" is probably very relative to everyone's knowledge in this kind of area (and their needs as well)

I have enough database theory books in my shelves to understand how it's "supposed" to work from a maethematical point of view, but we've all seen cases where some queries are done in a certain way by the optimizer instead of the one we thought it ought to. The main drawbacks of these books is that they most of the time of course are RDBMS-independent, which is also their strength.

I'm pretty sure a book like the SQL Internals mentioned earlier is not entirely dedicated to that but I think that it can give some valuable information that may affect these decisions. Mind you, I haven't read it yet, nor do I own it.

Regards,

Greg