SQL optimization and Disjunctive Normal Form

Question

So I was writing a query in Visual Studio 2010 (by which I mean I opened the server explorer, right clicked the server and chose New Query). The query includes the condition

A AND B AND C AND D AND E AND F AND (G OR H)

which is conjunctive normal form (CNF). When I ran the query(attached to MSSQL Server 2008), it changed the text into

A AND B AND C AND D AND E AND F AND G OR
A AND B AND C AND D AND E AND F AND H

which is disjunctive normal form (DNF).

From the little I found on-line, it seems like DNF allows SQL to run the conjunctives separately and union them at the end.

However, for something like this, with so many repeated conditions, does DNF actually provide an advantage over CNF? If it doesn't, how can I force the optimizer to take the condition as is? If it does, should I write the query in my application code in CNF form because it's shorter and neater or in DNF form because it saves time for the optimizer?

Answer 1

I don't know about the relative advantages of DNF/CNF in this situation, or even how to force the optimizer in this fashion.

Generally speaking, you don't want to force the optimizer to take your 'perceived', 'current', optimization over the one it will generate (there are exceptions to this, but these are usually rare). This largely has to do with the fact that the 'best' optimization may change over time, as a side effect of other actions (like adding an index). If you're forcing the optimizer to adopt a particular optimization, you're locking it into that path, even if a new one may perform better.

Given that, you should write the query in the form that is easiest to read and maintain (CNF), and let the optimizer change it if necessary - this is the whole point of SQL being a declarative language, to allow the optimizer to muck with things as necessary.

Answer 2

Off the top of my head, I wonder about the indexing on G or H. If G was indexed, but H wasn't...maybe a disjunctive would make more sense.

anyway, you can run the perfmance analyzer yourself to see the net difference in performance.

Besides that, here is some research you might be able to get access to if you want to dive-in: Reserach Material: http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=842265&abstractAccess=no&userType=inst