I’ve seen a few questions on this topic already but I’m looking for some insight on the performance differences between these two techniques.
For example, lets say I am recording a log of events which will come into the system with a dictionary set of key/value pairs for the specific event. I will record an entry in an Events table with the base data but then I need a way to also link the additional key/value data. I will never know what kinds of Keys or Values will come in so any sort of predefined enum table seems out of the question.
This event data will be constantly streaming in so insert times is just as important as query times.
When I query for specific events I will be using some fields on the Event as well as data from the key/value data. For the XML way I would simply use a Attributes.exists(‘xpath’) statement as part of the where clause to filter the records.
The normalized way would be to use a Table with basically Key and Value fields with a foreign link to the Event record. This seems clean and simple but I worry about the amount of data that is involved.
The problem I think the key/value table approach is regarding the datatypes – if a value could be a datetime, or a string or a unicode string or an integer, then how do you define the column? This dilemma means the value column has to be a datatype which can contain all the different types of data in it which then begs the question of efficiency/ease of querying. Alternatively, you have multiple columns of specific datatypes, but I think this is a bit clunky.
For a true flexible schema, I can’t think of a better option than XML. You can index XML columns.
This article off MSDN discusses XML storage in more detail.