Short version of the question:
If you have a table with a large number of small rows and you want to retrieve a single record from this table via an index probably consisting of two columns is this likely to be something that wil be low cost and fast or high cost and slow
Longer version of question and background:
I am a consultant working with a software development company and I have an argument with them about the performance implications of a piece of functionality that I want to add to the application they are building (and I am designing).
At the moment, we write out a log record every time somebody retrieves a client record. I want to put the name and time of the last person prevously to access that record onto the client page each time that record is retrieved.
They are saying that the performance implications of this will be high but based on my reasonable but not expert knowledge of how B trees work, this doesn’t seem right even if the table is very large.
If you create an index on the GUID of the client record and the date/time of access (descending), then you ought to be able to retrieve the required record via an index scan which would just need to find the first entry for that GUID and then stop? And that with a b-tree index, most of the index would be cached so the number of physical disc accesses needed would be very small and the query time therefore significantly less than 1s.
Or have I got this completely wrong
You will have problems with GUID index fragmentation but because your rows do not increase in size (as you said in the comments) you will not have page-splitting problems. The random insert issue is fixable by doing reorganizing and rebuilding.
Besides that, there is nothing wrong with your approach. If the table is larger than RAM you will likely have a single disk IO per access (the intermediate index levels will be cached). If your data fits in RAM you will pay about 0.2 to 0.5ms per query. If your data is on a magnetic disk a seek will likely require 8-12ms. On an SSD you are back to 0.2ms to 0.5ms (maybe 0.05ms more).
Why don’t you just create some test data (by selecting a cross product from sys.object of 1M rows) and measure it. It takes little time and you will find out for sure.