torsdag 29 maj 2008

When to optimize

When getting an optimization task for someone I always try to start as wide as possibly. What type of machine are you running on? What is displayed by the task manager etc... Quickly start a trace to see what is actually happening and then view some DMV's to spot issues.

I've found that quite often the trace shows statements or procedures that by them selves eat up tons of resources. In case you find a top five of tasks that use in the range of 30% or more of cpu or io resources then optimize! Even if these tasks aren't causing locks or other problems it will be such a significant improvement to your client that they will notice.

As a side effect the entire database will run much smoother afterwards and with less load the risk of locks etc will decrease. These tasks are often easy to optimize since you will almost always benefit from trading insert/update/delete cost to select cost by removing or adding indexes.

torsdag 22 maj 2008

Performance monitor doesn't lie!

Yesterday I worked on optimizing a database for a client... I had a pretty nice optimization of a stored proceedure in place... that's what I thought at least. Using a computed column with an index I could do direct lookups instead of searching large amounts of text data and when testing the procedure in management studio the execution plan was perfect and IO was reduced from 50k to less than 50 :). But server load didn't drop as expected so I fired up performance monitor and started a trace... guess what the procedure was still running with poor performance.

After some tinkering it was obvious that the application code failed to get the correct execution plan for reasons still unknown (maybe access rights or some weird ansi-option related to the computed column, a did a simple workaround and it worked like out fine.

Lesson learned: There is a difference between successful optimization when running the procedure in management studio and actually applying it to a running application.

When in doubt trust performance monitor.

torsdag 15 maj 2008

Drop a database

The drop syntax is rarely useful since there is always someone using the database... and I always seem to forget the lines needed to set the database in single user mode.

This works:


USE master


Really basic stuff...

Remember that:
@Variable like Column
Is not the same as:
Column like @Variable

Really basic, but sometimes even basic stuff can cause a mess.