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How to properly read an Execution Plan/Query Plan?
Data flows from RIGHT (TOP) to LEFT in the query plan. That is it data centric view of the query plan.
Logic in the query plan flows from LEFT to RIGHT. This is logic centric view of the query plan.
Well there is nothing like correct or incorrect way of reading the plan. It’s just that we have two different ways of reading the plan.
All the operators used in the execution plan are called physical operators.
Let’s go through an example. We know that select and top operator cannot have rows from their own. They are dependent on Clustered Index scan. So when the execution starts the “Select” operator asks the “Top” operator that hey do you have any rows for me, Top operator says that I don’t have rows let me ask “Clustered Index Scan”. Hey “Clustered Index scan” do you have any rows for me and “Clustered index scan” says yes I have rows for you.
That is how the logic flows from left to right and data flows from right to left.
By reading the execution plan both the ways we can easily track down issues and bottlenecks very quickly. Basically we will easily understand what’s going on in the execution plan.
That’s all folks; I hope you’ve enjoyed learning about how we can read execution plan more effectively and I’ll see you soon with more “Performance Tuning” articles.
Pawan Kumar Khowal