1. Analyzing Execution Plans
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Guy Glantser
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2. A Few Words about Me… Name: Guy Glantser
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4. A Few Words about Me… Name: Guy Glantser
Address:
Twitter: @guy_glantser
Blog:
Podcast:
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5. Agenda
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6. Summary Compare Session Settings
Each combination of the session settings produces a different execution plan.
By default, the SQL Native Client sets ARITHABORT to False, while SQL Server Management Studio sets this option to True.
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7. Summary Cost is Always Estimated!
Even when you look at the actual execution plan, the cost is estimated. There is no such thing as actual cost.
If there is a difference between the estimated and the actual number of rows, then don’t trust the cost.
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8. Summary Compare Estimated vs. Actual Number of Rows
Most performance problems will have this as a symptom. Find the operator in which there is a big difference between the estimated and the actual number of rows, and in most cases this will lead you to the root of the problem.
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9. Summary Nested Loops can be Misleading
In the inner operator of a Nested-Loops join, the estimated number of rows reflects the number of rows per iteration, while the actual number of rows reflects the total number of rows.
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10. Summary Never Use Table Variables!
Table variables don’t have associated statistics, so the optimizer always assumes there is a single row in them. This leads in many cases to serious performance problems, especially when the table variable is involved in a join operation.
Prefer temporary tables whenever you can.
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11. Summary Read Execution Plans in Both Directions
Reading an execution plan from right to left is helpful in understanding the data flow, while reading an execution plan from left to right is helpful in understanding the runtime execution flow.
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12. Summary Tooltips are Just Tooltips You can also use:
Operator properties
ShowPlan XML
Trace flags
Debugger (only if your name is Paul White)
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13. Summary Avoid Using Scalar Functions
Although they make sense in terms of modularity and reusability, they really suck in terms of performance.
In many cases, it is possible to rewrite the scalar function as an inline function, thus enjoying modularity, reusability and good performance together.
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14. Questions?
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16. Stay In Touch… Name: Guy Glantser Email Address: guy@madeiradata.com
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