Merge pull request microsoft#128 from Microsoft/pmasl-patch-9

Create README.md

Author: pmasl

samples/demos/mssqltiger/Plan-Comparison/Inside-Query-Store/README.md

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+The plan comparison tool is used inside Query Store UI to allow plan comparison.
+
+###Prerequisites
+Restore QueryStoreTest database from provided .bacpac (/Databases/Import Data-tier application).
+
+###Query with plan regression
+1. Run QueryStoreSimpleDemo.exe with option R
+2. Open SSMS, and under QueryStoreTest database, expand Query Store.
+3. Open the *Top Resource Consuming Queries* report.
+4. For query id 1 there are two execution plans that SQL Server use alternately (switches between 2 plan almost randomly).
+5. Select one of the plans in the right pane, and while holding the Shift key, select the other plan.
+6. On the top ribbon in the same pane, click on *Compare the plans for the selected query in a seperate window* - this brings up Plan Comparison.
+7. In the *Properties* window you are able to spot some differences in the SELECT node. Expand the *Parameter List* and observe how the *Parameter Compiled Value* is different on both plans.
+This is known as Parameter Sniffing problem - plan gets generated based on parameter available at the compilation time. 
+When compilation happens frequently and randomly and data is skewed (not all parameter values are uniformly distributed).
+
+Knowing the cause, you can chose to force the perceived better plan for most use cases. 
+This can fix performance quickly and is fully transparent to running apps.