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Row Counts and Arrow Widths in SSMS

Hugo Kornelis takes us through one of the niceties SQL Server Management Studio (and subsequent tools like SentryOne Plan Explorer and Azure Data Studio) has for reading execution plans:

Because both the Actual Number of Rows and the Estimated Number of Rows are very valuable for many use cases, Microsoft has built their primary tool for viewing execution plans (SQL Server Management Studio) to highlight the value of at least one of these two properties, in a visual way, when you look at the graphical representation of an execution plan.

Look at the arrows that show which operator (is called by and) returns rows to which operator, and that therefore can be thought of as representing the data stream. In most execution plans, they are not all the same width. Management Studio uses the width of the arrow as a quick, simple visual representation of the value of Actual Number of Rows (in an execution plan plus run-time statistics, aka “execution plan plus” or “actual execution plan”), or of the value of the Estimated Number of Rows in an execution plan only (aka “estimated execution plan”).

This first post is the happy path post, where the information is correct and useful. Later posts promise to cover cases in which this does not hold.

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