Cardinality, basically the number of rows being processed by an operation with the optimizer, is a calculation predicated on the statistics available for the columns in question. The statistics used are generally either the values from the histogram or the density. Prior to SQL Server 2014, and going all the way back to SQL Server 7.0 (in the Dark Ages when we had to walk uphill to our cubicles through 15 feet of snow battling Oracle DBAs and Fenris the whole way), there’s been one cardinality estimator (although you can modify the behavior somewhat with a traceflag in 2008R2 and 2012). Not any more. There’s a possibility for really complex, edge-case queries, that you may run into a regression from this.
You control whether or not you get the new cardinality estimator by setting the Compatibility Level of the database to SQL Server 2014 (120 for the picky amongst us). This could lead to regression issues. So, you’re going to pretty quickly want to know if your execution plan is using the new Cardinality Estimation Model, right? It’s tricky. Just look at the properties of the first operator in the plan (I told you to use that first operator). You’ll find one value there that will tell you what you need to know:
Just check this value (which you can also get from the XML behind the graphical plan) to see what calculations the optimizer used to arrive at the plan you’re observing.
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