Setting CE TraceFlags on a query-by-query (or session) basis

Cardinality - SeniorDBA

Using TraceFlags  on a query-by-query basis as described by Kimberly Tripp. This article does a great job of explaining this technique.

While the new cardinality estimator can offer benefits for some queries, it might not be perfect for ALL queries. Having said that, there is an OPTION clause that allows you to set the CE for that query. The setting to use depends on the CE that you’re running under currently. And, as of SQL Server 2016, even determining this can be difficult. At any given time, there are multiple settings that might affect your CE.

In SQL Server 2014, your CE was set by the database compatibility model. If you’re running with compatibility mode 120 or higher, then you’re using the new CE. If you’re running with compatibility mode 110 or lower, then you’re using the Legacy CE. In SQL Server 2016, the database compatibility mode is not the only setting that can affect the CE that you’re using. In SQL Server 2016, they added ‘database scoped configuations’ and introduced:

ALTER DATABASE SCOPED CONFIGURATION LEGACY_CARDINALITY_ESTIMATION = { ON | OFF | PRIMARY }

Having said that, an administrator can always override this setting by setting one of the CE trace flags globally [using DBCC TRACEON (TF, -1) – but I don’t recommend this!].

To use the LegacyCE when the database is set to the new CE, use Trace Flag 9481.
To use the New CE when the database is set to the LegacyCE, use Trace Flag 2312.

Generally, I recommend that most people STAY with the LegacyCE until they’ve thoroughly tested the new CE. Then, and only then, change the compatibility mode. But, even with extensive testing, you might still want some queries to run with the LegacyCE while most run with the new CE (or, potentially the opposite). What I like most about the addition of the new CE is that we have the ability to set EITHER!

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