Legacy Cardinality Estimation In SQL Server

Kellyn Pot’vin-Gorman explains what the Legacy Cardinality Estimation setting does in SQL Server:

Oracle DBAs have used the CARDINALITY hint for some time and it should be understood that this may appear to be similar, but is actually quite different.  As hinting in TSQL is a bit different than PL/SQL, we can compare similar queries to assist:

TSQL

SELECT CustomerId, OrderAddedDate 
FROM OrderTable 
WHERE OrderAddedDate >= '2016-05-01';
OPTION (USE HINT ('FORCE_LEGACY_CARDINALITY_ESTIMATION'));
go

PL/SQL

Where you might first mistake the CE hint for the following CARDINALITY hint in Oracle:

SELECT /*+ CARDINALITY(ORD,15000) */ ORD.CUSTOMER_ID, ORD.ORDER_DATE 
FROM ORDERS ORD WHERE ORD.ORDER_DATE >= '2016-05-01';

This would be incorrect and the closest hint in Oracle to SQL Server’s legacy CE hint would be the optimizer feature hint:

SELECT /*+ optimizer_features_enable('9.2.0.7') */ ORD.CUSTOMER_ID, ORD.ORDER_DATE FROM ORDERS ORD 
WHERE ORD.ORDER_DATE >= '2016-05-01';

If you’re wondering why I chose a 9i version to force the optimizer to, keep reading and you’ll come to understand.

Read on for the comparative explanation as well as more details on SQL Server’s legacy cardinality estimator hint and database-scoped configuration setting.