Before these releases, we can only know the number of rows returned from an operator, but we cannot know the actual number of rows that the operator processed/read. In case of "residual predicate pushdown", there's a hidden filter, aka Predicate or Residual Predicate which being evaluated for each row output by the table/index scan operator. In this case, the number of rows returned and the number of rows that the operator processed/read can have a big difference.
Let's see an example as below:
SET NOCOUNT ON;
GO
IF OBJECT_ID(N'ActualRowTbl') IS NOT NULL
DROP TABLE ActualRowTbl;
GO
CREATE TABLE ActualRowTbl (
id INT IDENTITY NOT NULL PRIMARY KEY,
firstName varchar(100) DEFAULT ('Ken'),
lastName varchar(100) DEFAULT ('Lam')
)
GO
INSERT ActualRowTbl DEFAULT VALUES;
GO 999
INSERT ActualRowTbl VALUES ('Jeremy','Po');
GO
CREATE INDEX IX_ActualRowTbl_1 ON ActualRowTbl (firstName, lastName);
GO
SELECT firstName FROM ActualRowTbl WHERE lastName = 'Po'
GO
By checking the actual execution plan of the last select statement, you can see there's a big difference between the number of rows returned (Actual Number of Rows) and Number of Rows Read, inside the Index Scan operator.
