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The Compute Scalar, called a Project in the Query Optimizer, is a simple operator that attempts to declare a set of columns, compute some value, or perhaps restrict columns from other operators in the query tree. These correspond to the SELECT list in the SQL language. These are actually not overly interesting operations to the Query Optimizer there is not much that the Query Optimizer needs to do with them. The Query Optimizer ends up moving them around the query tree during optimization, trying to separate them from the rest of the Query Optimizer logic that deals with join order, index selection, and other optimizations.
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The Query Optimizer
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Compute Sequence is known as a Sequence Project in the Query Optimizer, and this operator is somewhat similar to a Compute Scalar in that it computes a new value to be added into the output stream. The key difference is that this works on an ordered stream and contains state that is preserved from row to row. Ranking functions use this operator, for example. This is implemented using a different physical operation and it imposes additional restrictions on how the Query Optimizer can reorder this expression. This operator is usually seen in the ranking and windowing functions.
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Semi-Join
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The term semi-join comes from the academic database literature, and it is used to describe an operator that performs a join but returns only values from one of its inputs. The query processor uses this internal mechanism to handle most subqueries. SQL Server represents subqueries in this manner because it makes it easier to reason about the set of possible transformations for the query and because the run-time implementation of a semi-join and a regular join are similar. Contrary to popular belief, a subquery is not always executed and cached in a temporary table. It is treated much like a regular join. In fact, the Query Optimizer has transformation rules that can transform regular joins to semi-joins. One common misconception is that it is inherently incorrect to use subqueries. Like most generalizations, this is not true. Often a subquery is the most natural way to represent what you want in SQL, and that is why it is part of the SQL language. Sometimes, a subquery is blamed for a poorly indexed table, missing statistics, or a predicate that is written in a way that is too obtuse for the Query Optimizer to reason about using its domain constraint property framework. Like everything in life, it is possible to have too many subqueries in a system, especially if they are duplicated many times in the same query. So, if your company s development practices say, No subqueries, then examine your system a little closely these are blamed for many other problems that might lie right under the surface. Listing 8-1 is an example of where a subquery would be appropriate. Let s say that we need to ask a sales tracking system for a store to show me each customer who has made an order in the last 30 days so that we can send them a thank-you e-mail. Figures 8-7, 8-8, and 8-9 show the query plans for the three different approaches to try to submit queries to answer this question.
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LISTING 8-1 Common Errors in Writing Subquery Plans
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CREATE CREATE INSERT INSERT INSERT INSERT INSERT TABLE Customers(custid int IDENTITY, name NVARCHAR(100)); TABLE Orders (orderid INT IDENTITY, custid INT, orderdate DATE, INTO Customers(name) VALUES ('Conor Cunningham'); INTO Customers(name) VALUES ('Paul Randal'); INTO Orders(custid, orderdate, amount) VALUES (1, '2008-08-12', INTO Orders(custid, orderdate, amount) VALUES (1, '2008-08-14', INTO Orders(custid, orderdate, amount) VALUES (2, '2008-08-12', amount MONEY);
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49.23); 65.00); 123.44);
Microsoft SQL Server 2008 Internals
-- Let's find out customers who have ordered something in the last month -- Semantically wrong way to ask the question - returns duplicate names (See Figure 8-7) SELECT name FROM Customers C INNER JOIN Orders O ON C.custid = O.custid WHERE DATEDIFF("m", O.orderdate, '2008-08-30') < 1 -- and then people try to "fix" by adding a distinct (See Figure 8-8) SELECT DISTINCT name FROM Customers C INNER JOIN Orders O ON C.custid = O.custid WHERE DATEDIFF("m", O.orderdate, '2008-08-30') < 1; -- this happens to work, but it is fragile, hard to modify, and it is usually not done properly. -- the subquery way to write the query returns one row for each matching Customer SELECT name FROM Customers C WHERE EXISTS ( SELECT 1 FROM Orders O WHERE C.custid = O.custid AND DATEDIFF("m", O.orderdate, '2008-08-30') < 1 ); -- note that the subquery plan has a cheaper estimated cost result -- and should be faster to run on larger systems
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