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The Query Execution Component
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The query executor runs the execution plan that was produced by the optimizer, acting as a dispatcher for all the commands in the execution plan. This module loops through each command step of the execution plan until the batch is complete. Most of the commands require interaction with the storage engine to modify or retrieve data and to manage transactions and locking.
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Communication Between the Relational Engine and the Storage Engine
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The relational engine uses OLE DB for most of its communication with the storage engine. The following description of that communication is adapted from the section titled Database Server in Books Online. It describes how a SELECT statement that processes data from local tables only is processed: 1. The relational engine compiles the SELECT statement into an optimized execution plan. The execution plan defines a series of operations against simple rowsets from the individual tables or indexes referenced in the SELECT statement. (Rowset is the OLE DB term for a result set.) The rowsets requested by the relational engine return the amount of data needed from a table or index to perform one of the operations used to build the SELECT result set. For example, this SELECT statement requires a table scan if it references a table with no indexes:
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2. The relational engine implements the table scan by requesting one rowset containing all the rows from ScanTable. This next SELECT statement needs only information available in an index:
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SELECT DISTINCT LastName FROM Northwind.dbo.Employees
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3. The relational engine implements the index scan by requesting one rowset containing the leaf rows from the index that was built on the LastName column. The following SELECT statement needs information from two indexes:
SELECT CompanyName, OrderID, ShippedDate FROM Northwind.dbo.Customers AS Cst JOIN Northwind.dbo.Orders AS Ord ON (Cst.CustomerID = Ord.CustomerID)
4. The relational engine requests two rowsets: one for the clustered index on Customers and the other for one of the nonclustered indexes on Orders. The relational engine then uses the OLE DB API to request that the storage engine open the rowsets. As the relational engine works through the steps of the execution plan and needs data, it uses OLE DB to fetch the individual rows from the rowsets it requested the storage engine to open. The storage engine transfers the data from the data buffers to the relational engine. The relational engine then combines the data from the storage engine rowsets into the final result set transmitted back to the user. Not all communication between the relational engine and the storage engine uses OLE DB. In fact, some commands cannot be expressed in terms of OLE DB rowsets. The most obvious and common example of non-rowset-expressible return values is when the relational engine processes data definition language (DDL) requests to create a table or other SQL Server object.
The Access Methods Manager
When SQL Server needs to locate data, it calls the access methods manager. The access methods manager sets up and requests scans of data pages and index pages and then prepares the OLE DB rowsets to return to the relational engine. It contains services to open a table, retrieve qualified data, and update data. The access methods manager does not actually retrieve the pages; it makes the request of the buffer
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manager, which ultimately serves up the page already in its cache or reads it to cache from disk. When the scan is started, a look-ahead mechanism qualifies the rows or index entries on a page. Retrieving rows that meet specified criteria is known as a qualified retrieval. The access methods manager is employed not only for queries (selects) but also for qualified updates and deletes (for example, UPDATE with a WHERE clause). A session opens a table, requests and evaluates a range of rows against the conditions in the WHERE clause, and then closes the table. A session descriptor data structure (SDES) keeps track of the current row and the search conditions for the object being operated on (which is identified by the object descriptor data structure, or DES).
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