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DEALLOCATE PREPARE statement provides this capability. When the DBMS processes this statement, it can free the resources associated with the compiled statement, which will usually include some internal representation of the application plan for the statement. The statement named in the DEALLOCATE PREPARE statement must match the name specified in a previously executed PREPARE statement. In the absence of a capability like that provided by DEALLOCATE PREPARE, the DBMS has no way of knowing whether a previously prepared statement will be executed again or not, and so must retain all of the information associated with the statement. In practice, some DBMS brands maintain the compiled version of the statement only until the end of a transaction; in these systems, a statement must be reprepared for each subsequent transaction where it is used. Because of the overhead involved in this process, other DBMS brands maintain the compiled statement information indefinitely. The DEALLOCATE PREPARE can play a more important role in these systems, where a database session might last for hours. Note, however, that the SQL2 standard explicitly says that whether a prepared statement is valid outside of the transaction in which it is prepared is implementation dependent. The SQL2 extension to the DB2-style EXECUTE statement may be even more useful in practice. It allows the EXECUTE statement to be used to process simple singleton SELECT statements that return a single row of query results. Like the DB2 EXECUTE statement, the SQL2 statement includes a USING clause that names the host variables
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that supply the values for parameters in the statement being executed. But the SQL2 statement also permits an optional INTO clause that names the host variables that receive the values returned by a single-row query. Suppose you have written a program that dynamically generates a query statement that retrieves the name and quota of a salesperson, with the salesperson s employee number as an input parameter. Using DB2-style dynamic SQL, even this simple query involves the use of a SQLDA, cursors, a FETCH statement loop, and so on. Using SQL2 dynamic SQL, the statement can be executed using the simple two-statement sequence:
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PREPARE qrystmt FROM :statement_buffer; EXECUTE qrystmt USING :emplnum INTO :name, :quota;
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As with any prepared statement, this single-row query could be executed repeatedly after being prepared once. It still suffers from the restriction that the number of returned columns, and their data types, must know when the program is written, since they must match exactly the number and data types of the host variables in the INTO clause. This restriction is removed by allowing the use of a SQLDA-style descriptor area instead of a list of host variables, as described in the next section.
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Although its support for PREPARE/EXECUTE processing closely parallels that of DB2 dynamic SQL, the SQL2 standard diverges substantially from DB2 style in the area of dynamic query processing. In particular, the SQL2 standard includes major changes to the DB2 SQL Data Area (SQLDA), which is at the heart of dynamic multirow queries. Recall that a SQL Data Area (SQLDA) provides two important functions: I A flexible way to pass parameters to be used in the execution of a dynamic SQL statement (passing data from the host program to the DBMS), as described earlier in the section EXECUTE with SQLDA. I The way that the query results are returned to the program in the execution of a dynamic SQL query (passing data from the DBMS back to the host program), as described earlier in the section The Dynamic FETCH Statement. The DB2-style SQLDA handles these functions with flexibility, but it has some serious disadvantages. It is a very low-level data structure, which tends to be specific to a particular programming language. For example, the variable-length structure of a DB2-style SQLDA makes it very difficult to represent in the FORTRAN language. The SQLDA structure also implicitly makes assumptions about the memory of the computer system on which the dynamic SQL program is running, how data items in a structure are aligned on such a system, and so on. For the writers of the SQL2 standard, these lowlevel dependencies were unacceptable barriers to portability. Therefore, they replaced the
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Dynamic SQL *
DB2 SQLDA structure with a set of statements for manipulating a more abstract structure called a dynamic SQL descriptor. The structure of a SQL2 descriptor is shown in Figure 18-18. Conceptually, the SQL2 descriptor is parallel to, and plays exactly the same role as, the DB2-style SQLDA shown in Figure 18-7. The fixed part of the SQL2 descriptor specifies a count of the number of items in the variable part of the descriptor. Each item in the variable part contains information about a single parameter being passed, such as its data type, its length, an indicator telling whether a NULL value is being passed, and so on. But unlike the DB2 SQLDA, the SQL2 descriptor is not an actual data structure within the host program. Instead, it is a collection of data items owned by the DBMS software. The host program manipulates SQL2 descriptors creating them, destroying
Fixed part COUNT TYPE LENGTH OCTET_LENGTH RETURNED_LENGTH RETURNED_OCTET_LENGTH PRECISION SCALE DATETIME_INTERVAL_CODE DATETIME_INTERVAL_PRECISION NULLABLE INDICATOR DATA NAME UNNAMED Figure 18-18. number of items described data type of item length of item length of item (in 8-bit octets) length of returned data item length of returned data (in 8-bit octets) precision of data item scale of data item type of date/time interval data precision of date/time interval data can item be NULL is data item NULL (indicator value) data item itself
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