Preprocessing of Dynamic Non-Queries

There are two methods for dynamic preprocessing of a non-query:

Using EXECUTE IMMEDIATE.
Using PREPARE and EXECUTE.

The first method can be used with any non-query; the second is only for those non-query commands that use sections at execution time.

Using EXECUTE IMMEDIATE

If you know in advance that a dynamic command will not be a query, you can dynamically preprocess and execute the command in one step, using the EXECUTE IMMEDIATE command. Figure 10-3 “Procedure Hosting Dynamic Non-Query Commands” illustrates a procedure hosting a dynamic UPDATE STATISTICS command that can be handled in this fashion.

Procedure UpdateStatistics 1 prompts the user for a table name 2 . The table name entered is assigned to the host variable CmdLine 3 to complete the UPDATE STATISTICS command. After the command is prepared and executed 4 , the transaction is terminated with a COMMIT WORK command 5 or a ROLLBACK WORK command 6 , depending on the value in SQLCA.SQLCODE. Terminating the transaction before accepting another table name and re-executing the UPDATE STATISTICS command releases any locks obtained and improves concurrency.

If you do not know in advance whether a dynamic command will be a query or a non-query, you must use the PREPARE command to dynamically preprocess the command, the DESCRIBE command to distinguish between queries and non-queries, and the EXECUTE or EXECUTE IMMEDIATE command to execute the dynamic non-query. The program examined later in this chapter under "Program Using Dynamic Commands of Unknown Format" illustrates how to handle this situation.

Figure 10-3 Procedure Hosting Dynamic Non-Query Commands

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var

  EXEC SQL BEGIN DECLARE SECTION;

  CmdLine         : String[100];

  EXEC SQL END DECLARE SECTION;



  TableName       : String[50];

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procedure UpdateStatistics;                                        1 

begin



repeat



prompt('Enter name of table or / to terminate > ');                2 

readln(TableName);

if TableName[1] <> '/' then

  begin

    CmdLine := 'UPDATE STATISTICS FOR TABLE ' + TableName + ';';   3 



    EXEC SQL EXECUTE IMMEDIATE :CmdLine;                           4 



    if SQLCA.SQLCODE = 0 then

      EXEC SQL COMMIT WORK;                                        5 

    else

      EXEC SQL ROLLBACK WORK;                                      6 



  end;                (* end of if TableName *)

until TableName[1] = '/';



end;                  (* end of UpdateStatistics procedure *)

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Using PREPARE and EXECUTE

Use PREPARE command syntax to create and store a temporary section for the dynamic command:

   PREPARE CommandName FROM CommandSource

Because the PREPARE command operates only on sections, it can be used to dynamically preprocess only SQL commands executed by using sections. The DBE session management and transaction management commands can only be dynamically preprocessed by using EXECUTE IMMEDIATE.

With PREPARE, ALLBASE/SQL creates a temporary section for the command that you can execute one or more times in the same transaction by using the EXECUTE command:

   EXEC SQL PREPARE MyNonQuery FROM :DynamicCommand;



   for i := 1 to MaxIterations do

   EXEC SQL EXECUTE MyNonQuery;

As soon as you process a COMMIT WORK or ROLLBACK WORK command, the temporary section is deleted.

Preprocessing of Dynamic Non-Queries

Technical documentation

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Using EXECUTE IMMEDIATE

Using PREPARE and EXECUTE