143 lines
8.3 KiB
HTML
143 lines
8.3 KiB
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<meta name="DC.Title" content="Improve implementation of procedures and functions" />
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<meta name="abstract" content="These recommendations can be seen as simple coding techniques that can help reduce the processing time of a function or procedure. These tips are especially important to follow in functions, as a function will tend to be called multiple times from many different procedures." />
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<title>Improve implementation of procedures and functions</title>
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<body id="rbafyimplement"><a name="rbafyimplement"><!-- --></a>
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<!-- Java sync-link --><script language="Javascript" src="../rzahg/synch.js" type="text/javascript"></script>
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<h1 class="topictitle1">Improve implementation of procedures and functions</h1>
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<div><p>These recommendations can be seen as simple coding techniques that
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can help reduce the processing time of a function or procedure. These tips
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are especially important to follow in functions, as a function will tend to
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be called multiple times from many different procedures. </p>
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<div class="section"><ul><li>Use the NOT FENCED option so UDFs run in the same thread
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as the caller</li>
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<li>Use the DETERMINISTIC option on procedures and UDFs that return the same
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results for identical inputs. This allows the optimizer to cache the results
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of a function call or order where the function is called in the execution
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stream to reduce the run time.</li>
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<li>Use the NO EXTERNAL ACTION option on UDFs that do not take an action outside
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the scope of the function. An example of an external action is a function
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that initiates a different process to fulfill a transaction request.</li>
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</ul>
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</div>
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<div class="section"><p>Coding techniques used for the SQL routine body can have a major
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impact on the runtime performance of the generated C program. By writing your
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routine to allow greater use of C code for assignments and comparisons, the
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overhead of an equivalent SQL statement is avoided. The following tips should
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help your routine generate more C code and fewer SQL statements. </p>
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<ul><li>Declare host variables as NOT NULL when possible. This saves the generated
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code from having to check and set the null value flags. Do not automatically
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set all variables to NOT NULL. When you specify NOT NULL, you need to also
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give a default value. If a variable is always used in the routine, a default
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value might help. However, if a variable is not always used, having a default
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value set may cause additional initialization overhead that is not needed.
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A default value is best for numeric values, where an additional database call
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to process the assignment of the default value is not needed.</li>
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<li>Avoid character and date data types when possible. An example of this
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is a variable used as a flag with a value of 0, 1, 2, or 3. If this value
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is declared as a single character variable instead of an integer, it causes
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calls to the database engine that can be avoided.</li>
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<li>Use integer instead of decimal with zero scale, especially when the variable
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is used as a counter.</li>
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<li>Do not use temporary variables. Look at the following example: <pre>IF M_days<=30 THEN
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SET I = M_days-7;
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SET J = 23
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RETURN decimal(M_week_1 + ((M_month_1 - M_week_1)*I)/J,16,7);
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END IF</pre>
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This example can be rewritten without the temporary variables: <pre>IF M_days<=30 THEN
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Return decimal(M-week_1 + ((M_month_1 - M_week_1)* (M_days-7))/23,16,7);
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END IF</pre>
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</li>
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<li>Combine sequences of complex SET statements into one statement. This
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applies to statements where C code only cannot be generated because of CCSIDS
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or data types. <pre>SET var1 = function1(var2);
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SET var2 = function2();</pre>
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Can be rewritten into one statement: <pre>SET var1 = function1(var2), var2 = function2();</pre>
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</li>
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<li>Use IF () ELSE IF () ... ELSE ... constructs instead of IF (x AND y) to
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avoid unnecessary comparisons.</li>
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<li>Do as much in SELECT statements as possible: <pre>SELECT A INTO Y FROM B;
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SET Y=Y||'X';</pre>
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Rewrite this example: <pre>SELECT A || 'X' INTO Y FROM B</pre>
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</li>
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<li>Avoid doing character or date comparisons inside of loops when not necessary.
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In some cases the loop can be rewritten to move a comparison to precede the
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loop and have the comparison set an integer variable that is used within the
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loop. This causes the complex expression to be evaluated only one time.
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An integer comparison within the loop is more efficient since it can be done
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with generated C code.</li>
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<li>Avoid setting variables that might not be used. For example, if a variable
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is set outside of the an IF statement, be sure that the variable will actually
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be used in all instances of the IF statement. If not, then set the variable
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only in the portion of the IF statement that is it actually used.</li>
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<li>Replace sections of code with a single SELECT statement when possible.
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Look at the following code snippet: <pre>SET vnb_decimal = 4;
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cdecimal:
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FOR vdec AS cdec CURSOR FOR
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SELECT nb_decimal
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FROM K$FX_RULES
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WHERE first_currency=Pi_curl AND second_currency=P1_cur2
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DO
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SET vnb_decimal=SMALLINT(cdecimal.nb_decimal);
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END FOR cdecimal;
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IF vnb_decimal IS NULL THEN
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SET vnb_decimal=4;
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END IF;
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SET vrate=ROUND(vrate1/vrate2,vnb_decimal);
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RETURN vrate;</pre>
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This code snippet can be more efficient if rewritten in the following
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way: <pre>RETURN( SELECT
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CASE
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WHEN MIN(nb_decimal) IS NULL THEN ROUND(Vrate1/Vrate2,4)
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ELSE ROUND(Vrate1/Vrate2,SMALLINT(MIN(nb_decimal)))
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END
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FROM K$FX_RULES
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WHERE first_currency=Pi_curl AND second_currency=Pi_cur2);</pre>
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</li>
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<li>C code can only be used for assignments and comparisons of character data
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if the CCSIDs of both operands are the same, if one of the CCSIDs is 65535,
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if the CCSID is not UTF8, and if truncation of character data is not possible.
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If the CCSID of the variable is not specified, the CCSID is not determined
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until the procedure is called. In this case, code must be generated to determine
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and compare the CCSID at runtime. If an alternate collating sequence is specified
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or if *JOBRUN is specified, C code cannot be generated for character comparisons.</li>
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<li>Use the same data type, length and scale for numeric variables that are
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used together in assignments. C code can only be generated if truncation
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is not possible. <pre>DECLARE v1, v2 INT;
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SET v1 = 100;
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SET v1 = v2;</pre>
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</li>
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</ul>
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</div>
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</div>
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<div>
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<div class="familylinks">
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<div class="parentlink"><strong>Parent topic:</strong> <a href="rbafyudfperf.htm" title="When creating stored procedures and user-defined functions (UDFs), the SQL procedural language processor on the iSeries does not always generate the most efficient code. However, you can do some changes to reduce the number of database engine calls needed and improve performance.">Improve performance of procedures and functions</a></div>
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</div>
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</div>
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</body>
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</html>
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