Oracle Magazine - September/October 2007 - Marcelo Machado

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this case, if the table T is modified (which would change the answer from my cached function, hence I need it to be re-executed). Note that for effect, I’ve put a 1-second sleep in both functions and that that sleep call will make it painfully obvious when the function is actually invoked and when the results are reused. I start by calling the conventional (not_cached) function three times with timing on: SQL> exec dbms_output.put_line( not_ cached( ‘SCOTT’ ) ); 6 PL/SQL procedure successfully completed. Elapsed: 00:00:01.93 SQL> exec dbms_output.put_line( not_ cached( ‘SCOTT’ ) ); 6 PL/SQL procedure successfully completed. Elapsed: 00:00:01.29 SQL> exec dbms_output.put_line( not_ cached( ‘SCOTT’ ) ); 6 PL/SQL procedure successfully completed. Elapsed: 00:00:01.07 As you can see, each invocation takes at least one second—the work done by the function plus the SQL it executes take a little more than a second. Now I try out the cached version of this function: SQL> exec dbms_output.put_line( cached( ‘SCOTT’ ) ); 6 PL/SQL procedure successfully completed. Elapsed: 00:00:01.09 SQL> exec dbms_output.put_line( cached( ‘SCOTT’ ) ); 6 PL/SQL procedure successfully completed. Elapsed: 00:00:00.01 SQL> exec dbms_output.put_line( cached( ‘SCOTT’ ) ); 6 PL/SQL procedure successfully completed. Elapsed: 00:00:00.01 If you have a query that is executed over and over again against slowly or never-changing data, you will find the new server results cache to be of great interest. As you can see, the first execution takes a little more than a second, just like the noncached versions, but the subsequent ones run at light speed, simply because the cached function is, in fact, not being executed. If I modify the table this function relies on or change the inputs I use, I can see that the database does the right thing: SQL> update t set owner = initcap(owner) where rownum = 1; 1 row updated. SQL> commit; Commit complete. SQL> exec dbms_output.put_line( cached( ‘SCOTT’ ) ); 6 PL/SQL procedure successfully completed. Elapsed: 00:00:01.25 This first execution of the cached function takes more than a second, because it must refresh the result cache, but subsequent executions benefit from this cached result: SQL> exec dbms_output.put_line( cached( ‘SCOTT’ ) ); 6 PL/SQL procedure successfully completed. Elapsed: 00:00:00.01 SQL> exec dbms_output.put_line( cached( ‘SCOTT’ ) ); 6 PL/SQL procedure successfully completed. Elapsed: 00:00:00.01 And just to show that the PL/SQL function results cache understands that different inputs result in different outputs, I can call the cached function with a different username: SQL> exec dbms_output.put_line( cached( ‘SYS’ ) ); 29339 PL/SQL procedure successfully completed. Elapsed: 00:00:01.21 SQL> exec dbms_output.put_line( cached( ‘SYS’ ) ); 29339 PL/SQL procedure successfully completed. Elapsed: 00:00:00.01 Note that the first execution takes a relatively long time (to build the result cache), a subsequent execution is fast, and calling the function with a different username does not invalidate or throw away other cached results: SQL> exec dbms_output.put_line( cached( ‘SCOTT’ ) ); 6 PL/SQL procedure successfully completed. Elapsed: 00:00:00.00 Function calls that use SCOTT as the input will remain fast until the cache must be invalidated or until it runs out of room and must make way for other elements. Again, this is a feature that can be implemented without massive rearchitecting of your applications—in fact, without rearchitecting at all. The compiler parameter RESULT_CACHE can activate this feature, and any client that invokes the function will benefit. For example, informal tests with Oracle Application Express show an approximately 15 percent decrease in runtime in a standard benchmark harness—your mileage may vary, of course! WHEN OTHERS THEN NULL REDUX In the July/August 2007 issue of Oracle Magazine (at otn.oracle.com/oramag/ oracle/07-jul/o47asktom.html), I wrote about a certain PL/SQL programming construct, the WHEN OTHERS ORACLE MAGAZINE SEPTEMBER/OCTOBER 2007 87
ASK TOM exception handler. I wrote There’s a programming construct, WHEN OTHERS, in PL/SQL that I wish weren’t there. Well, I knew Oracle could never really remove this language feature—but Oracle certainly has made it safer. Just to reiterate why I wish this programming construct did not exist, I’ll quote myself again: But the problem I see is that far too many people use the WHEN OTHERS clause without following it by a RAISE or RAISE_APPLICATION_ERROR call. That effectively hides the error. In reality, the error happened, but it was not handled in any meaningful way; it was just ignored— silently. The invoker of your code has no idea that something went horribly wrong and that your code failed, and he or she typically thinks that everything was, in fact, successful. Many times—many more times than I can recall or count—“quirky” behavior in a PL/SQL program unit is found to be caused by the improper use of a WHEN OTHERS clause—one that is not followed by RAISE. The error was hidden, error handling was effectively disabled, and the outcome is typically logically corrupt data (the application did not do its job) or the wrong answer. Now, in Oracle Database 11g, I can easily find the offending bits of code. When I suspect that an exception is being raised and hidden in this fashion, I’ll be able to verify my hunch rapidly. Consider this innocuous procedure: SQL> create table t( x varchar2(4000) ); Table created. SQL> create or replace 2 procedure maintain_t 3 ( p_str in varchar2 ) 4 as 5 begin 6 insert into t 7 ( x ) values 8 ( p_str ); 9 exception 10 when others 11 then 12 -- call some log_error() routine 13 null; 14 end; 15 / Procedure created. It’s as simple as can be. Nothing should go wrong, but in case it does, I’ll log the error by using a utility routine I’ve developed. That’ll write a nice error message somewhere (hopefully!), but whoever invokes this routine will have no idea that an exception that was not expected—and that could not be fixed—happened. (I see code like this time after time after time.) Now when someone calls this routine, SQL> exec maintain_t( rpad( ‘x’, 4001, ‘x’ ) ); PL/SQL procedure successfully completed. it sure looks successful, but it wasn’t: SQL> select * from t; no rows selected This is where confusion comes in: people get on the phone; they write e-mails; they say “Oracle Database is broken; the transaction succeeds, but the data is wrong.” In fact, the problem is the hidden error. Now, to find these errors in Oracle Database 11g, I will simply ask people to perform the following operation on their code first: SQL> alter procedure maintain_t compile 2 PLSQL_Warnings = ‘enable:all’ 3 reuse settings 4 / SP2-0805: Procedure altered with compilation warnings SQL> show errors procedure maintain_t Errors for PROCEDURE MAINTAIN_T: LINE/COL ERROR -----------––––– ––––––––––––––––––––––––––––––––––––––– 9/8 PLW-06009: procedure “MAINTAIN_T” OTHERS handler does not end in RAISE or RAISE_APPLICATION_ERROR And instantly you’ll have a list of applications—and the source code 88 SEPTEMBER/OCTOBER 2007 ORACLE.COM/ORACLEMAGAZINE lines—to immediately go to and add a simple RAISE to see where this hidden error is coming from. Job done. IT’S THE LITTLE THINGS. . . . There’s a saying that goes, “It’s the little things in life that really make a difference.” So I’ll close with a little thing that will make your coding life a bit easier and the PL/SQL language a bit more complete: SQL> create sequence this_is_nice; Sequence created. SQL> declare 2 n number; 3 begin 4 n := this_is_nice.nextval; 5 dbms_output.put_line( n ); 6 end; 7 / 1 PL/SQL procedure successfully completed. Compare that to what would have happened in Oracle Database 10g and prior releases. In Oracle Database 11g, there’s no more selecting a sequence FROM DUAL. PL/SQL is that much more complete. Tom Kyte has worked for Oracle since 1993. He is a vice president in the Oracle Public Sector group and the author of Expert Oracle Database Architecture: 9i and 10g Programming Techniques and Solutions (Apress, 2005) and Effective Oracle by Design (Oracle Press, 2003), among others. ASK Tom Oracle Vice President Tom Kyte answers your most difficult technology questions. Highlights from that forum appear in this column. asktom.oracle.com READ more about Oracle Database 11g otn.oracle.com/products/database READ more Tom Expert Oracle Database Architecture: 9i and 10g Programming Techniques and Solutions amazon.com/exec/obidos/tg/detail/-/ 1590595300/ DOWNLOAD Oracle Database 11g otn.oracle.com/technology/software
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Oracle Magazine - September/October 2007 - Marcelo Machado

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