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Procedure-Level Atomicity
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It is interesting to note that Oracle considers PL/SQL anonymous blocks to be statements as well. Consider the previous stored procedure and reset example tables: ops$tkyte%ORA11GR2> create or replace procedure p 2 as 3 begin 4 insert into t values ( 1 ); 5 insert into t values (-1 ); 6 end; 7 / Procedure created. ops$tkyte%ORA11GR2> delete from t; 0 rows deleted. ops$tkyte%ORA11GR2> update t2 set cnt = 0; 1 row updated. ops$tkyte%ORA11GR2> commit; Commit complete. ops$tkyte%ORA11GR2> select * from t; no rows selected ops$tkyte%ORA11GR2> select * from t2; CNT ---------0 So, we have a procedure we know will fail, and the second INSERT will always fail in this case. Let s see what happens if we run that stored procedure:
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CHAPTER 8 TRANSACTIONS
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ops$tkyte%ORA11GR2> begin 2 p; 3 end; 4 / I fired and updated 1 rows I fired and updated 1 rows begin * ERROR at line 1: ORA-02290: check constraint (OPS$TKYTE.SYS_C0018095) violated ORA-06512: at "OPS$TKYTE.P", line 5 ORA-06512: at line 2 ops$tkyte%ORA11GR2> select * from t; no rows selected ops$tkyte%ORA11GR2> select * from t2; CNT ---------0 As you can see, Oracle treated the stored procedure call as an atomic statement. The client submitted a block of code BEGIN P; END; and Oracle wrapped a SAVEPOINT around it. Since P failed, Oracle restored the database back to the point right before it was called.
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Note The preceding behavior statement-level atomicity relies on the PL/SQL routine not performing any commits or rollbacks itself. It is my opinion that COMMIT and ROLLBACK should not be used in general in PL/SQL; the invoker of the PL/SQL stored procedure is the only one that knows when a transaction is complete. It is a bad programming practice to issue a COMMIT or ROLLBACK in your developed PL/SQL routines.
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Now, if we submit a slightly different block, we will get entirely different results: ops$tkyte%ORA11GR2> begin 2 p; 3 exception 4 when others then 5 dbms_output.put_line( 'Error!!!! ' || sqlerrm ); 6 end; 7 / I fired and updated 1 rows I fired and updated 1 rows Error!!!! ORA-02290: check constraint (OPS$TKYTE.SYS_C0018095) violated PL/SQL procedure successfully completed. ops$tkyte%ORA11GR2> select * from t;
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CHAPTER 8 TRANSACTIONS
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X ---------1 ops$tkyte%ORA11GR2> select * from t2; CNT ---------1 ops$tkyte%ORA11GR2> rollback; Rollback complete. Here, we ran a block of code that ignored any and all errors, and the difference in outcome is huge. Whereas the first call to P effected no changes, this time the first INSERT succeeds and the CNT column in T2 is incremented accordingly. Oracle considered the statement to be the block that the client submitted. This statement succeeded by catching and ignoring the error itself, so the If error then rollback... didn t come into effect and Oracle didn t roll back to the SAVEPOINT after execution. Hence, the partial work performed by P was preserved. The reason this partial work is preserved in the first place is that we have statementlevel atomicity within P: each statement in P is atomic. P becomes the client of Oracle when it submits its two INSERT statements. Each INSERT either succeeds or fails entirely. This is evidenced by the fact that we can see that the trigger on T fired twice and updated T2 twice, yet the count in T2 reflects only one UPDATE. The second INSERT executed in P had an implicit SAVEPOINT wrapped around it.
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THE WHEN OTHERS CLAUSE
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I consider virtually all code that contains a WHEN OTHERS exception handler that does not also include a RAISE or RAISE_APPLICATION_ERROR to re-raise the exception to be a bug. It silently ignores the error and it changes the transaction semantics. Catching WHEN OTHERS and translating the exception into an oldfashioned return code changes the way the database is supposed to behave. In fact, when Oracle 11g Release 1 was still on the drawing board, I was permitted to submit three requests for new features in PL/SQL. I jumped at the chance, and my first suggestion was simply remove the WHEN OTHERS clause from the language. My reasoning was simple, the most common cause of developer-introduced bugs I see the most common cause is a WHEN OTHERS not followed by a RAISE or RAISE_APPLICATION_ERROR. I felt the world would be a safer place without this language feature. The PL/SQL implementation team could not do this, of course, but they did the next best thing. They made it so that PL/SQL will generate a compiler warning if you have a WHEN OTHERS that is not followed by a RAISE or RAISE_APPLICATION_ERROR call. For example:
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ops$tkyte%ORA11GR2> alter session set 2 PLSQL_Warnings = 'enable:all' 3 / Session altered. ops$tkyte%ORA11GR2> create or replace procedure some_proc( p_str in varchar2 ) 2 as 3 begin 4 dbms_output.put_line( p_str ); 5 exception
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