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CHAPTER 7 CONCURRENCY AND MULTI-VERSIONING
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This raises several interesting questions. Can we observe this Can we see this actually happen And if so, so what What does this mean to us as developers We ll address these questions in turn now.
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Seeing a Restart
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It is easier to see a restart than you might, at first, think. We ll be able to observe one, in fact, using a simple one-row table. This is the table we ll use to test with: ops$tkyte%ORA11GR2> create table t ( x int, y int ); Table created. ops$tkyte%ORA11GR2> insert into t values ( 1, 1 ); 1 row created. ops$tkyte%ORA11GR2> commit; Commit complete. To observe the restart, all we need is a trigger to print out some information. We ll use a BEFORE UPDATE FOR EACH ROW trigger to print out the before and after image of the row as the result of an update: ops$tkyte%ORA11GR2> create or replace trigger t_bufer 2 before update on t for each row 3 begin 4 dbms_output.put_line 5 ( 'old.x = ' || :old.x || 6 ', old.y = ' || :old.y ); 7 dbms_output.put_line 8 ( 'new.x = ' || :new.x || 9 ', new.y = ' || :new.y ); 10 end; 11 / Trigger created. Now we ll update that row: ops$tkyte%ORA11GR2> set serveroutput on ops$tkyte%ORA11GR2> update t set x = x+1; old.x = 1, old.y = 1 new.x = 2, new.y = 1 1 row updated. So far, everything is as we expect: the trigger fired once, and we see the old and new values. Note that we have not yet committed, however the row is still locked. In another session, we ll execute this update: ops$tkyte%ORA11GR2> set serveroutput on ops$tkyte%ORA11GR2> update t set x = x+1 where x > 0; This will immediately block, of course, since the first session has that row locked. If we now go back to the first session and commit, we ll see this output (the update is repeated for clarity) in the second session:
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CHAPTER 7 CONCURRENCY AND MULTI-VERSIONING
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ops$tkyte%ORA11GR2> update t set x = x+1 where x > 0; old.x = 1, old.y = 1 new.x = 2, new.y = 1 old.x = 2, old.y = 1 new.x = 3, new.y = 1 1 row updated... As you can see, that row trigger saw two versions of that row here. The row trigger was fired two times: once with the original version of the row and what we tried to modify that original version to, and again with the final row that was actually updated. Since this was a BEFORE FOR EACH ROW trigger, Oracle saw the read-consistent version of the record and the modifications we would like to have made to it. However, Oracle retrieved the block in current mode to actually perform the update after the BEFORE FOR EACH ROW trigger fired. It waits until after this trigger fires to get the block in current mode, because the trigger can modify the :NEW values. So Oracle can t modify the block until after this trigger executes, and the trigger could take a very long time to execute. Since only one session at a time can hold a block in current mode, Oracle needs to limit the time we have it in that mode. After this trigger fired, Oracle retrieved the block in current mode and noticed that the column used to find this row, X, had been modified. Since X was used to locate this record and X was modified, the database decided to restart our query. Notice that the update of X from 1 to 2 did not put this row out of scope; we ll still be updating it with this UPDATE statement. Rather, it is the fact that X was used to locate the row, and the consistent read value of X (1 in this case) differs from the current mode read of X (2). Now, upon restart, the trigger sees the value of X=2 (following modification by the other session) as the :OLD value and X=3 as the :NEW value. So, this shows that these restarts happen. It takes a trigger to see them in action; otherwise, they are generally undetectable. That does not mean you can t see other symptoms such as a large UPDATE statement rolling back work after updating many rows and then discovering a row that causes it to restart just that it is hard to definitively say, This symptom is caused by a restart. An interesting observation is that triggers themselves may cause restarts to occur even when the statement itself doesn t warrant them. Normally, the columns referenced in the WHERE clause of the UPDATE or DELETE statement are used to determine whether or not the modification needs to restart. Oracle will perform a consistent read using these columns and, upon retrieving the block in current mode, it will restart the statement if it detects that any of them have changed. Normally, the other columns in the row are not inspected. For example, let s simply rerun the previous example and use WHERE Y>0 to find the rows in both sessions, the output we ll see in the first session (the one that gets blocked) would be: ops$tkyte%ORA11GR2> update t set x = x+1 where y > 0; old.x = 1, old.y = 1 new.x = 2, new.y = 1 old.x = 2, old.y = 1 new.x = 3, new.y = 1 1 row updated. So why did Oracle fire the trigger twice when it was looking at the Y value Does it examine the whole row As you can see from the output, the update was, in fact, restarted and the trigger again fired twice, even though we were searching on Y>0 and did not modify Y at all. But, if we recreate the trigger to simply print out the fact that it fired, rather than reference the :OLD and :NEW values ops$tkyte%ORA11GR2> create or replace trigger t_bufer 2 before update on t for each row 3 begin 4 dbms_output.put_line( 'fired' ); 5 end;
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