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Furthermore, you should be aware that SQL Server does not support detached databases (mdf files); therefore, if you are migrating from a SQL Express instance (for example when your hosting provider supports only SQL Server), you will need to manually import the contents of the detached database file into a SQL Server database (either by attaching it from the hosting.

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, change the point in time at which the update is as of ), and instead of updating the data again, it will go into SELECT FOR UPDATE mode and attempt to lock all of the rows WHERE Y=5 for your session Once it does this, it will run the UPDATE against that locked set of data, thus ensuring this time that it can complete without restarting But to continue on with the but what happens if train of thought, what happens if, after restarting the update and going into SELECT FOR UPDATE mode (which has the same read-consistent and read current block gets going on as an update does), a row that was Y=5 when you started the SELECT FOR UPDATE is found to be Y=11 when you go to get the current version of it That SELECT FOR UDPDATE will restart and the cycle begins again..

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.

provider s administrative console or creating a backup from the Express database and importing it); in other words, there is no way to use the .mdf database with SQL Server directly.

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:

ops$tkyte%ORA11GR2> update t set x = x+1 where x > 0; oldx = 1, oldy = 1 newx = 2, newy = 1 oldx = 2, oldy = 1 newx = 3, newy = 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.