Shai Berger, this explanation is pure gold! Definetely better than MySQL's one.
Now I may agree on that changing the isolation level should be probably left to a major release, or either have a huge warning on the patch notes. I personally can't think of any project I have done that this would be an issue since database errors are usually well handled (which I think doing this change would increase, since you will be reading more modified data from other transactions if I read correct) and critical operations should be handled with either a select for update or transactional operations (such as doing col = col + 1). El lunes, 21 de marzo de 2016, 4:27:59 (UTC-3), Shai Berger escribió: > > First of all, I would like to say that I strongly support the move to READ > COMITTED, including backporting it to 1.8.x. > > But we also need to explain: REPEATABLE READ is a higher transaction > isolation > level than READ COMMITTED. If you have problematic code, it should lead to > more deadlocks and/or transactions failing at commit time (compared to > READ > COMMITTED), not to data loss. The reason we get data losses is MySql's > unique > interpretation of REPEATABLE READ. If you're interested in the details > (and if > you use MySql, you should be), read on. > > With MySql's REPEATABLE READ, the "read" operations -- SELECT statements > -- > indeed act like they act in the usual REPEATABLE READ: Once you've read > some > table, changes made to that table by other transactions will not be > visible > within your transaction. But "write" operations -- UPDATE, DELETE, INSERT > and > the like -- act as if they're under READ COMMITTED, affecting (and > affected by) > changes committed by other transactions. The result is, essentially, that > within a transaction, the reads are not guaranteed to be consistent with > the > writes [1]. > > In particular, in the bug[2] that caused this discussion, we get the > following > behavior in one transaction: > > (1) BEGIN TRANSACTION > > (2) SELECT ... FROM some_table WHERE some_field=some_value > (1 row returned) > > (3) (some other transactions commit) > > (4) SELECT ... FROM some_table WHERE some_field=some_value > (1 row returned, same as above) > > (5) DELETE some_table WHERE some_field=some_value > (answer: 1 row deleted) > > (6) SELECT ... FROM some_table WHERE some_field=some_value > (1 row returned, same as above) > > (7) COMMIT > (the row that was returned earlier is no longer in the > database) > > Take a minute to read this. Up to step (5), everything is as you would > expect; > you should find steps (6) and (7) quite surprising. > > This happens because the other transactions in (3) deleted the row that is > returned in (2), (4) & (6), and inserted another one where > some_field=some_value; that other row is the row that was deleted in (5). > The > row that this transaction selects was not seen by the DELETE, and hence > not > changed by it, and hence continues to be visible by the SELECTs in our > transaction. But when we commit, the row (which has been deleted) no > longer > exists. > > I have expressed elsewhere my opinion of this behavior as a general > database > feature, and feel no need to repeat it here; but I think that, if > possible, it > is Django's job as a framework to protect its users from it, at least as a > default. > > On Monday 21 March 2016 02:25:37 Cristiano Coelho wrote: > > What performance changes can you expect doing this change? It is > probably > > that default on MySQL for a good reason. > > The Django project is usually willing to give up quite a lot of > performance in > order to prevent data losses. I agree that this default on MySql is > probably > for a reason, but I don't think it can be a good reason for Django. > > Have fun, > Shai. > > [1] https://dev.mysql.com/doc/refman/5.7/en/innodb-consistent-read.html > [2] https://code.djangoproject.com/ticket/26347 > -- You received this message because you are subscribed to the Google Groups "Django developers (Contributions to Django itself)" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send email to [email protected]. Visit this group at https://groups.google.com/group/django-developers. To view this discussion on the web visit https://groups.google.com/d/msgid/django-developers/9a0de040-4943-4a57-9153-312b30ee8524%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
