Saturday, November 11, 2017

Fun with Bugs #57 - On MySQL Bug Reports I am Subscribed to, Part I

I've decided to stop reviewing MySQL Release Notes in this series, but it does not mean that I am not interested in MySQL bugs any more. At the moment I am subscribed to 91 active MySQL bugs reported by other MySQL users, and in this blog post I am going to present 15 of them, the most recently reported ones. I'd really want to see them fixed or at least properly processed as soon as possible.

In some cases I am going to add my speculations on how the bug had better be handled, or maybe highlight some important details about it. It is not my job any more to process/"verify" any community bug reports for any kind of MySQL, but I did that for many years and I've spent more than 5 years "on the other side", being a member of Community, so in some cases I let myself to share some strong opinion on what may be done differently from the Oracle side.

As a side note, I started to subscribe to MySQL bugs mostly after I left Oracle, as before that I got email notification about each and every change in every MySQL bug report ever created...

Here is the list, starting from the most recent ones:
  • Bug #88422 - "MySQL 5.7 innodb purge thread get oldest readview could block other transaction". It is one of that bug reports without a test case from reporter. It is tempting to set it to "Verified" just "based on code review", as the code in 5.7 is quite obviously shows both holding the trx_sys->mutex and linear complexity of search depending on number of read views in the worst case (when most of them are closed):
    Get the oldest (active) view in the system.
    @return oldest view if found or NULL */

    MVCC::get_oldest_view() const
            ReadView*       view;


            for (view = UT_LIST_GET_LAST(m_views);
                 view != NULL;
                 view = UT_LIST_GET_PREV(m_view_list, view)) {

                    if (!view->is_closed()) {

    But probably current Oracle bugs verification rules do not let to just mark it as verified. After all, somebody will have to create a test case... So, my dear old friend Sinisa Milivojevic decided to try to force bug reporter to provide a test case instead of spending some time trying to create one himself. I am not going to blame him for that, why to try the easy way :) But I consider this his statement in the comment dated [10 Nov 16:21]:
    "... 5.7 methods holds no mutex what so ever..."
    a bit wrong, as we can see the mutex is acquired when get_oldest_view() method is called:
    MVCC::clone_oldest_view(ReadView* view)

            ReadView*       oldest_view = get_oldest_view();

            if (oldest_view == NULL) {
  • Bug #88381 - "Predicate cannot be pushed down "past" window function". Here bug reporter had provided enough hints for a test case. One can probably just check 'Handler%' status variables before and after query execution to come to the conclusion. Moreover, it seems Oracle developer,  Dag Wanvik, accepted this as a known limitation, but the bug still remains "Open" and nobody knows if it was copied to the internal bugs database, got prioritized and if any work on this is planned any time soon. We shell see. You may also want to monitor MDEV-10855.
  • Bug #88373 - "Renaming a column breaks replication from 5.7 to 8.0 because of impl. collation". This bug was quickly verified by Umesh Shastry. I expect a lot of "fun" for users upgrading to MySQL 8.0 when it becomes GA, especially in replication setups.
  • Bug #88328 - "Performance degradation with the slave_parallel_workers increase". There is no test case, just some general description and ideas about the root case when semi-sync replication is used. I expect this bug to stay "Open" for a long time, as it is a topic for a good research and blog posts like this one, that is, a work for real expert!
  • Bug #88223 - "Replication with no tmpdir space and InnoDB as tmp_storage_engine can break". Here we have clear and simple test case from Sveta Smirnova (no wonder, she also worked at bugs verification team in MySQL, Sun and Oracle). I hope Umesh will verify it soon. As a side note, it is explained (in the comments) elsewhere that InnoDB as internal_tmp_disk_storage_engine may not be the best possible option. We do not have this variable and do not plan to support InnoDB for internal temporary tables in MariaDB 10.2+.
  • Bug #88220 - "compressing and uncompressing InnoDB tables seems to be inconsistent". See also other, older bug reports mentioned there that are duplicates/closely related, but were not getting proper attention.
  • Bug #88150 - "'Undo log record is too big.' error occurring in very narrow range of str length". It was reported by my colleague Geoff Montee and is already fixed in recent versions of MariaDB (see MDEV-14051 for the details and some nice examples of gdb usage by a developer)!
  • Bug #88127 - "Index not used for 'order by' query with utf8mb4 character set". Here I am just curious when bugs like that would be caught up by Oracle QA before any public releases.
  • Bug #88071 - "An arresting Performance degradation when set sort_buffer_size=32M". here the test case is clear - just run sysbench oltp test at high concurrency with different values of sort_buffer_size. Still, Sinisa Milivojevic decided to explain when RAM limit may play a role instead of just showing how it works great (if it does) on any server with enough RAM... Let's see how this attempt to force bug reporter to work/explain more may end up...
  • Bug #87947 - "Optimizer chooses ref over range when access when range access is faster". Nice example of a case when optimizer trace may be really useful. Øystein Grøvlen kindly explained that "range access and ref access are not comparable costs". I wish we get better cost model for such cases in MySQL one day.
  • Bug #87837 - "MySQL 8 does not start after upgrade to 8.03". It is expected actually, and even somewhat documented in the release notes that MySQL 8.0.3 is not compatible to any older version. So, it is more like MySQL Installer (that I do not care much about) bug, but I still subscribed to it as yet another source of potential fun during further upgrade attempts.
  • Bug #87716 - "SELECT FOR UPDATE with BETWEEN AND gets row lock excessively". I think I already studied once why with IN() rows are locked differently by InnoDB comparing to BETWEEN that selects the same rows. But I'd like to know what's the Oracle's take on this, and I'd like to study this specific test case in details one day as well.
  • Bug #87670 - "Force index for group by is not always honored". Clear and simple test case, so no wonder it was immediately verified.
  • Bug #87621 - "Huge InnoDB slowdown when selecting strings without indexes ". I'd like to check with perf one day where the time is spent mostly during this test. For now I think this is a result of the way "long" data are stored on separate pages in InnoDB. What;'s interesting here is also a test case where R is used to generate data set.
  • Bug #87589 - "Documentation incorrectly states that LOAD DATA LOCAL INFILE does not use tmpdir". This was yet another report from my colleague Geoff Montee. lsof is your friend, maybe I have to talk about it one day at FOSDEM (call for papers is still open :) I like to find and follow bugs and missing details in MySQL manual, maybe because I would never be able to contribute to it as a writer directly...

So, this list shows my typical recent interests related to MySQL bugs - mostly InnoDB, optimizer, replication problems, fine manual and just some fun details like the way some Oracle engineers try to avoid working extra hard while processing bugs... I am also happy to know that in some cases MariaDB is able to deliver fixes faster.

No comments:

Post a Comment