Sunday, May 19, 2019

MySQL Support Engineer's Chronicles, Issue #9

My previous post from this series was published more than 1.5 years ago. I had never planned to stop writing about my everyday work on a regular basis, but sometimes it's not easy to pick up something really interesting for wider MySQL audience and when in doubts I always prefer to write about MySQL bugs...

In any case, any long way starts from the first step, so I decided to write one post in this series per week and try to summarize in it whatever findings, questions, discussions, bugs and links I've collected over the week. My work experience differs week after week, so some of these posts may be boring or less useful, but I still want to try to create them on a regular basis.

I was working on (upcoming) blog post (inspired by one customer issue) on the impact of innodb_default_row_format setting for importing tablespaces (and related checking of the row format really used in both .frm and .ibd files) and found FSP Header description in this old post by Jeremy Cole useful for further checks in the InnoDB source code. MySQL manual is not very informative (and MariaDB KB page is just wrong/incomplete) when describing flags for the table or tablespace, unfortunately, so I've reported MDEV-19523 to get this improved.

If you ever wonder what MariaDB plans to do with InnoDB in the future, please, check MDEV-11633 among other sources.

This week we in Support got customer (on MySQL 8.0.x) complaining that they can not start server any more on Windows 10 after moving datadir to other drive. Check this blog post by my colleague Nil on the reason, explanations and way to fix/prevent this from happening. One of those cases when MySQL Forums give useful hint.

If you build MariaDB (and MySQL) from source on a regular basis (as I do), you may wonder at times how to disable some storage engine plugin at build time (for example, NOT to be affected by some temporary bugs in it when you do not really need it for testing or production use). Save this as hint:
This is what you have to add to cmake command line to prevent building Mroonga, for example. Same approach applies to TokuDB etc. See this KB page also for more details.

I never noted before that "Explain Analyzer" service exists at, but it seems some customers use it and even prefer to share its output instead of plain text EXPLAIN. Just copy/paste any EXPLAIN ...\G there and decide if the result is useful. For Support purposes and queries accessing less than 10 tables or so I'd prefer usual text output.

Yet another public service at I noted this week by pure chance is "MariaDB CI" page with buildbot status and ways to check what is building now, what failed etc. MariaDB Foundation works in a true open manner at all levels.

If you ever cares to find out what exact versions of MariaDB (or MySQL) contain specific commit you can find out using git tag --contains commit_hash command.

I still do not care about Kubernetes at all, but it seems customers start to use it in production, so here is the hint for myself on how run specific command in a running container:
kubectl exec -it <pod> --container <container> -- vi grastate.dat
I may have to write or speak about some details of MySQL and MariaDB architecture soon, so I was looking for related pictures and texts. I found useful details in the following places:
If you are interested in different storage engines and efficiency of indexing, check this blog post by Mark Callaghan

The last but not the least, I've nominated the following bugs:
  • Bug #95269 - "binlog_row_image=minimal causes assertion failure". I really wonder why this combination was missed in any regular testing of debug builds (that I hope Oracle does).
  • Bug #90681 - "MySQL 8.0 fails to install and start from Oracle .debs on debian 9 x86_64". It seems proper documentation is missing for users to know what conflicting packages to remove, what paths to clean up (if any) etc. Maybe this is no longer a concern (I do no use Oracle .deb packages, so I don't know), but in any case having this bug just "Open" helps nobody.
  • Bug #87312 - "Test main.events_time_zone is fundamentally unstable". It's even more strange to see this bug report about unstable test case "Open" for more than 2 years. Is it really hard to run MTR many times or check the code and improve, or just agree to disable it?
  • Bug #95411 - "LATERAL produces wrong results (values instead of NULLs) on 8.0.16". This regression bug in optimizer of 8.0.16 (vs 8.0.14) leads to wrong results, but so far nobody cared to verify it (even though it has simple and clear "How to repeat" instructions). This is sad.
for bug of the day on Twitter this week. I've also participated in a discussion there. As a result I ended up reading some recent MEB 8.0 manual pages (and more here). MySQL Enterprise Backup really provides a lot of potentially useful options that mariabackup may benefit from one day...

I spent first two weeks of May properly last year, on vacation in UK. Battersea Park here.
That's more or less all I had written down for further review this week that I am ready top share. Stay tuned for what may come up next week!

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