Fun with Bugs #97 - On MySQL Bug Reports I am Subscribed to, Part XXXI

Time for the next blog post about MySQL bugs! The previous one covered some bugs I considered interesting in March, this one will be about community bug reports that I've picked up since March 26, 2020. I'd better review bug fixed in MySQL 5.7.30 instead of this, but it is still not released, even though we know it must get some important security fixes based on "Oracle Critical Patch Update Advisory - April 2020" recently published.

As usual, I am mostly interested in replication, InnoDB, optimizer and few more categories. Here is the list:

• Bug #99082 - "Problem with replication: XA transaction, temp tables and row based binlog". As noted by Libor Laichmann, creating temporary tables inside XA transaction leads to broken replication in MySQL 5.7.29, with somewhat misleading error message. This is unfortunate. We do not see any evidence, yet, if 5.6 and 8.0 branches are similarly affected.
• Bug #99094 - "coredump when install information schema plugin". Bug reporter, Lou Shuai, tried to create simple plugin for the information_schema and it crashed MySQL server 8.0.19 upon installation attempt. Bot the plugin source code and patch for the bug were contributed.
• Bug #99100 - "GROUP BY will throw table is full when temptable memory allocation exceed limit". More fun with TempTable storage engine that I blamed previously. This bug was reported by Dean Zhou, who had performed detailed analysis in gdb and suggested a fix. It took some efforts for the bug reporter to get it "Verified" as a regression bug in MySQL 8.0.19 (without a "regression" tag, surely).
• Bug #99101 - "SELECT .. LOCK IN SHARE MODE requires LOCK TABLES grant". Simple regression in MySQL 8.0.11+ (or incompatible change in behavior, if you prefer) vs MySQL 5.7 was found and reported by Matthew Boehm.
• Bug #99136 - "TempTable wastes 1MB for each connection in thread cache". Nikolai Ikhalainen demonstrated this additional memory usage in MySQL 8.0.16 comparing to 5.7 quite clearly.
• Bug #99174 - "Prepare transaction won't rollback if server crash after online ddl prepare stage". This bug was reported by Zhang Xiaojian. Additional debugging code (one DBUG_EXECUTE_IF statement) was added to the source to demonstrate the problem easily and it caused some questionable arguments of a kind:
  

  "First of all, changing our server code in order to cause a bug, can not be considered a repeatable test case."

  But the bug reporter provided more details and clear test case, and had not agreed with the above. The bug was soon "Verified", even though no attempts to check (or explain) if 5.7 may be similarly affected were made in public, so we still have to wonder if this is a regression.
• Bug #99180 - "Accessing freed memory in perfschema when aggregating status vars". Let me just quote Manuel Ung:
  

  "When aggregate_thread_status is called for other threads, it's possible for that thread to have exited and freed the THD between the time we check that the thread was valid, until the time we call get_thd_status_var."

  Ironically, he had to add some conditional my_sleep() call to server code to get a repeatable test case, and this was NOT a problem for a good Oracle engineer to verify the bug immediately. There are still bugs in my beloved Performance Schema. Who could imagine that?
• Bug #99200 - "CREATE USER get stuck on SHOW PROCESSLIST and ps.threads on slave". So, slave may disclose some sensitive information. As demonstrated by Marcelo Altmann, if a query had been rewritten by the parser because it contains sensitive information, it won't be cleaned up when slave's SQL thread applies it, making it visible in SHOW PROCESSLIST and performance_schema.threads. Both 5.7.29 and 8.0.19 are affected. Bug reporter had contributed fixes for both versions. See also another bug in Performance Schema that he reported and contributed a fix for, Bug #99204 - "performance_schema threads table PROCESSLIST_INFO incorrect behaviour".
• Bug #99205 - "xa prepare write binlog while execute failed". Then XA PREPARE from the binary log is executed on slave and... we are in troubles. The bug was reported by Phoenix Zhang. Unfortunately it is still not clear from this verified bug report if MySQL 5.6 and 5.7 are similarly affected (probably they are).
• Bug #99206 - "lock_wait_timeout is taking twice time while adding new partition in table". Nice finding by Lalit Choudhary. MySQL 8.0.19 is not affected by this bug.
• Bug #99244 - "bad performance in hash join because join with no join condition between tables". This optimizer bug (wrong join order when new hash join optimization is used) was reported by Chunyang Xu. But note also a simple test case and perf outputs contributed later by Shane Bester.
• Bug #99257 - "Inconsistent output and wrong ORDER BY Sorting for query with LIMIT". Yet another 5.7-only optimizer bug found by Lalit Choudhary. My quick test presented in a comment shows that MySQL 5.6.27 produced correct results, so this is a regression bug.
• Bug #99273 - "Item_ref in Having Clause points to invalid outer table field". This may lead to wrong results for simple enough queries. This regression bug that affects both MySQL 5.7.29 and 8.0.19 was reported by Shanshan Ying.
• Bug #99286 - "Concurrent update cause crash in row_search_mvcc". This great bug report with code analysis, fix suggested and a test case with detailed instructions was created by Zkong Kong. It was promptly verified, but I still miss any documented attempt to check (by running the test case or by code analysis if the bug applies only to 5.7.29 or MySQL 8.0.x is also potentially affected. For now I'll try to remember this assertion line:
  

  InnoDB: Assertion failure in thread 47758491551488 in file rem0rec.cc line 586

  and "crash with row_search_mvcc in backtrace". Who know when I hit something similar and what fork/version it will be...

Rainy birthday at the seaside in Cap-d'Ail. Almost 15 years of my 50 were spent checking new MySQL bug reports almost every day.

To summarize:

1. We still see many regression bugs in recent versions of MySQL 5.7.x and 8.0.x, often without "regression" tag.
2. There are cases when the bug is verified, but there are no clearly documented checks if all GA versions are affected.
3. XA transactions are (and had always been) a real disaster for modern MySQL versions in replication environments - all kinds of replication breakage and inconsistencies are possible.
4. Check "MySQL Bug Reporter Hall of Fame" if you want to know who from MySQL Community had contributed a lot of bug reports over last 10 years.

Fun with Bugs #96 - On MySQL Bug Reports I am Subscribed to, Part XXX

My weekdays are still busy even during this lockdown period, but weekend is a perfect time for yet another blog post about MySQL bugs I considered interesting. Very few followers read my posts on other topics anyway, especially if they have the "M....DB" word...

So, here is a new list of optimizer, replication, InnoDB and few other bugs in GA versions of MySQL that I've subscribed to since the end of February, 2020:

• Bug #98675 - "LIMIT clause in derived table garbles EXPLAIN Note". It's not a big deal and in specific case presented by Øystein Grøvlen optimizer works properly (note text aside), but some clarifications to the note and/or documentation would still help. Comments are useful to read and shows a great example of proper cooperation from Oracle side.
• Bug #98719 - "Parallel CREATE TABLE statement binlogged in a wrong order". This bug report from Song Libing shows that Oracle engineers readily accept and verify bug reports where some code modification (adding debug sync point, for example) is needed to demonstrate the problem with a repeatable, deterministic test case. I've seen other bug reports where code modification was considered a problem and bug was not verified immediately as a result. Note that this bug seem to be not repeatable on MySQL 8.0.19 and this was explicitly tested, with test results shared in public. Good job, Umesh Shastry!
• Bug #98734 - "Same digest for different queries if we use integer value in ORDER BY caluse". Different column numbers in ORDER BY often cause totally different execution plans, so assuming these queries are the same ("ORDER BY ?") for the purpose of digesting is misleading. Moreover, as noted by Lalit Choudhary from Percona, with column names instead of numbers such queries are not considered the same, so Performance Schema has to be more consistent.
• Bug #98739 - "TempTable storage engine slow if mmap is required". Take care when large temporary tables are used for your queries in MySQL 8.0.x. By default (without temptable_use_mmap = OFF) when the table is larger than temptable_max_ram you may notice a very slow query execution. Nice finding by Marcelo Altmann. This is not the only related performance regression I've seen reported recently. Looks like TempTable storage engine problems is a real main topic of this post!
• Bug #98750 - "SHOW FIELDS command regression on MySQL 8.0". This performance regression bug was reported by Juan Arruti. See also similar Bug #92195 - "checking permissions 90 time" that was wrongly(!) declared "Not a bug" back in 2018 just because of the way the problem was demonstrated. This time Przemyslaw Malkowski helped to make the point based on Performance Schema instrumentation, so there was no other option but to accept this as a real performance regression bug. Take care if you use SHOW statements with MySQL 8!
• Bug #98782 - "Using TempTable engine for GROUP BY is slower than using MEMORY engine". In this bug report Øystein Grøvlen demonstrated that MEMORY engine is about 10% faster for temporary tables in MySQL 8. Make sure to use internal_tmp_mem_storage_engine=MEMORY if you care about performance.
• Bug #98869 - "Temp ibt tablespace truncation at disconnection stuck InnoDB under large BP". Bug reporter, Fungo Wang, used different methods to show the performance impact of the regression probably introduced by WL#11613. From pt-pmp to perf and other OS level tools. Make sure to check all comments that point out to other bugs and problems.
• Bug #98974 - "InnoDB temp table could hurt InnoDB perf badly". Yet another bug report by Fungo Wang. This time it took a lot of time and efforts from the bug reporter and many MySQL Community members (including me) to get this bug properly processed, even though it started with a detailed source code analysis, properly described test case, stack traces analysis and perf profiling details shared. The ideal bug report got far from ideal treatment, unfortunately.
• Bug #98976 - "Case of referenced column of foreign key not corrected on import". Tim Düsterhus found that in versions with the fix for Bug #88718 - "Foreign key is always in lower case" the case is not always correct in the output of subsequent mysqldump.
• Bug #98980 - "A state inside mysql_real_connect_nonblocking still blocks". This bug report by Jay Edgar was verified surprisingly fast. This is why it ended up in my list.
• Bug #98990 - "avg_count_reset for monitor set owner is always NULL in I_S.INNODB_METRICS ". In this case Fungo Wang had not only found a bug, but also provided a patch that was accepted by Oracle after signing the OCA.
• Bug #99006 - "GTID Synchronisation speed when doing START SLAVE". Simon Mudd noted that in some cases with GTIDs START SLAVE may take a lot of time:
  

  root@myhost [(none)]> start slave;
  Query OK, 0 rows affected (3 min 17.74 sec)

  and what's worse nothing is logged to the error log in the process to show the reason, progress or anything useful. Probably it's expected that with many binary logs finding proper GTID in them takes time, but some feedback would be useful. The bug does not have any clear public test case and is still under analysis.
• Bug #99010 - "The mtr case --send command not use ps-protocol when run with --ps-protocol ". Does not sound like a big deal for anyone but developers who write MTR test cases, but in this report Ze Yang had provided source code analysis and quite detailed how to repeat steps and still got useless requests for additional feedback and nothing else till today. This is, again, unfortunate.
• Bug #99039 - "PSI thread info not getting updated after statement execution". Who could imagine that regression bugs may be introduced even into Performance Schema? But Pranay Motupalli found one introduced into 8.0.18, 5.7.28+ and 5.6.47+ by this commit! I hope to see it fixed soon.
• Bug #99051 - "XA commit may do engine commit before MYSQL_BIN_LOG::ordered_commit". XA transactions is one of my favorite weak areas of MySQL. In this bug report Dennis GAO described a case when XA COMMIT operation may do a engine commit before binlog flush on Ubuntu 18.04. He had contributed the fix based on the assumption that the best way is to ensure the sequence in the plugin_hash, so that the binlog plugin should be always before all the transnational engine plugins. I only hope that one day a long list of XA bugs will be fixed in one of forks if not in MySQL itself.

This is my favorite point of view this year.

To summarize:

1. I see some really good examples of bugs verification by Oracle engineers recently. All supported versions are tested, nobody tries to argue against the bug reporter approach used to demonstrate the problem, even if it includes source code modifications. Looks really promising.
2. Even ideal bug reports sometimes are not processed properly without extra efforts from MySQL Community, unfortunately.
3. Percona engineers still contribute a lot to MySQL with both new bug reports and useful comments and clarifications. I hope Oracle appreciates that.
4. Looks like code review and regression testing in Oracle still may benefit from some improvements, as we still see new regression bugs...

Fun with Bugs #94 - On MySQL Bug Reports I am Subscribed to, Part XXVIII

I may get a chance to speak about proper bugs processing for open source projects later this year, so I have to keep reviewing recent MySQL bugs to be ready for that. In my previous post in this series I listed some interesting MySQL bug reports created in December, 2019. Time to move on to January, 2020! Belated Happy New Year of cool MySQL Bugs!

As usual I mostly care about InnoDB, replication and optimizer bugs and explicitly mention bug reporter by name and give link to his other active reports (if any). I also pick up examples of proper (or improper) reporter and Oracle engineers attitudes. Here is the list:

• Bug #98103 - "unexpected behavior while logging an aborted query in the slow query log".  Query that was killed while waiting for the table metadata lock is not only get logged, but also lock wait time is saved as query execution time. I'd like to highlight how bug reporter, Pranay Motupalli, used gdb to study what really happens in the code in this case. Perfect bug report!
• Bug #98113 - "Crash possible when load & unload a connection handler". The (quite obvious) bug was verified based on code review, but only after some efforts were spent by Oracle engineer on denial to accept the problem and its importance. This bug was reported by Fangxin Flou.
• Bug #98132 - "Analyze table leads to empty statistics during online rebuild DDL ". Nice addition to my collections! This bug with a nice and clear test case was reported by Albert Hu, who also suggested a fix.
• Bug #98139 - "Committing a XA transaction causes a wrong sequence of events in binlog". This bug reported by Dehao Wang was verified as a "documentation" one, but I doubt documenting current behavior properly is an acceptable fix. Bug reporter suggested to commit in the binary log first, for example. Current implementation that allows users to commit/rollback a XA transaction by using another connection if the former connection is closed or killed, is risky. A lot of arguing happened in comments in the process, and my comment asking for a clear quote from the manual:
  

  Would you be so kind to share some text from this page you mentioned:
  
  https://dev.mysql.com/doc/refman/8.0/en/xa.html
  
  or any other fine MySQL 8 manual page stating that XA COMMIT is NOT supported when executed from session/connection/thread other than those prepared the XA transaction? I am doing something wrong probably, but I can not find such text anywhere.

  was hidden. Let's see what happens to this bug report next.
• Bug #98211 - "Auto increment value didn't reset correctly.". Not sure what this bug reported by Zhao Jianwei has to do with "Data Types", IMHO it's more about DDL or data dictionary. Again, some sarcastic comments from Community users were needed to put work on this bug back on track...
• Bug #98220 - "with log_slow_extra=on Errno: info not getting updated correctly for error". This bug was reported by lalit Choudhary from Percona.
• Bug #98227 - "innodb_stats_method='nulls_ignored' and persistent stats get wrong cardinalities". I think category is wrong for this bug. It's a but in InnoDB's persistent statistics implementation, one of many. The bug was reported by Agustín G from Percona.
• Bug #98231 - "show index from a partition table gets a wrong cardinality value". Yet another by report by Albert Hu. that ended up as a "documentation" bug for now, even though older MySQL versions provided better cardinality estimations than MySQL 8.0 in this case (so this is a regression of a kind). I hope the bug will be re-classified and properly processed later.
• Bug #98238 - "I_S.KEY_COLUMN_USAGE is very slow". I am surprised to see such a bug in MySQL 8. According to the bug reporter, Manuel Mausz, this is also a kind of regression comparing to older MySQL version, where these queries used to run faster. Surely, no "regression" tag in this case was added.
• Bug #98284 - "Low sysbench score in the case of a large number of connections". This notable performance regression of MySQL 8 vs 5.7 was reported by zanye zjy. perf profiling pointed out towards ppoll() where a lot of time is spent. There is a fix suggested by Fangxin Flou (to use poll() instead), but the bug is still "Open".
• Bug #98287 - "Explanation of hash joins is inconsistent across EXPLAIN formats". This bug was reported by Saverio M and ended up marked as a duplicate of Bug #97299 fixed in upcoming 8.0.20. Use EXPLAIN FORMAT=TREE in the meantime to see proper information about hash joins usage in the plan.
• Bug #98288 - "xa commit crash lead mysql replication error". This bug report from Phoenix Zhang (who also suggested a patch) was declared a duplicate of Bug #76233 - "XA prepare is logged ahead of engine prepare" (that I've already discussed among other XA transactions bugs here).
• Bug #98324 - "Deadlocks more frequent since version 5.7.26". Nice regression bug report by Przemyslaw Malkowski from Percona, with additional test provided later by Stephen Wei . Interestingly enough, test results shared by Umesh Shastry show that MySQL 8.0.19 is affected in the same way as 5.7.26+, but 8.0.19 is NOT listed as one of versions affected. This is a mistake to fix, along with missing regression tag.
• Bug #98427 - "InnoDB FullText AUX Tables are broken in 8.0". Yet another regression in MySQL 8 was found by Satya Bodapati. Change in default collation for utf8mb4 character set caused this it seems. InnoDB FULLTEXT search was far from perfect anyway...

The are clouds in the sky of MySQL bugs processing.

To summarize:

1.  Still too much time and efforts are sometimes spent on arguing with bug reporter instead of accepting and processing bugs properly. This is unfortunate.
2. Sometimes bugs are wrongly classified when verified (documentation vs code bug, wrong category, wrong severity, not all affected versions are listed, ignoring regression etc). This is also unfortunate.
3. Percona engineers still help to make MySQL better.
4. There are some fixes in upcoming MySQL 8.0.20 that I am waiting for :)
5. XA transactions in MySQL are badly broken (they are not atomic in storage engine + binary log) and hardly safe to use in reality.

Fun with Bugs #93 - On MySQL Bug Reports I am Subscribed to, Part XXVII

No matter what I write and present about dynamic tracing, blog posts about MySQL bugs are more popular based on statistics. So, to make more readers happy, I'd like to continue my review of interesting bugs reported in November with this post on bugs reported during December, 2019.

As usual, I'll try to concentrate on bug reports related to InnoDB, replication and optimizer, but some other categories also got my attention:

• Bug #97911 - "[FATAL] Semaphore wait has lasted > 600 seconds. We intentionally crash the serv...". This bug got marked as a duplicate of other, older long semaphore wait bug (in "Can't repeat" status!) without much analysis. I think all Oracle engineers who added comments to that bug missed one interesting point:
  

  ... [ERROR] [MY-012872] [InnoDB] Semaphore wait has lasted > 39052544 seconds.

  even though bug reporter highlighted it in a comment. Reported wait time is a problem and is surely a bug, no matter how to reproduce the long wait itself and what is its root cause!
• Bug #97913 - "Undo logs growing during partitioning ALTER queries". This bug (affecting only MySQL 5.7.x) was reported by Przemyslaw Malkowski from Percona, who also presented useful examples of monitoring queries to the information_schema.innodb_metrics and performance_schema. Check also comments that may explain why 8.0 is not affected in a similar way.
• Bug #97935 - "Memory leak in client connection using information_schema". It took some efforts (starting from but not limited to Valgrind Massif profiling of heap memory usage) and time for Daniel Nichter to prove the point and get this bug "Verified". It is also not clear if MySQL 8 is also affected.
• Bug #97950 - "buf_read_page_handle_error can trigger assert failure". Bug reporter, Shu Lin, tried his best to make the point. It's clear enough how to repeat this, and one could use one of documented test synchonisation methods if gdb is too much for bug verification. I do not think this bug was handled properly or got the level of attention it truly deserved.
• Bug #97966 - "XA COMMIT in another session will not write binlog event". This bug was reported by Lujun Wang and immediately verified, but again with no documented check if MySQL 8 is affected. This happens too often, unfortunately.
• Bug #97971 - "Roles not handling column level privileges correctly; Can SELECT, but not UPDATE". Clear and simple bug report with a workaround from Travis Bement. It was immediately verified.
• Bug #98014 - "Lossy implicit conversion in conditional breaks ONLY_FULL_GROUP_BY". Yet another case of (IMHO) improper bug processing. The argument presented (from the manual):
  

  "MySQL 5.7.5 and later also permits a nonaggregate column not named in a GROUP BY clause when ONLY_FULL_GROUP_BY SQL mode is enabled, provided that this column is limited to a single value"

  does not apply, as "single value" for = 0 is NOT selected, we have multiple Host values matching it due to conversion. This is how proper version (guess what it is) works:
  

  mysql> SELECT User, Host, COUNT(*) FROM mysql.user WHERE Host = 0 GROUP BY 1;
  ERROR 1055 (42000): 'mysql.user.Host' isn't in GROUP BY
  mysql> select @@sql_mode;
  +--------------------+
  | @@sql_mode         |
  +--------------------+
  | ONLY_FULL_GROUP_BY |
  +--------------------+
  1 row in set (0.001 sec)
  
  mysql> set session sql_mode='';
  Query OK, 0 rows affected (0.029 sec)
  
  mysql> SELECT User, Host, COUNT(*) FROM mysql.user WHERE Host = 0 GROUP BY 1;+---------------+-----------+----------+
  | User          | Host      | COUNT(*) |
  +---------------+-----------+----------+
  | data_engineer |           |        1 |
  | en            | localhost |        1 |
  | ro1           |           |        1 |
  | ro2           |           |        1 |
  | role-1        |           |        1 |
  | root          | ::1       |        3 |
  ...
  | user1         | %         |        1 |
  +---------------+-----------+----------+
  13 rows in set, 17 warnings (0.003 sec)

  I think this bug reported by Joshua Varner must be verified.
• Bug #98046 - "Inconsistent behavior while logging a killed query in the slow query log". Bug reporter, Pranay Motupalli, provided a clear test case and a detailed analysis, including the gdb debugging session that proves the point. Nice bug report.
• Bug #98055 - "MySQL Optimizer Bug not picking right index". Both the bug reporter (Sudheer Gadipathi) and engineer who verified the bug stated that MySQL 8.0.x is similarly affected (UNIQUE key is preferred for the partitioned table, even though there is a better non-unique index). But 8.0.x is NOT listed in the "Version:" filed. Weird.
• Bug #98068 - "SELECT FOR UPDATE not-exist table in PERFORMANCE SCHEMA reports confusing error". This is a funny (but still a regression) bug report by William ZHANG. Proper versions work like this:
  

  mysql> select database();+--------------------+
  | database()         |
  +--------------------+
  | performance_schema |
  +--------------------+
  1 row in set (0.001 sec)
  
  mysql> select * from not_exist_table;
  ERROR 1146 (42S02): Table 'performance_schema.not_exist_table' doesn't exist
  mysql> select * from not_exist_table for update;
  ERROR 1146 (42S02): Table 'performance_schema.not_exist_table' doesn't exist

• Bug #98072 - "innochecksum summary shows blob pages as other type of page for 8.0 tables". The bug was reported by SERGEY KUZMICHEV. This time "regression" tag is missing, even though it's clearly stated that MySQL 5.7 worked differently. This is from the proper version:
  

  ...
  File::..\data\test\blob_test.ibd
  ================PAGE TYPE SUMMARY==============
  #PAGE_COUNT     PAGE_TYPE
  ===============================================
         1        Index page
         0        Undo log page
         1        Inode page
         0        Insert buffer free list page
       508        Freshly allocated page
         1        Insert buffer bitmap
         0        System page
         0        Transaction system page
         1        File Space Header
         0        Extent descriptor page
        64        BLOB page
         0        Compressed BLOB page
         0        Page compressed page
         0        Page compressed encrypted page
         0        Other type of page
  ...
  

• Bug #98083 - "Restarting the computer when deleting the database will cause directory residues". One would expect that MySQL 8 with a data dictionary should have some means to figure out the remaining database directory for a dropped database upon startup (as it stores information about databases elsewhere) and do proper cleanup. I think this bug reported by Jinming Liao must be verified and fixed. There is no "... manual creation or deletion of tables or databases..." involved in this case.
• Bug #98091 - "InnoDB does not initialize raw disk partitions". As simple as that and both 5.7.29 and 8.0.219 are surely affected. It was not always the case, I've used raw devices myself with older MySQL versions, so this bug reported by Saverio M is a regression. Still, "regression" tag is missing.

That's all bugs reported in December, 2019 that I cared to subscribe to  and mention here. Next time I'll check bugs reported in January, 2020. There are at least 16 in my list already, so stay tuned.

Follow the links in this post to get more details about profiling and creating off-CPU FlameGraphs for MySQL. This post is devoted to bugs, though :)

To summarize:

1. I am happy to see bug reports from people whom I never noticed before. MySQL Community is alive.
2. Some flexibility in following common sense based bugs verification procedures is still visible. Bugs reported for 5.7 are not checked on 8.0 (or the results of this check are not documented in public), nobody cares to read what bug reporter says carefully or go extra mile, "regression" tag not added, and so on. 
3. Probably at this stage my writings are mostly ignored by Oracle's decision makers. But I keep watching them all anyway.

Fun with Bugs #92 - On MySQL Bug Reports I am Subscribed to, Part XXVI

I'd like to continue reviewing MySQL bug reports from Community users that I considered interesting and subscribed to. Unlike in the previous post in this series, I am not going to check test cases on any competitor product, but will use only recently released MySQL 5.7.29 and 8.0.19 for checks, if any. This time I'll concentrate on bugs reported in November 2019.

As usual, I mostly care about optimizer, InnoDB and replication related bugs. Here is the list:

• Bug #97476 - "Range optimizer skips rows". This bug reported by Ilya Raudsepp looks like a clear regression in MySQL 8.0.x comparing to MySQL 5.7.x at least. I get the following correct results with 5.7.29:
  

  mysql> SELECT t.id
      -> FROM Test t
      -> JOIN (
      ->     SELECT item_id, MAX(created_at) AS created_at
      ->     FROM Test t
      ->     WHERE (platform_id = 2) AND (item_id IN (3,2,111)) AND (type = 'Default')
      ->     GROUP BY item_id
      -> ) t2 ON t.item_id = t2.item_id
      ->   t.item_id = t2.item_id
      ->   AND t.created_at = t2.created_at
      ->   AND t.type = 'Default'
      -> WHERE t.platform_id = 2;
  +----+
  | id |
  +----+
  |  6 |
  |  3 |
  |  5 |
  +----+
  3 rows in set (0,03 sec)
  
  mysql> select version();
  +-----------+
  | version() |
  +-----------+
  | 5.7.29    |
  +-----------+
  1 row in set (0,02 sec)

• Bug #97531 - "5.7 replication breakage with syntax error with GRANT management". This tricky bug reported by Simon Mudd applies also to MySQL 8.0.x. It is closed as fixed, but the fix had not made it to recent 5.7.29 and 8.0.19 releases, so you'll have to wait for few more months.
• Bug #97552 - "Regression: LEFT JOIN with Impossible ON condition performs slowly". Yet another optimizer regression in MySQL 8 (comparing to 5.7.x) that is fixed only in MySQL 8.0.20+. The bug was reported by Fredric Johansson.
• Bug #97648 - "Bug in order by clause in union clause". Yet another regression (at least from user's point of view) in recent MySQL 5.7.x and 8.0.x comparing to 5.6.x. This time without a "regression" tag. The bug was reported by Andrei Mart.
• Bug #97662 - "MySQL v8.0.18 FIPS mode is no longer supported". According to Ryan L, MySQL 8.0.18+ is no longer supporting ssl_fips_mode=STRICT, as OpenSSL 1.1.1 is not FIPS-compatible and MySQL Server must be compiled using OpenSSL 1.1.1 or higher. That's interesting. Check also this link.
• Bug #97682 - "Handler fails to trigger on Error 1049 or SQLSTATE 42000 or plain sqlexception". This regression (comparing to MySQL 5.7) was reported by Jericho Rivera. It is fixed in MySQL 8.0.20. The patch was provided by Kamil Holubicki.
• Bug #97692 - "Querying information_schema.TABLES issue". I do not see any documented attempt to check on MySQL 8.0, so I had to add a comment to the bug report. From what I see, in MySQL 8.0.19 we still get different (empty) result from the second query, but at least now we have a warning:
  

  mysql> SELECT ts.TABLE_SCHEMA
      -> FROM information_schema.TABLES ts
      -> WHERE ts.TABLE_TYPE ='VIEW'
      -> AND ts.TABLE_SCHEMA NOT IN ('sys')
      -> AND ts.TABLE_COMMENT LIKE '%invalid%';
  +--------------+
  | TABLE_SCHEMA |
  +--------------+
  | test         |
  +--------------+
  1 row in set, 1 warning (0,00 sec)
  
  mysql> show warnings\G
  *************************** 1. row ***************************
    Level: Warning
     Code: 1356
  Message: View 'test.v' references invalid table(s) or column(s) or function(s) or definer/invoker of view lack rights to use them
  1 row in set (0,00 sec)
  
  mysql> select version();
  +-----------+
  | version() |
  +-----------+
  | 8.0.19    |
  +-----------+
  1 row in set (0,00 sec)

  The bug was reported by Vinicius Malvestio Grippa.

• Bug #97693 - "ALTER USER user IDENTIFIED BY 'password' broken by invalid authentication_string". The bug was reported by Nikolai Ikhalainen. MySQL 8.0.19 is still affected.
• Bug #97694 - "MySQL 8.0.18 fails on STOP SLAVE/START SLAVE stress test". For some reason I do not see any documented attempt to verify this on MySQL 5.7 also. The bug was reported by Przemysław Skibiński, who also suggested a fix.
• Bug #97734 - "Document the correct method to stop slaving with MTS without a warning or error". I can only agree with this request from Buchan Milne. Please. do :)
• Bug #97735 - "ALTER USER IF EXISTS ... WITH_MAX_USER_CONNECTIONS 9999 not applied correctly". yet another bug report by Simon Mudd in this list. For some reason, again, I do not see any documented attempt to verify the bug on MySQL 8.0.x, while there is no clear reason to think it is not affected.
• Bug #97742 - "bad item ref from correlated subquery to outer distinct table". This bug was reported by Song Zhibai, who also had contributed a patch. Based on further comments from  Øystein Grøvlen and these results:
  

  mysql> EXPLAIN SELECT f3 FROM t1 HAVING (SELECT 1 FROM t2 HAVING f2 LIMIT 1);
  +----+--------------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
  | id | select_type        | table | partitions | type  | possible_keys | key     | key_len | ref  | rows | filtered | Extra       |
  +----+--------------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
  |  1 | PRIMARY            | t1    | NULL       | ALL   | NULL          | NULL    | NULL    | NULL |    3 |   100.00 | NULL        |
  |  2 | DEPENDENT SUBQUERY | t2    | NULL       | index | NULL          | PRIMARY | 4       | NULL |    1 |   100.00 | Using index |
  +----+--------------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
  2 rows in set, 2 warnings (0,00 sec)
  
  mysql> show warnings\G
  *************************** 1. row ***************************
    Level: Note
     Code: 1276
  Message: Field or reference 'f2' of SELECT #2 was resolved in SELECT #1
  *************************** 2. row ***************************
    Level: Note
     Code: 1003
  Message: /* select#1 */ select `test`.`t1`.`f3` AS `f3` from `test`.`t1` having (/* select#2 */ select 1 from `test`.`t2` having `test`.`t1`.`f2` limit 1)
  2 rows in set (0,00 sec)
  
  mysql> select version();
  +-----------+
  | version() |
  +-----------+
  | 5.7.29    |
  +-----------+
  1 row in set (0,00 sec)

  I'd say that MySQL 5.7.x is also affected, but for some reason nobody documented any attempt to verify it there. So, I've added a comment.
• Bug #97777 - "separate global variables (from hot variables) using linker script (ELF)". Beautiful bug report from Daniel Black.With a lot of details, perf and readelf outputs and patch contributed. See also his Bug #97822 - "buf_page_get_gen buf_pool->stat.n_page_gets++ is a cpu waste", with perf analysis up to a single assembler instruction level and fix suggested.
• Bug #97825 - "dd_mdl_acquire in dd_table_open with dict_sys->mutex hold may cause deadlock". Here I am really puzzled by no visible attempt to check the arguments of bug reporter, Dave Do, who tried to perform lock order analysis by code review. All we see as a result is this:
  

  "Lock order could be different, but it is irrelevant, since these are locks on totally different levels and can't, in themselves, cause any deadlock."

  What a great argument! Not a bug, surely... We trust you.

"What bugs are you talking about? I have no bugs, neither does MySQL 8!"

To summarize:

1. MySQL 8 introduces some optimizer (and some other) regressions. They seem to be fixed fast enough, but I wonder why only Community users were able to find them not Oracle's QA...
2. MySQL 8.0.19 is surely great, but I see many serious bugs fixed o0nly in 8.0.20+.
3. Percona, Booking and Facebook engineers still continue contributing high quality bug reports, comments/verification details and patches. Oracle is lucky to have such nice partners in making MySQL better.
4. I still see problems with following proper verification procedures and documenting the results. Too often the bug reported for 8.0.x is NOT checked on 5.7.x as well, regression tag is not set, and so on. Sometimes reports are closed as "Not a bug" without any attempt to follow the analysis provided or prove the point. This is sad and wrong.

Fun with Bugs #90 - On MySQL Bug Reports I am Subscribed to, Part XXIV

Previous post in this series was published 3 months ago and the last Bug #96340 from it is already closed as fixed in upcoming MySQL 8.0.19. I've picked up 50+ more bugs to follow since that time, so I think I should send quick status update about interesting public MySQL bug reports that are still active.

As usual I concentrate mostly on InnoDB, replication and optimizer bugs. Here is the list, starting from the oldest:

• Bug #96374  - "binlog rotation deadlock when innodb concurrency limit setted". This bug was reported by Jia Liu, who used gdb to show threads deadlock details. I admit that recently more bug reporters use gdb and sysbench with custom(ized) Lua scripts to prove the point, and I am happy to see this happening!
• Bug #96378 - "Subquery with parameter is exponentially slower than hard-coded value". In my primitive test with user variables replaced by constants (on MariaDB 10.3.7) I get the same plan for the query, so I am not 100% sure that the analysis by my dear friend Sinisa Milivojevic was right and it's about optimization (and not comparing values with different collations, for example). But anyway, this problem reported by Jeff Johnson ended up as a verified feature request. Let's see what may happen to it next.
• Bug #96379 - "First query successful, second - ERROR 1270 (HY000): Illegal mix of collations ". This really funny bug was reported by Владислав Сокол.
• Bug #96400 - "MTS STOP SLAVE takes over a minute when master crashed during event logging". Nice bug report by Przemyslaw Malkowski from Percona, who used sysbench and dbdeployer to demonstrate the problem. Later Przemysław Skibiński (also from Percona) provided a patch to resolve the problem.
• Bug #96412 - "Mess usages of latch meta data for InnoDB latches (mutex and rw_lock)". Fungo Wang had to make a detailed code analysis to get this bug verified. I am not sure why it ended up with severity S6 (Debug Builds) though.
• Bug #96414 - "CREATE TABLE events in wrong order in a binary log.". This bug was reported by Iwo P. His test case to demonstarte the problem included small source code modification, but (unlike with some other bug reports) this had NOT prevented accepting it as a true, verified bug. The bug not affect MySQL 8.0.3+ thanks to WL#6049 "Meta-data locking for FOREIGN KEY tables" implemented there.
• Bug #96472 - "Memory leak after 'innodb.alter_crash'". Yet another bug affecting only MySQL 4.7 and not MySQL 8.0. It was reported by Yura Sorokin from Percona.
• Bug #96475 - "ALTER TABLE t IMPORT TABLESPACE blocks SELECT on I_S.tables.".  Clear and simple "How to repeat" instructions (using dbdeployer) by Jean-François Gagné. See also his related Bug #96477 - "FLUSH TABLE t FOR EXPORT or ALTER TABLE t2 IMPORT TABLESPACE broken in 8.0.17" for MySQL 8. The latter is a regression bug (without a regression tag), and I just do not get how the GA releases with such new bugs introduced may happen.
• Bug #96504 - "Refine atomics and barriers for weak memory order platform". Detailed analysis, with links to code etc from Cai Yibo.
• Bug #96525 - "Huge malloc when open file limit is high". Looks more like a systemd problem (in versions < 240) to me. Anyway, useful report from Andreas Hasenack.
• Bug #96615 - "mysql server cannot handle write operations after set system time to the past". A lot of arguments were needed to get this verified, but Shangshang Yu was not going to give up. First time I see gstack used in the bug report to get a stack trace quickly. It's a part of gdb RPM on CentOS 6+. I have to try it vs gdb and pstack one day and decide what is the easiest and most efficient way to get backtraces of all threads in production...
• Bug #96637 - "Clone fails on just upgraded server from 5.7". I had not used MySQL 8 famous clone plugin yet in practice, but I already know that it has bugs. This bug was reported by Satya Bodapati, who also suggested a patch.
• Bug #96644 - "Set read_only on a master waiting for semi-sync ACK blocked on global read lock". Yet another problem (documented limitation) report from Przemyslaw Malkowski. Not sure why it was not verified on MySQL 8.0. Without a workaround to set master to read only it is unsafe to use long rpl_semi_sync_master_timeout values, as we may end up with that long downtime.
• Bug #96677 - ""SELECT ... INTO var_name FOR UPDATE" not working in MySQL 8". This regression bug was reported by Vinodh Krish. Some analysis and patch were later suggested by Zsolt Parragi.
• Bug #96690 - "sql_require_primary_key should not apply to temporary tables". This bug was also reported by Przemyslaw Malkowski from Percona. It ended up as a verified feature request, but not everyone in community is happy with this. Let me quote:
  

  "[30 Aug 8:08] Jean-François Gagné
  Could we know what was the original severity of this bug as reported by Przemyslaw ? This is now hidden as it has been reclassified as S4 (Feature Request).
  
  From my point of view, this is actually a bug, not a feature request and it should be classified as S2. A perfectly working application would break for no reason when a temporary table does not have a Primary Key, so this is actually a big hurdle for using sql_require_primary_key, hence serious bug in the implementation of this otherwise very nice and useful feature."

That's all about bugs I've subscribed to in summer.

Winter is coming, so why not to remember nice warm sunny days and interesting MySQL bugs reported back then.

To summarize:

1. We still see some strange "games" played during bugs processing and trend to decrease severity of reports. I think this is a waste of time for both Oracle engineers and community bug reporters.
2. I am still not sure if Oracle's QA does not use ASan or just ignore problems reported for MTR test cases. Anyway, Percona engineers do this for them, and report related bugs :)
3. dbdeployer and sysbench are really popular among MySQL bug reporters recently!
4. Importing of InnoDB tablespaces is broken in MySQL 8.0.17+ at least.
5. There are many interesting MySQL bugs reported during last 3 months, so I epxect more posts in this series soon.

Time in Performance Schema

I've seen questions like this:

"Is there a way to know when (date and time) the last statement captured in ... was actually ran?"

more than once in some discussions (and customer issues) related to Performance Schema. I've seen answers provided by my colleagues and me after some limited testing. I've also noticed statements that it may not be possible.

Indeed, examples of wall clock date and time in the output of queries from the performance_schema are rare (and usually come from tables in the information_schema.  sys.format_time() function converts time to a nice, human readable format, but it still remains relative - it is not a date and time when something recorded in performance_schema happened.

In this post I'd like to document the answer I've seen and have in mind (and steps to get it) here, to save time for readers and myself faced with similar questions in the future. I'll also show the problem with this answer that I've noticed after testing for more than few minutes.

Let's start with simple setup of testing environment. In my case it is good old MariaDB 10.3.7 running on this netbook under Windows. First, let's check if Performance Schema is enabled:

 MariaDB [test]> select version(), @@performance_schema;
+--------------------+----------------------+
| version()          | @@performance_schema |
+--------------------+----------------------+
| 10.3.7-MariaDB-log |                    1 |
+--------------------+----------------------+
1 row in set (0.233 sec)

Then let's enable recording of time for everything and enable all consumers:

MariaDB [test]> update performance_schema.setup_instruments set enabled='yes', timed='yes';
Query OK, 459 rows affected (0.440 sec)
Rows matched: 707  Changed: 459  Warnings: 0

MariaDB [test]> update performance_schema.setup_consumers set enabled='yes';
Query OK, 8 rows affected (0.027 sec)
Rows matched: 12  Changed: 8  Warnings: 0

Now we can expect recently executed statements to be recorded, like this:

MariaDB [test]> select now(), event_id, timer_start, timer_end, sql_text from performance_schema.events_statements_current\G*************************** 1. row ***************************
      now(): 2019-11-03 17:42:51
   event_id: 46
timer_start: 22468653162059216
  timer_end: 22468697203533224
   sql_text: select now(), event_id, timer_start, timer_end, sql_text from performance_schema.events_statements_current
1 row in set (0.045 sec)

Good, but how we can get a real time when the statement was executed (like now() reports)? We all know from the fine manual that timer_start and timer_end values are in "picoseconds". So we can easily convert them into seconds (or whatever units we prefer):

MariaDB [test]> select now(), event_id, timer_start/1000000000000, sql_text from performance_schema.events_statements_current\G
*************************** 1. row ***************************
                    now(): 2019-11-03 17:54:02
                 event_id: 69
timer_start/1000000000000: 23138.8159
                 sql_text: select now(), event_id, timer_start/1000000000000, sql_text from performance_schema.events_statements_current
1 row in set (0.049 sec)

This value is related startup time one might assume, and we indeed can expect that timer in Performance Schema is initialized at some very early stage of startup. But how to get date and time of server startup with SQL statement?

This also seems to be easy, as we have a global status variable called Uptime measured in seconds. Depending on fork and version used we can get the value of Uptime either from the Performance Schema (in MySQL 5.7+) or from the Information Schema (in MariaDB and older MySQL versions). For example:

MariaDB [test]> select variable_value from information_schema.global_status where variable_name = 'Uptime';
+----------------+
| variable_value |
+----------------+
| 23801          |
+----------------+
1 row in set (0.006 sec)

So, server startup time is easy to get with a date_sub() function:

MariaDB [test]> select @start := date_sub(now(), interval (select variable_value from information_schema.global_status where variable_name = 'Uptime') second) as start;
+----------------------------+
| start                      |
+----------------------------+
| 2019-11-03 11:28:18.000000 |
+----------------------------+
1 row in set (0.007 sec)

In the error log of MariaDB server I see:

2019-11-03 11:28:18 0 [Note] mysqld (mysqld 10.3.7-MariaDB-log) starting as process 5636 ...

So, I am sure the result is correct. Now, if we use date_add() to add timer value converted to seconds, for example to the server startup time, we can get the desired answer, date and time when the statement recorded in performance_schema was really executed:

MariaDB [test]> select event_id, @ts := date_add(@start, interval timer_start/1000000000000 second) as ts, sql_text, now(), timediff(now(), @ts) from performance_schema.events_statements_current\G
*************************** 1. row ***************************
            event_id: 657
                  ts: 2019-11-03 18:24:00.501654
            sql_text: select event_id, @ts := date_add(@start, interval timer_start/1000000000000 second) as ts, sql_text, now(), timediff(now(), @ts) from performance_schema.events_statements_current
               now(): 2019-11-03 18:24:05
timediff(now(), @ts): 00:00:04.498346
1 row in set (0.002 sec)

I was almost ready to publish this blog post a week ago, before paying more attention to the result (that used to be perfectly correct in earlier simple tests) and executing a variation of statement presented above. The problem I noticed is that when Uptime of the server is not just few minutes (as it often happens in quick test environments), but hours or days, timestamp that we get for a recent event from the performance_schema using the approach suggested may differ from current timestamp notably (we see 4.5+ seconds difference highlighted above). Moreover, this difference seem to fluctuate:

MariaDB [test]> select event_id, @ts := date_add(@start, interval timer_start/1000000000000 second) as ts, sql_text, now(), timediff(now(), @ts) from performance_schema.events_statements_current\G
*************************** 1. row ***************************
            event_id: 682
                  ts: 2019-11-03 18:24:01.877763
            sql_text: select event_id, @ts := date_add(@start, interval timer_start/1000000000000 second) as ts, sql_text, now(), timediff(now(), @ts) from performance_schema.events_statements_current
               now(): 2019-11-03 18:24:07
timediff(now(), @ts): 00:00:05.122237
1 row in set (0.002 sec)

and tend to grow with Uptime. This make the entire idea of converting timer_start and timer_end Performance Schema "counters" in "picoseconds" questionable and unreliable for the precise real timestamp matching and comparing with other timestamp information sources in production.

Same as with this photo of sunset at Brighton taken with my Nokia dumb phone back in June, I do not see a clear picture of time measurement in Performance Schema...

After spending some more time thinking about this I decided to involve MySQL team somehow and created the feature request, Bug #97558 - "Add function (like sys.format_time) to convert TIMER_START to timestamp", that ended up quickly "Verified" (so I have small hope that I had not missed anything really obvious - correct me if I am wrong). I'd be happy to see further comments there and, surely, the function I asked about implemented. But I feel there is some internal problem with this and some new feature at server side may be needed to take the "drift" of time in Performance Schema into account.

There is also a list of currently open questions that I may try to answer in followup posts:

1. Is the problem of time drift I noticed a MariaDB 10.3.7-specific, or recent MySQL 5..x and 8.0.x are also affected?
2. Is this difference in time growing monotonically with time or really fluctuating?
3. When exactly Performance Schema time "counter" starts, where is it in the code?
4. Are there any other, better or at least more precise and reliable ways to get timestamps of some specific events that happen during MySQL server work? I truly suspect that gdb and especially dynamic tracing on Linux with tools like bpftrace may give us more reliable results...

Fun with Bugs #88 - On MySQL Bug Reports I am Subscribed to, Part XXII

It's Saturday night. I have a week of vacation ahead that I am going to spend at home, working on things I usually do not have time for. I already did something useful (created a couple of test cases for MariaDB bugs here and there, among other things), so why not to get some fun and continue my previous review of recent interesting MySQL bug report...

So, here is the list of 15 more MySQL community bug reports that I've subscribed to back in May and June 2019:

• Bug #95491 - "The unused wake up in simulator AIO of function reserve_slot". I am impressed by the fact that these days many people just study InnoDB code, find problematic parts, improve them, test and prove the improvement, and submit patches to Oracle. Chen Zongzhi, who reported this bug, is one of such great people.
• Bug #95496 - "Replication of memory tables". As pointed out by Iwo P, MySQL manual describes what happens to MEMORY tables replicated from master to slave in details, including the case of master restart. But one detail is missing - when slave with MEMORY tables restarts, even with super_read_only set, it also generates even to delete data from the table into its own binary log, with GTID, and this leads to problems in case of failover.
• Bug #95507 - "innodb_stats_method is not honored when innodb_stats_persistent=ON". This bug was reported by my colleague Sergei Petrunia. See also related MDEV-19574  (still "Open").
• Bug #95547 - "MySQL restarts automatically after ABORT_SERVER due to disk full". I am not sure this is a replication bug, more a problem of too primitive mysqld_safe or systemd startup scripts that do not check free space on disk or reason why mysqld process committed suicide. Anyway, this report by Przemyslaw Malkowski from Percona was accepted as a replication bug.
• Bug #95582 - "DDL using bulk load is very slow under long flush_list". As found and proved by Fungo Wang, due to optimized away redo log writing for bulk load, to make sure such DDL is crash safe, dirtied pages must be flushed to disk synchronously. Currently this is done inefficiently, as shown in this bug report. See also a year old (and still "Open") Bug #92099 - "provide an option to record redo log for bulk loading" from the same reporter, where he asks for a way to get redo logging back (but still benefit from other "online" ALTER features). MariaDB has that option after the fix of MDEV-16809.
• Bug #95612 - "replication breakage due to Can not lock user management caches for processing". Simon Mudd noted that in some cases simple DROP USER IF EXISTS ... breaks parallel replication in MySQL 8.0.16. Good to know that START SLAVE allows to continue :)
• Bug #95698 - "JSON columns are returned with binary charset". This bug was reported by Daniel Book. Note that according to the original issue, DBD::MariaDB (I had no idea this exists!) works properly with MariaDB 10.3 or 10.4. I had not checked.
• Bug #95734 - "Failing SET statement in a stored procedure changes variables". This really weird and unexpected bug was reported by Przemysław Skibiński.
• Bug #95863 - "MTS ordered commit may cause deadlock ". Great finding by Lalit Choudhary from Percona!
• Bug #95895 - "Shutdown takes long time when table uses AUTOINC and FOREIGN KEY constraints". It was reported by Satya Bodapati from Percona, who later contributed as patch. See also his related Bug #95898 - "InnoDB releases memory of table only on shutdown if table has FK constraint". I truly hope to see these fixed in next minor releases of MySQL. Note that originally bug (PS-5639) was reported by Sveta Smirnova.
• Bug #95928 - "InnoDB crashes on compressed and encrypted table when changing FS block size". Make sure to check and write original filesystem block size somewhere... This bug was reported by Sergei Glushchenko from Percona.
• Bug #95929 - "Duplicate entry for key 'PRIMARY' when querying information_schema.TABLES". This nice feature of new data dictionary of MySQL 8.0.16 under concurrent load was found by Manuel Rigger.
• Bug #95934 - "innodb_flush_log_at_trx_commit = 0 get performance regression on 8 core machine". If I had access to 8 cores box, I'd try to check this sysbench-based complete test case immediately. I should do this on my quadcore box at least, next week. But here we see some 12 days just wasted and then bug report from Chen Zongzh ended up "Analyzing", till today, with no further feedback.
• Bug #95954 - "CAST of negative function return value to UNSIGNED malfunctions with BIGINT". Yet another weird bug found by Manuel Rigger. There are patches (separate for 5.6, 5.7 and 8.0) contributed by Oleksandr Peresypkin.

Montjuïc is a place to enjoy nice view of Barcelona and forget about MySQL bugs. So, I could miss some interesting ones at the end of May, 2019...

To summarize:

1. Percona engineers still contribute a lot of useful and interesting MySQL bug reports, often - with patches.
2. I am impressed by quality of MySQL bug reports in 2019. Often we see detailed analysis, proper complete test cases and patches.
3. There is a recent fork of DBD::mysql for MariaDB (and MySQL), DBD::MariaDB!
4. I badly need some new hardware for running benchmarks and tests from recent bug reports...
5. I should spend more time in May in Barcelona :)