Saturday, May 30, 2020

Fun with Bugs #99 - On MySQL Bug Reports I am Subscribed to, Part XXXIII

In my previous post in this series I've commented on some interesting MySQL bug reports that were added during the second half of April. Time to move on to bugs reported in May, 2020, as we are quickly approaching MySQL Bug #100000 soon and I want to write a separate post for this great achievement :)

Here is the list:
  • Bug #99432 - "Improving memory barrier during rseg allocation". Nice contribution by my former colleague in Percona, Krunal Bauskar, who now works on making MySQL better for ARM processors. According to his findings, the use of a relaxed memory model improves performance on ARM by up to 2%. See also yet another bug report with a contribution that matters for ARM, Bug #99556 - "Avoid sequentially consistent atomics for atomic counters" (contributed by Sergey Glushchenko from Percona).
  • Bug #99444 - "New HASH JOIN order problem". One should not expect and rely on any specific order unless explicit ORDER BY is used, so formally this report by Gabor Aron is "Not a Bug". I put it into this list as several other community members helped him a lot in understanding why results with HASH_JOIN optimization in newer versions are still valid and what are the ways to get the results with the desired ordering efficiently. Guilhem Bichot, for instance, suggested two different ways, using window function and lateral table. Useful reading in any case!
  • Bug #99458 - "i_s_fts_index_cache_fill_one_index() is not protect by the lock". Looks like even crashes are possible as a result, based on comments. Nice finding by Haiqing Sun.
  • Bug #99459 - "SQL run with GROUP_MIN_MAX may infinite loop and never return". After some discussion around the validity and severity of bug reports where test case involved adding DEBUG_SYNC() to show the problem in a predictable way, this great bug report by Ze Yang was verified. All MySQL GA versions are affected, including 8.0.20! As a side note, I'd prefer NOT to read such discussions any more. They are wasting time of all parties involved.
  • Bug #99499 - "Incorrect result when constant equailty expression is used in LEFT JOIN condition". This bug that affects MySQL 5.7.x only (it was fixed in MySQL 8.0.17+ and in 5.6 code was different) was reported by Marcos Albe from Percona. 
  • Bug #99504 - "Generated column incorrect on INSERT when based on column w/ expression DEFAULT". Several problems are highlighted in the complex enough test case submitted by Brad Lanier.
  • Bug #99582 - "Reduce logging of new doublewrite buffer initialization which is confusing". 180 lines or so are added when --log-error-verbosity is set to 3. As a workaround one can add:
    to the [mysqld] section of the .cnf file. This problem was reported by Simon Mudd. Make sure to read all comments.
  • Bug #99591 - "Option --tc-heuristic-recover documentation wrong, missing details". In reality it does not work with more than one XA-capable engine installed. I wish fine manual documents the reality, not the good intentions of the past. This documentation request was added by Sami Ahlroos.
  • Bug #99593 - "Performance issues in 8.0.20". It seems to be yet another TempTable engine problem that caused regression comparing to MySQL 5.7. At least this:

    • SET GLOBAL internal_tmp_mem_storage_engine=MEMORY;

    is a workaround. The bug (a duplicate of internal Bug #30562964) was reported by billy noah and is fixed in upcoming MySQL 8.0.21.
  • Bug #99601 - "Broken Performance using EXIST function, increasing execution time each loop". This regression bug (without tag, but who cares...) in MySQL 8.0 was reported by Ronny Görner and minimal test case demonstrating that the problem is actually with function call was contributed by Shane Bester.
  • Bug #99643 - "innobase_commit_by_xid/innobase_rollback_by_xid is not thread safe". This bug was reported by Zhai Weixiang, who had also suggested the fix in the code.
  • Bug #99717 - "Performance regression of parallel count". Great bug report with code analysis and ready to use MTR test case from Ze Yang. Sunny Bains already confirmed that the problematic code is going to be removed.

To summarize:
  1. I am happy to see Oracle engineers explaining community bug reporters the reasons and possible solutions for the problems they hit that are not actually caused by any bug in MySQL. I tried to do this as well, whenever possible, while working on MySQL bugs...
  2. We can still find speculations that if the bug is repeatable only by adding DEBUG_SYNC() or similar debug lines, then it can not be verified or gets lower severity... IMHO this is nonsense, as there are many high severity verified real bug reports where this method is used to demonstrate the problem clearly. Just stop it!

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