Saturday, November 24, 2018

Fun with Bugs #73 - On MySQL Bug Reports I am Subscribed to, Part X

It's time to continue my review of MySQL bug reports that I considered interesting for some reason recently. I had not got any notable reaction from Oracle engineers to my previous post about recent regression bugs in MySQL 8.0.13, so probably this topic is not really that hot. In this boring post I'll just review some bugs I've subscribed to since August that are still not closed, starting from the oldest.

Let me start with a couple of bug reports that remain "Open":
  • Bug #91959 - "UBSAN: signed integer overflow in lock_update_trx_age". It's really unusual to see bug reported by Shane Bester himself just "Open" for months. As he noted, it's really strange to see age defined as 32-bit value here when age_updated two lines later is 64-bit. On the other hand, from comments in the code it seems the age value is supposed to be small enough and if it grows too much this is a problem.
  • Bug #92964 - "Slave performance degrades over time". As description says:
    "When mysql slave is running gtid based asynchronous multi threaded replication, performance degrades over time in significant way when session_track_gtids is set to OWN_GTID; "
    The bug is assigned, but there are no public comments, while the way to reproduce seem to be defined clearly.
Same as the weather at Seven Sisters Cliffs back on September 14, 2018, some recent MySQL bug reports do not add confidence to those few visitors who explore them...

Now let me continue with some "Verified" bugs:
  • Bug #91981 - "Inconsistent user@host definitions for definer/grantor/grantee columns". It seems consistency is not a high priority these days.
  • Bug #92020 - "Introduce new SQL mode rejecting queries with results depending on query plan". I already mentioned this report while reviewing recent MySQL optimizer bugs. I have nothing to add to my conclusion there:
    "So, current MySQL considers different results when different execution plans are used normal!"
  • Bug #92032 - "Restrict usage of session foreign_key_checks". It's a great request from Federico Razzoli and I am happy to see it verified as a bug, fast.
  • Bug #92093 - "Replication crash safety needs relay_log_recovery even with GTID." Recently Jean-François Gagné pays special attention to MySQL replication crash safety in his bug reports, talks and blog posts. This specific report ended up as a request for a more clear documentation that should not provoke (false) safety feelings. See also his Bug #92109 - "Please make replication crash safe with GTID and less durable setting (bis)." for the request to improve MySQL in this regard one day.
  • Bug #92131 - "ASan: Direct leak of 272 byte(s) in main.mysqlpump_partial_bkp MTR test case". This kind of reports make me nervous. They make me think that still there is no regular testing of ASan-enabled builds in Oracle. Lucky we are, Percona engineers (like Yura Sorokin) do this for Oracle.
  • Bug #92133 - "DICT_SYS mutex contention causes complete stall when running with 40 mill tables". Proper data dictionary in MySQL 8.0 was supposed to solve some performance problems for instances with a lot of tables. As this perfect bug report from Alexander Rubin shows, this is not yet the case. We can still push MySQL instance over the limits and single instance for thousands of different small databases is hardly usable.
  • Bug #92209 - "AVG(YEAR(datetime_field)) makes an error result because of overflow". Perfect analysis from Pin Lin. All recent MySQL (and MariaDB) versions are affected.
  • Bug #92252 - "Mysql generates gtid gaps because of option slave-skip-errors". Yet another bug report from Pin Lin
  • Bug #92364 - "events_transactions_summary_global_by_event_name not working as expected". This bug was reported by Przemyslaw Malkowski from Percona. We all know Performance Schema is near perfect, but it turned out that "...performance_schema.events_transactions_summary_global_by_event_name behavior for instrumenting transactions seems completely broken.".
  • Bug #92398 - "point in time recovery using mysqlbinlog broken with temporary table -> errors". Let me quote Shane Bester:
    "Running a multi-threaded workload involving temporary and non-temporary tables leads to binary log playback giving errors."
  • Bug #92421 - "Queries with views and operations over local variables don't use indexes". This is a kind of regression in MySQL 5.7. MySQL 8 allows to workaround the problem properly using ROW_NUMBER() function.
  • Bug #92540 - "Comparison between DATE and DECIMAL does not round nanoseconds". This bug was reported by Alexander Barkov from MariaDB. MariaDB 10.3.x is not affected based on my tests.
To summarize:
  1. It seems ASan-enabled builds are not tested by Oracle engineers on a regular basis.
  2. Percona engineers help a lot with regular MySQL QA and testing for various extreme use cases.
  3. There is a lot to do to make GTID-based replication crash-safe and working fast in common use cases.
 More bug review are coming soon, so stay tuned.

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