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Displaying posts with tag: optimizer (reset)
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
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Fun with Bugs #95 - On MySQL Bug Reports I am Subscribed to, Part XXIX

With conferences cancelled or postponed and people forced to stay at home due to COVID-19 wide spreading, what can be better than to give my readers a new list of MySQL bugs to check? Useful reading should help! So today I continue my previous post with a quick review of bugs I've subscribed to in February, 2020, while things were still going as usual for most of us...

Here is the list of InnoDB, replication, optimizer and some other bugs to note while working with recent MySQL 5.7.x and 8.0.19 releases:

  • Bug #98473 - "group replication will be block after lock table". This problem report by
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A must-know about NOT IN in SQL – more antijoin optimization

I will try to make it short and clear: if you are writing SQL queries with “NOT IN” like
SELECT … WHERE x NOT IN (SELECT y FROM …)
you have to be sure to first understand what happens when “x” or “y” are NULL: it might not be what you want!…

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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
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A new, simple way to figure out why your recursive CTE is running away

In MySQL 8.0.1, we introduced support for recursive common table expressions (CTE). There are quite a few blog entries showcasing the feature, starting from this one, and there is also a complete documentation. Today, I would like to present a solution to a problem which nearly everybody meets when writing queries with recursive CTE’s: when infinite recursion happens, how to debug ?…

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Antijoin in MySQL 8

In MySQL 8.0.17, we made an observation in the well-known TPC-H benchmark for one particular query. The query was executing 20% faster than in MySQL 8.0.16. This improvement is because of the “antijoin” optimization which I implemented. Here is its short mention in the release notes:

“The optimizer now transforms a WHERE condition having NOT IN (subquery), NOT EXISTS (subquery), IN (subquery) IS NOT TRUE, or EXISTS (subquery) IS NOT TRUE internally into an antijoin, thus removing the subquery.”

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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 …
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Hash join in MySQL 8

For a long time, the only algorithm for executing a join in MySQL has been variations of the nested loop algorithm. With the release of MySQL 8.0.18, the server can now execute joins using hash join. This blog post will have a look at how it works, when it is used, and how it compares to the old join algorithms in MySQL in terms of performance.…

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MySQL EXPLAIN ANALYZE

MySQL 8.0.18 was just released, and it contains a brand new feature to analyze and understand how queries are executed: EXPLAIN ANALYZE.

What is it?

EXPLAIN ANALYZE is a profiling tool for your queries that will show you where MySQL spends time on your query and why.…

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Fun with Bugs #89 - On MySQL Bug Reports I am Subscribed to, Part XXIII

I have to celebrate the anniversary of my last day in Oracle (that was 7 years ago!) somehow, and I think writing yet another blog post about Oracle MySQL bugs is a good way to do this. I am actually surprised (and happy) that public bugs database is still alive, maintained and considered important in Oracle, and I know who in Oracle was working hard all these years for this to happen!

In my previous post in this series I've stopped on Bug #95954 and had not completed review of interesting MySQL bug reports that I've subscribed to in June 2019. So, below I start with the next bug in my list, complete review for June and cover some bugs reported in July. There were many.

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