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Displaying posts with tag: optimizer (reset)
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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MySQL Optimizer: Naughty Aberrations on Queries Combining WHERE, ORDER BY and LIMIT

Sometimes, the MySQL Optimizer chooses a wrong plan, and a query that should execute in less than 0.1 second ends-up running for 12 minutes !  This is not a new problem: bugs about this can be traced back to 2014, and a blog post on this subject was published in 2015.  But even if this is old news, because this problem recently came yet again to my attention, and because this is still not fixed in MySQL 5.7 and 8.0, this is a subject worth writing about.

The MySQL Optimizer

Before looking at the problematic query, we have to say a few words about the optimizer.  The Query Optimizer is the part of query execution that chooses the query plan.  A Query Execution Plan is the way MySQL chooses to execute a specific query.  It …

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Fun with Bugs #86 - On Some Public Bugs Fixed in MySQL 5.7.27

This week new minor releases of MySQL server were announced. Of them all I am mostly interested in MySQL 5.7.27. I plan to concentrate on InnoDB, replication and optimizer bugs reported in public MySQL bugs database and fixed in MySQL 5.7.27. As usual I name bug reporters explicitly and give links to their remaining currently active bug reports, if any.

Let me start with InnoDB bug fixes:

  • Bug #94699 - "Mysql deadlock and bugcheck on aarch64 under stress test". This bug report with a fix for insufficient memory barriers in the rw-lock implementation was contributed by Cai Yibo.
  • Bug #93708 - "Page Cleaner will sleep for long …
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Supporting all kinds of outer references in derived tables (lateral, or not)

(Image credit: Pixabay).

In my earlier post, I showed how MySQL, since version 8.0.14, has support for LATERAL derived tables. With LATERAL, a JOIN can have a second table – a subquery-based derived table – be defined based on values from columns of the first table, and thus be re-calculated for each row of the first table.…

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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 …

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Fun with Bugs #85 - On MySQL Bug Reports I am Subscribed to, Part XX

We have a public holiday here today and it's raining outside for a third day in a row already, so I hardly have anything better to do than writing yet another review of public MySQL bug reports that I've subscribed to recently.

Not sure if these reviews are really considered useful by anyone but few of my readers, but I am still going to try in a hope to end up with some useful conclusions. Last time I've stopped on Bug #94903, so let me continue with the next bug in my list:

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Fun with Bugs #84 - On Some Public Bugs Fixed in MySQL 5.7.26

Oracle released minor MySQL Server versions in all supported branches on April 25, 2019. MySQL 5.7.26 is just one of them, but recently I prefer to ignore MySQL 8 releases (after checking that I can build them from source code at least somewhere, even if it takes 18G+ of disk space and that they work in basic tests), as there are more chances for MySQL 5.7 bug fixes to affect me (and customers I care about) directly.

So, in this yet another boring blog post (that would never be a reason for any award) I plan to concentrate …

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Fun with Bugs #83 - On MySQL Bug Reports I am Subscribed to, Part XIX

I have not much yet to say on a popular topic of upgrading everything to MySQL 8, so let me just continue reviewing public MySQL bug reports that I've subscribed to recently. After my previous post at least one bug, Bug #94747, got enough comments and clarifications (up to specific commit that introduced this regression pointed out by Daniel Black!) to have it re-classified and verified as InnoDB code bug. So, I see good reasons to continue attracting wide public attention …

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