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Displaying posts with tag: optimizer (reset)
Fun with Bugs #57 - On MySQL Bug Reports I am Subscribed to, Part I

I've decided to stop reviewing MySQL Release Notes in this series, but it does not mean that I am not interested in MySQL bugs any more. At the moment I am subscribed to 91 active MySQL bugs reported by other MySQL users, and in this blog post I am going to present 15 of them, the most recently reported ones. I'd really want to see them fixed or at least properly processed as soon as possible.

In some cases I am going to add my speculations on how the bug had better be handled, or maybe highlight some important details about it. It is not my job any more to process/"verify" any community bug reports for any kind of MySQL, but I did that for many years and I've spent more than 5 years "on the other side", being a member of Community, so in some cases I let myself to share some strong opinion on what may be done …

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Histogram statistics in MySQL

As of MySQL 8.0.3, you now have the ability to create histogram statistics in order to provide more statistics to the optimizer. In this blog post, we will have a look at how you can create histogram statistics, and we will explain when it might be useful to have histogram statistics.…

MySQL Support Engineer's Chronicles, Issue #8

This week is special and full of anniversaries for me. This week 5 years ago I left Oracle behind and joined Percona... Same week 5 years ago I had written something about MySQL in this blog for the first time in my life. 5 years ago I've created my Facebook account that I actively (ab)use for discussing work-related issues. So, to summarize, it's a five years anniversary of my coming out as a MySQL Entomologist, somebody who writes and speaks about MySQL and bugs in MySQL in public! These 5 years were mostly awesome.

I decided to celebrate with yet another post in this relatively new series and summarize in short what interesting things I studied, …

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Fun with Bugs #55 - On Some Public Bugs Fixed in MySQL 8.0.2

I do not care much about MySQL 8.0.x at the moment, as it's far from being GA and is work in progress. It is not yet used by customers whom I have to support. But I know about many interesting changes and improvements there that, eventually, are going to influence all main forks and kinds of MySQL. So, it would not be wise to ignore MySQL 8.0.c entirely even for me.

For this post I decided to briefly check what community reported bugs were fixed in the recent release, 8.0.2, based on release notes. For me it's a measure of community interest in MySQL 8.0.x and Oracle's interest in further working with MySQL Community. I ended up with the following, short enough list of bug fixes in the categories I usually care about (InnoDB, partitioning, replication and optimizer):

  • The very first InnoDB bug mentioned in …
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Fun with Bugs #54 - On Some Bugs Fixed in MySQL 5.7.19

More than 3 months after 5.7.18 we' ve got MySQL 5.7.19 released recently. This is my quick review of the release notes with interesting fixed bug (reported in public) highlighted in the areas I am usually interested in.

Let's start with InnoDB. The following bug fixes attracted my attention:

  • Bug #85043 is still private. You know how much I hate those. At least we can see it was about the fact that "The server allocated memory unnecessarily for an operation that rebuilt the table." Let's hope this is no longer the case.
  • Bug …
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Fun with Bugs #52 - On Some Bugs Fixed in MySQL 5.7.18

I had not reviewed MySQL 5.7 release notes for quite a some time in this series. Last time I checked it was MySQL 5.7.15. So, as soon as I noted new release, 5.7.18, I decided to check the release notes for any interesting fixed bug (reported by Community users in public) in the areas I am interested in: InnoDB, replication, optimizer and few others.

Note that recently most of the bugs fixed are internal only, found by Oracle engineers that never cared (or are not allowed, who knows) to report them in public, so this blog post is not even remotely a full review of what's fixed in MySQL 5.7.18 and is not a replacement for reading the detailed …

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Fun with Bugs #51 - My Bug Reports that Oracle doesn't Want to Fix

This week I noticed (yet another) customer issue related to the output produced by mysqladmin debug command (or when mysqld process gets SIGHUP signal). I mean the output generated by the mysql_print_status() function. In this issue the content of the output was misinterpreted. I've seen this in the past more than once, and requested to document the output properly, but it never happened for a reason that there is an internal feature request to put this information elsewhere, in Performance Schema or Information Schema. The bug ended up with "Won't fix" status.

Surely I complained in a comment and on Facebook, and then …

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Fun with Bugs #50 - On Bugs Tagged as "missing manual"

Back in January 2014, some time after many nice people kindly asked me to shut up stop writing about MySQL bugs on Facebook several times per day, I decided to start reading the fine MySQL Manual more carefully than before and report not only typos there, but also any topic or detail not properly explained. Usually these reports, tagged as "missing manual", were the result of careful study of the documentation based on real user question or customer issue. So, most of these reports came from real life, and missing information badly affected poor MySQL users.

Today, for this issue #50 in my series of posts about MySQL bugs, I decided to list and …

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When order of appearance of indexes matters in MySQL

Sometimes MySQL surprises you in ways you would have never imagined.

Would you think that the order in which the indexes appear in a table matters?
It does. Mind you, not the order of the columns - the order of the indexes.
MySQL optimizer can, in specific circumstances, take different paths, sometimes with nefarious effects.


Please consider the following table:

CREATE TABLE `mypartitionedtable ` (
  `HASH_ID` char(64) NOT NULL,
  `RAW_DATA` mediumblob NOT NULL,
  `EXPIRE_DATE` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
  KEY `EXPIRE_DATE_IX` (`EXPIRE_DATE`),
  KEY `HASH_ID_IX` (`HASH_ID`)
) ENGINE=TokuDB DEFAULT CHARSET=latin1 ROW_FORMAT=TOKUDB_UNCOMPRESSED
/*!50100 PARTITION BY RANGE (UNIX_TIMESTAMP(EXPIRE_DATE))
(PARTITION p2005 VALUES LESS THAN (1487847600) ENGINE = …

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Fun with Bugs #47 - On Some Bugs Found Using oprofile

Users had to identify the reasons for MySQL performance problems somehow well before famous Performance Schema appeared in MySQL 5.5.3, and even before Domas invented his Poor Man's Profiler. Poor users had to use some OS-level tools, and among these tools the most important one was, probably, oprofile.

oprofile is a system-wide statistical profiling tool for Linux available since 2001 and 2.4.x kernels. It was applied to MySQL for many …

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