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10 Newer Entries Showing entries 31 to 40 of 74 10 Older Entries

Displaying posts with tag: explain (reset)

Discover MyXplain, another way to learn, search, find and stroll
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I’m very glad to announce you that MyXplain is released today.

This project is the result of many weeks of work for my friend Max and I and we are obviously very proud to present it.

Please take a time to browse it and find all the surprises that awaiting you.

We wanted to make an unique place, a gathering place where you can find all you want to know about the most useful MySQL commands.
We started with the explain

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Covering or not covering, that is the question
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I’m currently working on a new project about the explain command output.
For my research I’ve read a lot of documentations, presentations and blog posts and I would like to focus on the join type in the explain output.
And in particular when type=index in this explain output.

Take a look at what says the official documentation about type=index :

Wait a second, only the index tree is scanned? really?
This description seems to be the definition of a



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Index merge annoyances fixed in MySQL 5.6
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While the index merge access types certainly are useful for a number of queries, there has been some frustration expressed both from customers and the community about how it...

  1. is not used when it should have been
  2. is used when ref access is obviously better
  3. merges suboptimal indexes
  4. is too restricted in which conditions can be used

I could come up with numerous examples of related bugs and feature requests dating back more than six years. To list a few: …


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Recent Presentations Buenos Aires MySQL/NoSQL/Cloud Conference
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The first annual Latin America MySQL/NoSQL/Cloud Conference was held in Buenos Aires Argentina from June 26-28. Kudos to Santiago Lertora from Binlogic who had the vision for the conference in his country and made it happen. I look forward to the second annual event.

My first presentation was “Improving Performance with Better Indexes”. This presentation details the six steps to SQL performance analysis, Capture, Identify, Confirm, Analyze, Optimize and Verify. An explanation of MySQL EXPLAIN, and working examples to create …

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Index Condition Pushdown to the rescue!
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A while ago, I explained how range access in a multiple-part index works and why MySQL can't utilize key parts beyond the first occurrence of some often used comparison operators. Luckily, there is a great improvement underway in MySQL 5.6 that will remedy much of this limitation. Meet Index Condition Pushdown.

How does ICP work?

Index Condition Pushdown is a new way for MySQL to evaluate conditions. Instead of evaluating conditions on rows read from a table, ICP makes it possible to evaluate …



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Understanding the unique_subquery optimization
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If you use the EXPLAIN SELECT statement to see how your subqueries are treated by MySQL, you may sometimes meet the "unique_subquery" optimization. Here is how the manual describes it:
"unique_subquery: this type replaces ref for some IN subqueries of the following form: value IN (SELECT primary_key FROM single_table WHERE some_expr); unique_subquery is just an index lookup function that replaces the subquery completely for better efficiency".Few weeks ago, while I was reviewing a patch fixing a …

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Explaining MySQL's EXPLAIN
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The MySQL explain plan is a great tool to help developers and database administrators to improve the performance of specific queries happening against a database. It is very easy to use, but its output can be confusing, so I thought I'd show a very simple example.

Let's begin by looking at a table called recipes

mysql> desc recipes;
+-------------+--------------+------+-----+---------+----------------+
| Field       | Type         | Null | Key | Default | Extra          |
+-------------+--------------+------+-----+---------+----------------+
| id          | int(11)      | NO   | PRI | NULL    | auto_increment |
| name        | varchar(400) | …
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Added a Table of Contents
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Not a big deal, but I just added a “Table of Contents” page to my blog to make finding older articles much easier.

I noticed most of my posts are quite lengthy, and it can take a bit of searching/clicking to find an older entry. So unless you happen to recall the ‘month/year’ it was published, which I don’t even remember that, then hopefully this will help.

Really simple, and looks just like this:

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When EXPLAIN estimates can go wrong!
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This is the title of my first blog post on MySQL Performance Blog. It deals with a customer case where the customer was facing a peculiar problem where the rows column in the EXPLAIN output of the query was totally off. The actual number of rows was 18 times more than the number of rows reported by MySQL in the output of EXPLAIN. Now this can be a real pain as MySQL uses “the number of rows” estimation to pick and choose indexes and it could really be picking up a wrong index simply because of the wrong estimate. You...

The post …

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Optimizer tracing: how to configure it
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In this blog post, my colleague Jørgen Løland described a new feature of MySQL 5.6: Optimizer Tracing. I recommend reading his article, as it presents this new feature in a simple, easy-to-read manner.

The Optimizer Tracing feature can help understanding what the Optimizer is doing; it is available since milestone 5.6.3, announced October 3rd at Oracle Open World (here is the changelog). It's good to see it mature now; I …

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10 Newer Entries Showing entries 31 to 40 of 74 10 Older Entries

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