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Displaying posts with tag: explain (reset)
Many-table joins in MySQL 5.6

I recently worked on an uncommon slow query: less than 100 rows were read and returned, the whole dataset was fitting in memory but the query took several seconds to run. Long story short: the query was a join involving 21 tables, running on MySQL 5.1. But by default MySQL 5.1 is not good at handling joins with such a large number of tables. The good news is that MySQL 5.6 brings welcome improvements.

Isolating the problem

As always with a slow query, finding the execution plan with EXPLAIN is the 1st step to understand where time is spent. Here the plan was very good with almost all joins using the primary key or a unique key, but perhaps the most interesting part was that EXPLAIN was very slow as well. This indicates that the optimizer takes a lot of time finding the optimal execution plan. To double check, we can run SHOW PROFILE:

mysql> set @@profiling = 1;
mysql> SELECT …
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Unittesting your indexes

During FOSDEM PGDay I watched the "Indexes: The neglected performance all-rounder" talk by Markus Winand. Both his talk and the "SQL Performance Explained" book (which is also available online) are great.

The conclusion of the talk is that we should put more effort in carefully designing indexes. But how can we make sure the indexes are really used now and in the future? We need to write some tests for it.

So I wrote a small Python script to test index usage per query. This uses the JSON explain format available in MySQL 5.6. It's just a proof-of-concept so don't expect too much of it yet (but please sent pull requests!).

A short example:


#!/usr/bin/python3
import indextest

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SHOW EXPLAIN in MariaDB 10.0 vs EXPLAIN FOR CONNECTION in MySQL 5.7

MariaDB 10.0 has SHOW EXPLAIN feature. It was coded by yours truly and first introduced about a year ago. Last release of MySQL 5.7 introduced EXPLAIN FOR CONNECTION, which looks rather similar to MariaDB’s SHOW EXPLAIN. I was wondering how these two compare.

The basics

The usage scenarios are similar. In both cases, you first need a connection id of a running query. It is typically obtained by running SHOW PROCESSLIST:

MariaDB [(none)]> show processlist;
+----+------+-----------+----------+---------+------+--------------+-----------------------------+----------+
| Id | User | Host      | db       | Command | Time | State        | Info                        | Progress | …
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Favorite MySQL 5.6 features: an optimizer perspective

There are so many exciting new features in MySQL 5.6 that I almost don't know where to start. To mention a few, MySQL's multi-core scalability has been significantly improved to meet modern hardware, InnoDB has better index statistics, much better performance, and online ALTER, replication has multi-threaded slaves and …

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Discover MyXplain, another way to learn, search, find and stroll

 

 

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 command because we consider it as one of the most important.
show processlist and show slave status will be the next proposed commands on MyXplain.

The concept is pretty simple, we would like to offer you a new documentation to access all needed …

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Covering or not covering, that is the question

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 covering index.
But a covering index is only characterized by a “Using index” in the extra column of the explain output.

Consider the t1 table, just …

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Index merge annoyances fixed in MySQL 5.6

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: 17673, 30151, 23322, 65274, …

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Recent Presentations Buenos Aires MySQL/NoSQL/Cloud Conference

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 indexes and better covering indexes in several examples are provided. A production example of a 13 table join is used to detail how covering indexes and partial column indexes can make a dramatic improvement in performance. …

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Index Condition Pushdown to the rescue!

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 conditions in the index and thereby avoid looking at the table if the condition is false.

Let's assume that we have a multiple-part index covering columns (keypart_1, ..., keypart_n). Further assume that we have a condition with a comparison operator on keypart_1 that does not allow …

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Understanding the unique_subquery optimization

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 bug in unique_subquery, I got a "simplification" pulsion. I told myself that:

  •  unique_subquery is an optimization for a special case of simple subqueries (single inner …
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