Many people consider this information in the MySQL Query Execution Plan (QEP) to indicate that the referenced table is using an index. It actually means that ONLY the index is used. This can for larger and more frequent queries provide a significant boost.

In a recent example, adding an index dropped a query from 190ms to 6ms. However adding a better index dropped that 6ms query to 1.2ms. When executed 100s/1000s of times per second these millisecond improvements can have a huge benefit in greater scalability. While people often tune slow running queries, in a well tuned system shaving milliseconds of queries, in this example making 6ms query 80% better is a far greater improvement.

You can get a detailed explanation of how to identify, create and verify covering indexes from my Percona Live presentation Improving performance …

While most people look at performance optimizations for SELECT statements, UPDATE and DELETE statements are often overlooked. These can benefit from the principles of analyzing the Query Execution Plan (QEP). You can only run an EXPLAIN on a SELECT statement, however it’s possible to rewrite an UPDATE or DELETE statement to perform like a SELECT statement.

To optimize an UPDATE, look at the WHERE clause. If you are using the PRIMARY KEY, no further analysis is necessary. If you are not, it is of benefit to rewrite your UPDATE statement as a SELECT statement and obtain a QEP as previously detailed to ensure optimal indexes are used. For example:

UPDATE t
SET     c1 = ‘x’, c2 = ‘y’, c3 = 100
WHERE c1 = ‘x’
AND     d = CURDATE()

You can rewrite this UPDATE statement as a SELECT statement for using EXPLAIN:

EXPLAIN SELECT c1, c2, c3 FROM  t WHERE c1 = ‘x’ AND    d = CURDATE()

You should …

I love cool toys, don’t you? Especially when they help me optimize ugly, nasty, evil looking queries :)
I use mk-visual explain to help me understand better what the h*ll is happening on my MySQL servers and how they are being butchered, but today I discovered a small yet neat tool, which I think might have been overlooked. It uses mk-visual-explain, but does it visually, enabling you to open and close sections of the explain tree structure. I’m talking about http://explain.plosquare.com/.

Thank you Jan Ploski

Because Sharing is Caring

When examining the execution plan of troublesome queries in MySQL, most users are aware of using EXPLAIN. However, an often overlooked, yet very helpful extension of EXPLAIN, is EXPLAIN EXTENDED coupled with the SHOW WARNINGS command.

The reason being is because it provides a little more information about how the optimizer processes the query, and thus it could help to quickly identify a problem that you might not otherwise recognize with just EXPLAIN.

For instance, here is a common query which could be inefficient:

SELECT id FROM t WHERE id='1';

And here is the CREATE TABLE output:

mysql> show create table tG
*************************** 1. row …

Today at Kaleidoscope I will be doing a 90-minute session comparing MySQL’s SQL syntax to the ANSI/ISO SQL:2003 standard, entitled What Do You Mean, “SQL Syntax Error”?

You can download the PDF slides now.

For those that may be following along the presentation later today (4 pm Eastern time), here are some links that I may throw out during the session:

• SQL 2003 standard – actually it is “Information taken from the Final Committee Draft (FCD) of ISO/IEC 9075-2:2003″ but it’s extremely close to the actual standard. The actual standard is a document that costs a non-trivial amount of money to get, and cannot be republished.
  
• …

The slides for my presentation at FOSDEM 2010 are now available online at slideshare. In this presentation I describe a successful client implementation with the result of 10x performance improvements. My presentation covers monitoring, reviewing and analyzing SQL, the art of indexes, improving SQL, storage engines and caching.

The end result was a page load improvement from 700+ms load time to a a consistent 60ms.

10x Performance Improvements – A Case Study View more presentations from Ronald Bradford.

This Thursday (February 4th, 14:00 UTC), Morgan Tocker will talk about Optimizing Queries with Explain. Morgan was a technical instructor at MySQL and works for Percona today.

For MySQL University sessions, point your browser to this page. You need a browser with a working Flash plugin. You may register for a Dimdim account, but you don't have to. (Dimdim is the conferencing system we're using for MySQL University sessions. It provides integrated voice streaming, chat, whiteboard, session recording, and more.) All MySQL University sessions are recorded, that is, slides and voice can be viewed as a Flash movie (.flv). You can find those recordings on the respective MySQL …

This Thursday (February 4th, 14:00 UTC), Morgan Tocker will talk about Optimizing Queries with Explain. Morgan was a technical instructor at MySQL and works for Percona today.

For MySQL University sessions, point your browser to this page. You need a browser with a working Flash plugin. You may register for a Dimdim account, but you don't have to. (Dimdim is the conferencing system we're using for MySQL University sessions. It provides integrated voice streaming, chat, whiteboard, session recording, and more.) All MySQL University sessions are recorded, that is, slides and voice can be viewed as a Flash movie (.flv). You can find those recordings on the respective MySQL …

This Thursday (February 4th, 14:00 UTC), Morgan Tocker will talk about Optimizing Queries with Explain. Morgan was a technical instructor at MySQL and works for Percona today.

For MySQL University sessions, point your browser to this page. You need a browser with a working Flash plugin. You may register for a Dimdim account, but you don't have to. (Dimdim is the conferencing system we're using for MySQL University sessions. It provides integrated voice streaming, chat, whiteboard, session recording, and more.) All MySQL University sessions are recorded, that is, slides and voice can be viewed as a Flash movie (.flv). You can find those recordings on the respective MySQL …

Here’s a quick tip for using explain: You may know this already, but mysql will actually execute some subqueries when you invoke explain.  Here’s an example: mysql> explain select id from projects where id = (select max(id) from projects where name like 'en%'); +----+-------------+----------+-------+---------------+---------+---------+-------+-------+-------------+ | id | select_type | table | type | possible_keys | … Continue reading how to prevent explain from executing subqueries →

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