Another post in the EXPLAIN FORMAT=JSON is Cool! series! In this post, we’ll discuss how the EXPLAIN FORMAT=JSON provides optimization details for 

ORDER BY

 and  

GROUP BY

 operations in conjunction with 

order_by_subqueries

 and  

group_by_subqueries

. 

EXPLAIN FORMAT=JSON

 can print details on how a subquery in

ORDER BY

 is optimized:

mysql> explain format=json select emp_no, concat(first_name, ' ', last_name) f2 from employees order by (select emp_no limit 1)G
*************************** 1. row ***************************
EXPLAIN: { …

The previous post in the EXPLAIN FORMAT=JSON is Cool! series showed an example of the query

select dept_name from departments where dept_no in (select dept_no from dept_manager where to_date is not null)

, where the subquery was materialized into a temporary table and then joined with the outer query. This is known as a semi-join optimization. But what happens if we turn off this optimization?

EXPLAIN FORMAT=JSON

 can help us with this investigation too. …

EXPLAIN FORMAT=JSON

The regular

EXPLAIN

 command already provides some information about subquery optimization. For example, you can find out if the subquery is dependent or not, and (since version 5.6) if it was materialized:

mysql> explain select dept_name from departments where dept_no in (select dept_no from dept_manager where to_date is not null)G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: departments
   partitions: NULL
         type: index
possible_keys: PRIMARY
          key: dept_name
      key_len: 42
          ref: NULL
         rows: 9
     filtered: 100.00
        Extra: Using where; Using index
*************************** 2. row ***************************
           id: 1
  select_type: SIMPLE
        table: …

In the previous post for this ongoing “EXPLAIN FORMAT=JSON is Cool!” series, we discussed covered indexes and how the 

used_columns

  array can help to choose them wisely. There is one more type of multiple-column indexes: composite indexes. Composite indexes are just indexes on multiple columns. Covered indexes are a subgroup of the larger set “composite indexes.” In this post we’ll discuss how “used_key_parts” can help show …

In the “MySQL Query tuning 101” video, Alexander Rubin provides an excellent example of when to use a covered index. On slide 25, he takes the query

select name from City where CountryCode = ’USA’ and District = ’Alaska’ and population > 10000

 and adds the index

cov1(CountryCode, District, population, name)

 on table

City

. With Alex’s query tuning experience, making the right index decision is simple – but what about us mere mortals? If a query is more complicated, or simply uses more than one table, how do we know what to do? Maintaining another index …

At the end of my talk “Troubleshooting MySQL Performance” at the LinuxPiter conference, a user asked me a question: “What does the EXPLAIN ‘filtered’ field mean, and how do I use it?” I explained that this is the percentage of rows that were actually needed, against the equal or bigger number of resolved rows. While the user was happy with the answer, I’d like to better illustrate this. And I can do it with help of EXPLAIN FORMAT=JSON and its

rows_examined_per_scan, rows_produced_per_join

  statistics.

Let’s take a simple query that searches information about the …

When you work with complicated queries, especially ones which contain subqueries, it is easy to make a typo or misinterpret column name. While in many cases you will receive a 

column not found

 error, sometimes you can get strange results instead.

Like finding 4079 countries in Antarctica:

mysql> select count(*) from City where CountryCode in (select CountryCode from Country where Continent = 'Antarctica');
+----------+
| count(*) |
+----------+
|     4079 |
+----------+
1 row in set (0.05 sec)

Or not finding any cities in Georgia:

mysql> select Name, Language from City join CountryLanguage using (CountryCode) where CountryCode in (select Code from Country where District='Georgia' and Continent='Asia');
Empty set (0.18 sec)

I used a standard …

When a new version of MySQL is about to be released we read a lot of blog posts about the performance and scalability improvements. That’s good but sometimes we miss some small features that can help us a lot in our day-to-day tasks. One good example is the blog post that Aurimas wrote about a new small feature in MySQL 5.6 that I didn’t know about until I read it: the Automatic InnoDB transaction log file size change. How cool is that?

I plan to write a series of blog posts that will show some of those small new features in MySQL 5.7 that are going to be really useful. I’m going to start with EXPLAIN FOR CONNECTION.

This feature allows us to run an EXPLAIN for an already running statement. Let’s say that you find a query …

I thought it was worth a moment to reiterate on the new Performance Schema related defaults that MySQL 5.7.7 brings to the table, for various reasons.

For one, most of you might have noticed that profiling was marked as deprecated in MySQL 5.6.7. So it is expected that you invest into learning more about Performance Schema (and Mark’s sys schema!).

Second, there are lots of virtual environments and appliances out there running Community Edition MySQL where Performance Schema can be a useful tool for analyzing performance. Thus, expect to see more articles about using PERFORMANCE_SCHEMA and SYS_SCHEMA from us!

Third, we have more and more junior readers who might benefit from light reads such as this.

The new defaults that I wanted to highlight are mentioned in the …

In version MySQL 5.7.7 Oracle presented a new promising feature: optimizer hints. However it did not publish any documentation about the hints. The only note which I found in the user manual about the hints is:

• It is now possible to provide hints to the optimizer by including /*+ ... */ comments following the SELECT, INSERT, REPLACE, …