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Displaying posts with tag: performance_schema (reset)
What is the best practice to get a list of all the queries running in MySQL (processlist) ?

If you are a MySQL DBA for a long time (like me), it’s very complicated to get rid of bad habits. One of them I really need to change is the way to retrieve the list of all the running queries (processlist).

Usually, I use SHOW FULL PROCESSLIST which is very convenient, but like querying the Information_Schema, this statement has negative performance consequences because it requires a mutex. Therefore, you should use Performance_Schema which doesn’t require a mutex and has minimal impact on server performance.

Let’s see the output of both commands:

mysql> show full processlist\G                                                                                                                   *************************** 1. row ***************************
     Id: 4
   User: event_scheduler
   Host: localhost
     db: NULL
Command: Daemon
   Time: 376338
  State: …
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How to know if a user never connected to the MySQL server since last boot ?

Performance_Schema is used most of the time to get metrics about queries and connections. But it can also provide other very useful information.

So today, I will show you how you can see a list of users that didn’t connect to MySQL since we restarted it (since last reboot).

SELECT DISTINCT mu.user FROM mysql.user mu
LEFT JOIN performance_schema.users psu
ON mu.user = psu.user
WHERE psu.user IS NULL
AND mu.user NOT IN ('mysql.infoschema', 'mysql.session', 'mysql.sys')
ORDER BY mu.user;

Example:

mysql> SELECT DISTINCT mu.user FROM mysql.user mu
    ->       LEFT JOIN performance_schema.users psu 
    ->       ON mu.user = psu.user  
    ->       AND mu.user NOT IN ('mysql.infoschema', 'mysql.session', 'mysql.sys') …
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MySQL 8.0: Statements Latency Histograms

At the end we all agree that what really matters is the Query Response Time, isn’t it ?

MySQL 8.0 engineers have added more visibility in the summary tables as explained in this previous post. But to add more details, now the Performance_Schema also collects the statements latency and provide their distribution thanks to the collected histogram data.

Two tables have been added to Performance_Schema:

  • events_statements_histogram_by_digest: details about latency related to schema and query digest
  • events_statements_histogram_global: global latency summary across all schemas and queries

Let’s have a look at what we can see: …

[Read more]
MySQL 8.0 : Digest Query Samples in Performance_Schema

Today, yet another blog post about improvements in MySQL 8.0 related to Performance_Schema. Before MySQL 8.0 it was not always easy to get an example of the queries you could find in Performance_Schema when looking for statements summaries. You had to link several tables (even from sys) to achieve this goal as I explained it in this post.

Now in MySQL 8.0, we have changed the table events_statements_summary_by_digest. This table now contains 6 extra columns:

  • QUANTILE_95 : stores the 95th percentile of the statement latency, in …
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MySQL 8.0 : meta-data added to Performance_Schema’s Instruments

In MySQL 8.0, the engineers have added useful meta-data to the table SETUP_INSTRUMENT. This table lists the classes of instrumented objects for which events can be collected.

To the NAME, ENABLES and TIMED columns, were added PROPERTIES, VOLATILITY and DOCUMENTATION.

Let’s have a quick look at these new columns:

PROPERTIES can have the following values

  • global_statistics: only global summaries are available for this instrument. Example: memory/performance_schema/metadata_locks that return the memory used for table performance_schema.metadata_locks
[Read more]
TOP 10 MySQL 8.0 features for developers

MySQL 8.0 RC2 has just been released with a lot of new features compared to MySQL 5.7. In this blog post, I will list the top 10 new features that should excite developers.

These functionalities are presented in descending order of preference of our MySQL Community.

TOP 10

  1. MySQL Document Store
  2. Default to utf8mb4
  3. JSON enhancements
  4. CTEs
  5. Window Functions
  6. Descending Indexes
  7. Better Optimizer Cost Model
  8. MySQL Server Components
  9. Improvement in GIS
  10. InnoDB NO WAIT & SKIP LOCKED

MySQL Document Store

This is the most expected and liked feature in MySQL 8.0 … and …

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MySQL Group Replication: who is the primary master – updated!

Some time ago I wrote this article explaining how to find which node was the Primary Master in a MySQL Group Replication cluster running in Single-Primary Mode.

In the latest release of Group Replication, MySQL 8.0.2 dmr, Jaideep improved the visibility of Group Replication extending the performance_schema tables (see his article).

Thanks to these improvements, it’s now very easy to find which host is acting a Primary-Master. This is the query you can use:

mysql-sql> SELECT MEMBER_HOST as `PRIMARY` 
           FROM performance_schema.replication_group_members 
           WHERE MEMBER_ROLE='PRIMARY';
+---------+
| PRIMARY |
+---------+
| mysql3  |
+---------+
1 row in set (0.00 sec) …
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InnoDB Locks Analysis: Why is Blocking Query NULL and How To Find More Information About the Blocking Transaction?

Consider the scenario that you execute a query. You expect it to be fast - typically subsecond - but now it take not return until after 50 seconds (innodb_lock_wait_timeout seconds) and then it returns with an error:

mysql> UPDATE world.City SET Population = Population + 999 WHERE ID = 130;
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction

You continue to investigate the issue using the sys.innodb_lock_waits view or the underlying Information Schema tables (INNODB_TRX, INNODB_LOCKS and INNODB_LOCK_WAITS).

Note:

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MySQL Group Replication: who is the primary master ??

As you know, MySQL Group Replication runs by default in single primary mode.

mysql2 mysql> show global variables like 'group_replication_single_primary_mode';
+---------------------------------------+-------+
| Variable_name                         | Value |
+---------------------------------------+-------+
| group_replication_single_primary_mode | ON    |
+---------------------------------------+-------+

But how can we easily find which member of the group is the Primary-Master ?

Of course you could check which one is not in read_only:

mysql2 mysql> select @@read_only;
+-------------+
| @@read_only |
+-------------+
|           1 |
+-------------+

But then you need to perform this on all the nodes one by one until you find the right one.

The primary …

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Quickly Troubleshoot Metadata Locks in MySQL 5.7

In a previous article, Ovais demonstrated how a DDL can render a table blocked from new queries. In another article, Valerii introduced performance_schema.metadata_locks, which is available in MySQL 5.7 and exposes metadata lock details. Given this information, here’s a quick way to troubleshoot metadata locks by creating a stored procedure that can:

  • Find out which thread(s) have the metadata lock
  • Determine which thread has been waiting for it the longest
  • Find other threads waiting for the metadata lock

Setting up …

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