Knowing the bits and bytes of an SQL query's order of operations can be very valuable, as it can ease the process of writing new queries, while also being very beneficial when trying to optimize an SQL query.

If you're looking for the short version, this is the logical order of operations, also known as the order of execution, for an SQL query:

1. FROM, including JOINs
2. WHERE
3. GROUP BY
4. HAVING
5. WINDOW functions
6. SELECT
7. DISTINCT
8. UNION
9. ORDER BY
10. LIMIT and OFFSET

But the reality isn't that easy nor straight forward. As we said, the SQL standard defines the order of execution for the different SQL query clauses. Said that, modern databases are already challanaging that default order by applying some optimization tricks which might change the actual order of execution, though they must end up returning …

The majority of DBA’s perform health checks on their databases every now and then. Usually, it would happen on a daily or weekly basis. We previously discussed why such checks are important and what they should include.

To make sure your systems are in a good shape, you’d need to go through quite a lot of information - host statistics, MySQL statistics, workload statistics, state of backups, database packages, logs and so forth. Such data should be available in every properly monitored environment, although sometimes it is scattered across multiple locations - you may have one tool to monitor MySQL state, another tool to collect system statistics, maybe a set of scripts, e.g., to check the state of your backups. This makes health checks much more time-consuming than they should be - the DBA has to put together the different pieces …

Prometheus 2 time series database (TSDB) is an amazing piece of engineering, offering a dramatic improvement compared to “v2” storage in Prometheus 1 in terms of ingest performance, query performance and resource use efficiency. As we’ve been adopting Prometheus 2 in Percona Monitoring and Management (PMM), I had a chance to look into the performance of Prometheus 2 TSDB. This blog post details my observations.

Understanding the typical Prometheus workload

For someone who has spent their career working with general purpose databases, the typical workload of Prometheus is quite interesting. The ingest rate tends to remain very …

MySQL privilege system is small, almost all administrative tasks can be completed using a handful of privileges. If we exclude generic ones as ALL, SHOW DATABASES and USAGE, create and drop permissions as CREATE USER, DROP ROLE or CREATE TABLESPACE, the number of privileges remaining is really limited: PROCESS, PROXY, RELOAD, REPLICATION_CLIENT, REPLICATION_SLAVE, SHUTDOWN and SUPER.

Having such a reduced list of privileges means that it is very difficult to control what a connected session can do. For example, if a user has privileges to stop replication, it also has privileges to start it, and also to configure it. Actually, it has rights to do almost everything as the privilege required to stop replication is …

The Tungsten Replicator is an extraordinarily powerful and flexible tool capable of moving vast volumes of data from source to target.

In this blog post we will discuss one specific aspect of the THL (Transaction History Log) – the METADATA unsafe_for_block_commit flag.

What do you mean, Unsafe?

In a recent customer support case, we were asked the meaning of the unsafe_for_block_commit flag. For example, list the event information for sequence number 3481394254:

[tungsten@tr2-mysql01 (sandbox) ~]$ thl list -seqno 3481394254 | more
SEQ# = 3481394254 / FRAG# = 0 (last frag)
- TIME = 2018-09-16 06:52:47.0
- EPOCH# = 3480364140
- EVENTID = mysql-bin.001068:0000000294739578;622252
- SOURCEID = tr2-mysql01.sandbox.yourdomain.com
- METADATA = [mysql_server_id=1;unsafe_for_block_commit;dbms_type=mysql;tz_aware=true;service=brm;shard=shard_1736]
- TYPE = …

As you may know, sometimes MySQL can be memory-hungry. Of course having data in memory is always better than disk… RAM is still much faster than any SSD disk.

This is the reason why we recommended to have the working set as much as possible in memory (I assume you are using InnoDB of course).

Also this why you don’t want to use Swap for MySQL, but don’t forget that a slow MySQL is always better than no MySQL at all, so don’t forget to setup a Swap partition but try to avoid using it. In fact, I saw many people just removing the Swap partition… and then OOM Killer did its job… and mysqld is often its first victim.

MySQL allocates buffers and caches to improve performance of database operations. That process is explained in details in the manual.

In this article series, I will provide you some …

When the NDB version is upgraded it requires the underlying MySQL server as well is upgraded. Internal table storage format is different in MySQL 5.7 and for new tables created on MySQL 5.6. Row_format, temporal data type (time, datetime etc.) storage are handled differently in 5.7. You can find the details on how MySQL handles temporal in these versions from the link below. The new format have time value in microseconds resolution, and also changes the way datetime is stored, which is more optimized
https://dev.mysql.com/doc/refman/5.7/en/time.html …

ProxySQL 1.4.10, released by ProxySQL, is now available for download in the Percona Repository along with an updated version of Percona’s proxysql-admin tool.

ProxySQL is a high-performance proxy, currently for MySQL and its forks (like Percona Server for MySQL and MariaDB). It acts as an intermediary for client requests seeking resources from the database. René Cannaò created ProxySQL for DBAs as a means of solving complex replication topology issues.

The ProxySQL 1.4.10 source and binary packages available at https://percona.com/downloads/proxysql …

Calculate the size of your dataset:

SELECT
sys.format_bytes(sum(data_length)) DATA,
sys.format_bytes(sum(index_length)) INDEXES,
sys.format_bytes(sum(data_length + index_length)) ‘TOTAL SIZE’
FROM information_schema.TABLES ORDER BY data_length + index_length;

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Alexey made this amazing tool that the majority of MySQL DBAs are using, but if you use sysbench provided with your GNU/Linux distribution or its repository on packagecloud.io you won’t be able to use it with the new default authentication plugin in MySQL 8.0 (caching_sha2_password).

This is because most of the sysbench binaries are compiled with the MySQL 5.7 client library or MariaDB ones. There is an issue on github where Alexey explains this.

So if you want to use sysbench with MySQL 8.0 …