Starting from Percona Toolkit 3.6.0, pt-online-schema-change supports the option –where, so you can not only alter your table definition live but copy only rows that satisfy certain criteria. It may happen that you may not need to copy all the data when changing table definition. For example, if the table is too big and you […]

Starting from Percona Toolkit 3.6.0, you can resume pt-online-schema-change if it was interrupted. This blog describes the prerequisites and usage of the new –resume option. To restart the job, you need to know where it failed. This is why the first option you must use is –history. It instructs pt-online-schema-change to store its progress in […]

Table modifications are a routine task for database administrators. The blog post Using Percona Toolkit to Alter Database Tables Online: A Controlled Approach provides insights into the process of altering tables online in a controlled manner, ensuring uninterrupted access for application users and preventing application downtime. We will focus here on utilizing the powerful “pt-online-schema-change” […]

Dear Dave,

My DBA told me that an account I use to talk to my MySQL database instance has TWO passwords! How does that happen? Do I have to provide both passwords every time? 

Sincerely,

A Confused User Who Does Not Want to Type Two Passwords All The Time


Dear Confused User,

Dual Password Support was added in MySQL 8.0.14 and it is a very handy thing. User accounts can now have a primary password and a secondary password. Image a scenario where you are rotating passwords as directed by your company policy but it takes a while to change the credentials in all your code despite your best EMACS magic. The ability to keep the servers up and active as your do your security due diligence is very helpful.
…

Overview

DDL (Data Definition Language) statements create, alter, and remove database objects. These types of changes can be a very dangerous action to take on such a critical piece of your infrastructure. You want to make sure that the command that you are executing has been given proper thought and testing.

In this post I go through multiple version of MySQL and verify the best course of action to take in regards to executing DDL statements.  There are many things that you have to consider when making these types of changes, such as disk space, load on the database server, slave replication, the type of DDL statement you are executing, and if it will lock the table. 

Because of these risks, there are tools that can be used to help mitigate some of the dangers. But unless you have tested and verified their functionality, these tools in themselves can cause trouble. Whenever in doubt, take the time to test …

Recently, I was working on a MySQL support ticket where a customer was facing an issue while transporting tablespace from MySQL 5.6 to MySQL 5.7.
After closely reviewing the situation, I saw that while importing tablespace they were getting errors such as:

ERROR 1808 (HY000): Schema mismatch (Table flags don't match, server table has 0x10 and the meta-data file has 0x1)

After some research, I found that there is a similar bug reported to MySQL for this issue (https://bugs.mysql.com/bug.php?id=76142), but no solution is mentioned. I tested the scenario locally and found a solution …

People using OpenStack Trove instances can hit a common issue in the MySQL world: how to perform schema change operations while minimizing the impact on the database server? Let’s explore the options that can allow online schema changes.

Summary

With MySQL 5.5, pt-online-schema-change from Percona Toolkit is your best option for large tables while regular ALTER TABLE statements are only acceptable for small tables. Also beware of metadata locks.

With MySQL 5.6, almost all types of schema changes can be done online. Metadata locks can also be an issue. pt-online-schema-change can still be worth using as it is also online on read replicas.

Regular ALTER TABLE with MySQL 5.5

If you are still using MySQL 5.5, almost all schema changes will require a table …

Benchmarking is a popular topic. People love drawing graphs as much as watching how X is 10% faster than Y; there must be something special in measurements.

For a DBA, however, more tangible improvements come from less popular area of database maintenance. While MariaDB spreads FUD around InnoDB (nonetheless still uses it) I have to admit InnoDB gets more friendly to DBAs.

In MySQL 5.6 new temporary table naming scheme was introduced – one of improvements. Invisible, yet important.

Temporary table names became more random and should not ever be reused.

Some time ago I wrote a post about how to remove …

Sometimes corruption is not the true corruption. Corruption in compressed InnoDB tables may be a false positive.

Compressed InnoDB table may hit false checksum verification failure. The bug (http://bugs.mysql.com/bug.php?id=73689) reveals itself in the error log as follows:

2014-10-18 08:26:31 7fb114254700 InnoDB: Compressed page type (17855); stored checksum in field1 0; calculated checksums for field1: crc32 4289414559, innodb 0, none 3735928559; page LSN 24332465308430; page number (if stored to page already) 60727; space id (if stored to page already) 448
InnoDB: Page may be an index page where index id is 516

InnoDB complains that a stored checksum is zero. If you look closely it’s suspicious that calculated checksum is zero too.

Every InnoDB page stores a checksum in first four bytes. When InnoDB reads a page it compares the checksum, stored in …

When ALTER TABLE crashes MySQL server it leaves orphaned records in InnoDB dictionary. It is annoying because next time you run the same ALTER TABLE query it will fail with error:

ERROR 1050 (42S01) at line 1: Table 'sakila/#sql-ib712' already exists

The post explains why it happens and how to fix it.

When you run ALTER table InnoDB follows the plan:

1. Block the original table
2. Create an empty temporary table with the new structure. The name of the new table is something like #sql-ib712.
3. Copy all records from the original table to the temporary one
4. Swap the temporary and original tables
5. Unblock the original table

The temporary table is a normal InnoDB table except it’s not visible to a user. InnoDB creates a record in the dictionary for the temporary table as for any other table.

If MySQL crashes in the middle of the …