A Wolf, a Dolphin and a Sphinx walk into a bar… nevermind. We’ll skip the jokes. This post is about using SSL to set up a secure connection between MySQL and Sphinx. Serious stuff! The Idea It simple. We’re going to use Sphinx to index some data from MySQL across a secure connection. So, we’ll [...]

Sometimes users ask for something that doesn’t really make sense. On the first glance. But then you start asking and realize that the user was right, you were wrong, and it is, actually, a perfectly logical and valid use case.

I’ve had one of these moments when I’ve heard about a request of making triggers to work on the slave in the row-based replication. Like, really? In RBR all changes made by triggers are replicated from the master to slaves as row events. If triggers would be fired on the slave they would do their changes twice. And anyway, assuming that one only has triggers one the slave (why?) in statement-based replication triggers would run on the slave normally, wouldn’t they?

Well, yes, they would, but one cannot always use statement-based replication. If one could, RBR would’ve never been implemented. There are many cases that statement-based replication cannot handle correctly. Galera requires RBR too. And as …

Let me start with a little story. You sit in your house near the fireplace in the living room and need a book from the library… Eh, no, sorry, wrong century. You’re building a robotic arm that will open your beer or brew your coffee or supply you with whatever other drinks of your choice… while you’ll be building the next robotic arm. So, you — soldering iron in one hand and Arduino in another — ask your little brother to bring a box with specific resistors (that you unexpectedly run out of) from the cellar. The problem — your brother is small and cannot tell a resistor from a respirator. You explain that it’s small thing with two wires sticking out of it. And he starts going back and forth brining you boxes after boxes of different small things with two wires.

This is approximately where we were in MySQL when NDB Cluster was just added. The use wants to find a row, say WHERE number_of_wires=2 AND size='small' AND …

I don’t think it’ll surprise anybody if I say that MariaDB or MySQL server knows a lot more about server system variables, then just their values. Indeed, every variable can be session or global only, read-only or writable, it has an associated help text (that is printed on mysqld --help --verbose), certain variables only accept values from a given set of strings (this set of allowed values is also printed in mysqld --help --verbose since MariaDB 10.1.0), numeric variables have lower and upper range boundaries of valid values (that are never printed anywhere), and so on. I always thought it’s kind of a waste that there is no way to query this information. That could’ve been very convenient, in particular for various GUI clients — they could show the help in tooltips, validate values and so on.

But recently we’ve got our users asking for it — precisely, for system variable metadata, whether a variable …

Introduction

When you e.g. delete rows, these rows are just marked as deleted not really physically deleted from indexes and free space introduced is not returned to operating system for later reuse. Purge thread will physically delete index keys and rows, but still free space introduced is not returned to operating system and this operation can lead holes on page. If you have variable length rows, this could lead to situation where this free space can’t be used for new rows (if these rows are larger than old ones). User may use OPTIMIZE TABLE or ALTER TABLE <table> ENGINE=InnoDB to reconstruct the table.

Unfortunately, running OPTIMIZE TABLE against an InnoDB table stored in the shared …

Introduction

Online DDL is a new feature in MariaDB 10.0. Online DDL is processed through below 4 tasks in sequence.

1. InnoDB::ha_prepare_inplace_alter_table(..)
2. InnoDB::ha_inplace_alter_table(..)
3. InnoDB::ha_commit_inplace_alter_table(..)
4. mysql_rename_table(..)

InnoDB storage engine allocates temporal memory buffer for transaction logging in phase 1 where row changes during this phase are logged. Size of this buffer is at start sort_buffer_size and it can be grown up to innodb_online_alter_log_max size. During phase 2 thread processing the ALTER statement will copy old table’s rows to a new altered table. After this MariaDB will take exclusive lock for target table and applies row log buffer to the new altered table.

This introduces a new unpredictable failure case row log buffer overflow. MariaDB server will rollback ALTER statement if row log buffer …

One of the most popular plugin types both in MariaDB and MySQL is INFORMATION_SCHEMA plugin type. INFORMATION_SCHEMA plugins add new tables to the INFORMATION_SCHEMA. There are lots of INFORMATION_SCHEMA plugins, because they can be used to show just anything to the user and are very easy to write.

MariaDB 10.1.1 comes with nine INFORMATION_SCHEMA plugin:

• Feedback — shows the anonymised server usage information and can optionally send it to the configured url.
• Locales — lists compiled-in server locales, implemented by Roberto Spadim
• METADATA_LOCK_INFO — Lists metadata locks in the server. Implemented by Kentoku Shiba
• …

Every now and then there is a need to execute certain SQL statements conditionally. Easy, if you do it from your PHP (or Java or whatever) application. But if all you have is pure SQL? There are two techniques that MariaDB and MySQL use in the mysql_fix_privilege_tables.sql script (applied by mysql_upgrade tool).

1. Create a stored procedure with IF statements inside, call it once and drop it. This requires the user to have the CREATE ROUTINE privilege and mysql.proc table must exist and be usable (which is not necessarily true — we’re doing it from mysql_upgrade, right?).
2. Use dynamic SQL, like
   

   SET @str = IF (@have_csv = 'YES',
                  'CREATE TABLE IF NOT EXISTS general_log (
                     event_time TIMESTAMP(6) NOT NULL,
                     user_host MEDIUMTEXT NOT NULL,
                     thread_id BIGINT(21) UNSIGNED NOT NULL, …

Yes! In MariaDB 10.1.1 tables in PERFORMANCE_SCHEMA do not use .frm files. These files are not created, not read — in fact, PERFORMANCE_SCHEMA tables never touch the disk at all.

This became possible due to a lesser-known feature of MariaDB — new table discovery (“old table discovery” was implemented in MySQL for NDB Cluster in 2004), implemented in MariaDB 10.0.2. Instead of reading and parsing .frm files, MariaDB simply asks PERFORMANCE_SCHEMA table, what structure it has, and because these tables always have a fixed structure, the table directly returns it to MariaDB with no need for any external data dictionary.

It also means, you never need to upgrade PERFORMANCE_SCHEMA tables, they always have the correct structure corresponding to the MariaDB version …

As you all know MariaDB supported roles since the MariaDB release 10.0.5. They were implemented almost exactly as specified in the SQL Standard 2003, features T331 “Basic roles” and T332 “Extended Roles”.

But we were often hearing complains, users were not satisfied with purely standard set of features. In particular, the standard specified that one had to do

SET ROLE foobar;

to be able to use privileges, granted to the role foobar. This was not always convenient and sometimes not even possible (imagine, you need to grant role privileges to an account used by a closed-source application). There had to be some way to enable a given role automatically, when a user connects.

To solve this issue we have introduced the concept of a default role. A default role for given user is automatically enabled when this user …