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## CHAR and spaces

MySQL Data Types June 12, 2012

I know about it, I knew about it all along, but… it’s so easy to fall for it; there’s just so much absurdity!

A CHAR type has a known number of characters. For example, the column:

>     CountryCode CHAR(3) CHARSET ascii NOT NULL

– is known to have exactly three characters. These could be ‘USA’, ‘FRA’, etc.

What happens with spaces? A SQL CHAR type ignores any trailing spaces; thus, the code ‘GB ‘ (the characters ‘G’, ‘B’, and the space ‘ ‘) is interpreted as ‘GB’. Trailing spaces are not regarded as part of the text. Want to see some absurdity?

>     CREATE TABLE `c_test` (
>       `c` char(1) DEFAULT NULL
>     );
>     INSERT INTO c_test VALUES ('a');
>     INSERT INTO c_test VALUES ('b');
>     INSERT INTO c_test VALUES (' ');

>     SELECT c, LENGTH(c), c = 'a', c = 'b', c = ' ', LENGTH(' ') FROM c_test;
>     +------+-----------+---------+---------+---------+-------------+
>     | c    | LENGTH(c) | c = 'a' | c = 'b' | c = ' ' | LENGTH(' ') |
>     +------+-----------+---------+---------+---------+-------------+
>     | a    |         1 |       1 |       0 |       0 |           1 |
>     | b    |         1 |       0 |       1 |       0 |           1 |
>     |      |         0 |       0 |       0 |       1 |           1 |
>     +------+-----------+---------+---------+---------+-------------+

Continue reading » “CHAR and spaces”

## Getting rid of huge ibdata file, no dump required, part II

MySQL Hack InnoDB MyISAM Refactoring May 30, 2012June 3, 2012

This post continues Getting rid of huge ibdata file, no dump required, part I, where I describe way of converting your single-tablespace InnoDB database into a file-per-table one, without the pain of exporting and importing everything at once.

In previous part we put aside the issue of foreign keys. We address this issue now.

#### What if my InnoDB tables have foreign keys?

MyISAM does not support them, so you can’t just ALTER an InnoDB table to MyISAM and back into InnoDB, and expect everything to work.

Alas, this calls for additional steps (i.e. additional ALTER commands). However, these still fall well under the concept of “do it one table at a time, then take time to recover your breath and replication lag”.

#### Save , drop and restore your Foreign Keys setup

You can use common_schema‘s sql_foreign_keys to get the full listing and create definition of your foreign keys. For example, assume we use the sakila database: Continue reading » “Getting rid of huge ibdata file, no dump required, part II”

## Getting rid of huge ibdata file, no dump required

MySQL Hack InnoDB MyISAM Refactoring May 22, 2012May 22, 2012

You have been told (guilty as charged), that the only way to get rid of the huge InnoDB tablespace file (commonly named ibdata1), when moving to innodb_file_per_table, is to do a logical dump of your data, completely erase everything, then import the dump.

To quickly reiterate, you can only delete the ibdata1 file when no InnoDB tables exist. Delete this file with an existing InnoDB table, even a table in its own tablespace, and nothing ever works anymore.

#### The problem with the dump-based solution

The impact of doing a logical dump is often overwhelming. Well, the dump may be tolerable, but the restore is much longer. The real pain is that you can’t do this one table at a time: you have to destroy everything before dropping the ibdata1 file; you then have to import everything.

Perhaps the most common scenario is that we do the changes on a slave, so as not to completely shut down our database. This is nice; no one is aware of the shutdown process. However, Huston, we have a problem: we need to make sure we can keep up the binary logs on the master for the duration of the entire process. Continue reading » “Getting rid of huge ibdata file, no dump required”

## Impact of foreign keys absence on replicating slaves

MySQL common_schema data integrity May 14, 2012

In this post I describe what happens when a slave’s Foreign Key setup is different from that of the master. I’m in particular interested in a setup where the slave has a subset of the master’s foreign keys, or no foreign keys at all. I wish to observe whether integrity holds.

#### Making the changes

Which foreign keys do we have and how do we drop them? If you want to do this by hand, well, good luck! Fortunately, common_schema provides with quite a few handy views and routines to assist us. Consider viewing the existing foreign keys on sakila:

>     master> SELECT create_statement FROM common_schema.sql_foreign_keys WHERE TABLE_SCHEMA='sakila';
>     +-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
>     | create_statement                                                                                                                                                                                |
>     +-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
>     | ALTER TABLE `sakila`.`address` ADD CONSTRAINT `fk_address_city` FOREIGN KEY (`city_id`) REFERENCES `sakila`.`city` (`city_id`) ON DELETE RESTRICT ON UPDATE CASCADE                             |
>     | ALTER TABLE `sakila`.`city` ADD CONSTRAINT `fk_city_country` FOREIGN KEY (`country_id`) REFERENCES `sakila`.`country` (`country_id`) ON DELETE RESTRICT ON UPDATE CASCADE                       |
>     | ALTER TABLE `sakila`.`customer` ADD CONSTRAINT `fk_customer_address` FOREIGN KEY (`address_id`) REFERENCES `sakila`.`address` (`address_id`) ON DELETE RESTRICT ON UPDATE CASCADE               |
>     | ALTER TABLE `sakila`.`customer` ADD CONSTRAINT `fk_customer_store` FOREIGN KEY (`store_id`) REFERENCES `sakila`.`store` (`store_id`) ON DELETE RESTRICT ON UPDATE CASCADE                       |
>     | ALTER TABLE `sakila`.`film` ADD CONSTRAINT `fk_film_language` FOREIGN KEY (`language_id`) REFERENCES `sakila`.`language` (`language_id`) ON DELETE RESTRICT ON UPDATE CASCADE                   |
>     | ALTER TABLE `sakila`.`film` ADD CONSTRAINT `fk_film_language_original` FOREIGN KEY (`original_language_id`) REFERENCES `sakila`.`language` (`language_id`) ON DELETE RESTRICT ON UPDATE CASCADE |
>     | ALTER TABLE `sakila`.`film_actor` ADD CONSTRAINT `fk_film_actor_actor` FOREIGN KEY (`actor_id`) REFERENCES `sakila`.`actor` (`actor_id`) ON DELETE RESTRICT ON UPDATE CASCADE                   |
>     | ALTER TABLE `sakila`.`film_actor` ADD CONSTRAINT `fk_film_actor_film` FOREIGN KEY (`film_id`) REFERENCES `sakila`.`film` (`film_id`) ON DELETE RESTRICT ON UPDATE CASCADE                       |
>     | ALTER TABLE `sakila`.`film_category` ADD CONSTRAINT `fk_film_category_category` FOREIGN KEY (`category_id`) REFERENCES `sakila`.`category` (`category_id`) ON DELETE RESTRICT ON UPDATE CASCADE |
>     | ALTER TABLE `sakila`.`film_category` ADD CONSTRAINT `fk_film_category_film` FOREIGN KEY (`film_id`) REFERENCES `sakila`.`film` (`film_id`) ON DELETE RESTRICT ON UPDATE CASCADE                 |
>     | ALTER TABLE `sakila`.`inventory` ADD CONSTRAINT `fk_inventory_film` FOREIGN KEY (`film_id`) REFERENCES `sakila`.`film` (`film_id`) ON DELETE RESTRICT ON UPDATE CASCADE                         |
>     | ALTER TABLE `sakila`.`inventory` ADD CONSTRAINT `fk_inventory_store` FOREIGN KEY (`store_id`) REFERENCES `sakila`.`store` (`store_id`) ON DELETE RESTRICT ON UPDATE CASCADE                     |
>     | ALTER TABLE `sakila`.`payment` ADD CONSTRAINT `fk_payment_customer` FOREIGN KEY (`customer_id`) REFERENCES `sakila`.`customer` (`customer_id`) ON DELETE RESTRICT ON UPDATE CASCADE             |
>     | ALTER TABLE `sakila`.`payment` ADD CONSTRAINT `fk_payment_rental` FOREIGN KEY (`rental_id`) REFERENCES `sakila`.`rental` (`rental_id`) ON DELETE SET NULL ON UPDATE CASCADE                     |
>     | ALTER TABLE `sakila`.`payment` ADD CONSTRAINT `fk_payment_staff` FOREIGN KEY (`staff_id`) REFERENCES `sakila`.`staff` (`staff_id`) ON DELETE RESTRICT ON UPDATE CASCADE                         |
>     | ALTER TABLE `sakila`.`rental` ADD CONSTRAINT `fk_rental_customer` FOREIGN KEY (`customer_id`) REFERENCES `sakila`.`customer` (`customer_id`) ON DELETE RESTRICT ON UPDATE CASCADE               |
>     | ALTER TABLE `sakila`.`rental` ADD CONSTRAINT `fk_rental_inventory` FOREIGN KEY (`inventory_id`) REFERENCES `sakila`.`inventory` (`inventory_id`) ON DELETE RESTRICT ON UPDATE CASCADE           |
>     | ALTER TABLE `sakila`.`rental` ADD CONSTRAINT `fk_rental_staff` FOREIGN KEY (`staff_id`) REFERENCES `sakila`.`staff` (`staff_id`) ON DELETE RESTRICT ON UPDATE CASCADE                           |
>     | ALTER TABLE `sakila`.`staff` ADD CONSTRAINT `fk_staff_address` FOREIGN KEY (`address_id`) REFERENCES `sakila`.`address` (`address_id`) ON DELETE RESTRICT ON UPDATE CASCADE                     |
>     | ALTER TABLE `sakila`.`staff` ADD CONSTRAINT `fk_staff_store` FOREIGN KEY (`store_id`) REFERENCES `sakila`.`store` (`store_id`) ON DELETE RESTRICT ON UPDATE CASCADE                             |
>     | ALTER TABLE `sakila`.`store` ADD CONSTRAINT `fk_store_address` FOREIGN KEY (`address_id`) REFERENCES `sakila`.`address` (`address_id`) ON DELETE RESTRICT ON UPDATE CASCADE                     |
>     | ALTER TABLE `sakila`.`store` ADD CONSTRAINT `fk_store_staff` FOREIGN KEY (`manager_staff_id`) REFERENCES `sakila`.`staff` (`staff_id`) ON DELETE RESTRICT ON UPDATE CASCADE                     |
>     +-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+

Most of the foreign key constraints use RESTRICT for DELETE (meaning you are not allowed to delete a parent row when children exist), and CASCADE for UPDATE (meaning changes to parent will propagate to children). This is good, since I want to test behavior of both RESTRICT and CASCADE.

Continue reading » “Impact of foreign keys absence on replicating slaves”

## Webinar review: Zero-Downtime Schema Changes In MySQL

MySQL openark kit May 3, 2012

Yesterday I attended the Zero-Downtime Schema Changes In MySQL webinar by Baron Schwartz, Percona (do you say “attended” for something you listened to from your home office?)

I was keen to learn about possible enhancements and improvements of pt-online-schema-change over oak-online-alter-table. Here are my impressions:

The base logic of pt-online-schema-change is essentially the same as of oak-online-alter-table. You create a ghost/shadow table, create complex triggers, copy in chunks, freeze and swap. Both work on any type of PRIMARY KEY (oak-online-alter-table can work with any UNIQUE KEY, I’m not sure about pt-online-schema-change on this), be it an INTEGER, other type, or a multi column one.

However, pt-online-schema-change also adds the following:

- It supports FOREIGN KEYs (to some extent). This is something I’ve wanted to do with oak-online-alter-table but never got around to it. Foreign keys are very tricky, as Baron noted. With child-side keys, things are reasonably manageable. With parent-side this becomes a nightmare, sometimes unsolvable (when I say “unsolvable”, I mean that under the constraint of having the operation run in a non-blocking, transparent way).
- Chunk size is auto-calculated by the script. This is a cool addition. Instead of letting the user throwing out numbers like 1,000 rows per chunk, in the hope that this is neither too small nor too large, the tool monitors the time it takes a chunk to complete, then adjusts the size of next chunk accordingly. Hopefully this leads to a more optimized run, where locks are only held for very short periods, yet enough rows are being processed at a time.
- The tool looks into replicating slaves to verify they’re up to the job. If the slave lags too far, the tool slows down the work. This is an excellent feature, and again, one that I always wanted to have. Great work!

So the three bullets above are what I understand to be the major advantages of Percona’s tool over oak-online-alter-table.

#### Q & A

The presentation itself was very good, and Baron answered some questions. There was one question he did not answer during the webinar, nor here, and I though I may pop in and answer it. Although I can’t speak for the coders of pt-online-schema-change, I safely assume that since the logic follows that of oak-online-alter-table, the same answer applies in the case of Percona’s toolkit. Continue reading » “Webinar review: Zero-Downtime Schema Changes In MySQL”

## Installing MySQL from source/binary tarball as a Linux service

MySQL Installation Linux May 1, 2012

I’ve written before I prefer to do a manual install of MySQL over a repository one. I still do: I typically install from binary tarball or by compiling from source.

I’d like to share my setup procedure for Linux installation and service setup. I’ve done this dozens of times, on different Linux flavors, and it works well for me.

#### Installing from source

To get this straight: you sometimes have to compile the source files. I, for example, happen to use the Sphinx MySQLSE extension. You can only use it if compiled with MySQL. You had to compile a “vanilla” 5.1 version without query cache in order to completely remove the cache’s mutex contention.

Anyway, I find the easiest way is to install onto a path associated with the server version. For example, I would install a 5.5 server onto /usr/local/mysql55

This way, a new version gets its own path, and no ambiguity.

To do that, use the prefix option on configuration step:

>     cd /path/to/extracted/source/tarball
>     sh BUILD/autorun.sh
>     ./configure --prefix=/usr/local/mysql55
>     make
>     sudo make install

Once this is complete, you have everything under /usr/local/mysql55. This means binaries, libraries, scripts, etc.

To install the MySQL server as a service, copy the mysql.server script to /etc/init.d:

>     sudo cp /usr/local/mysql55/support-files/mysql.server /etc/init.d/mysql55

Again, I’m naming the script after the MySQL version. This avoids conflict with possible past or future installations of the MySQL server, which typically create a service named mysql or mysqld. Continue reading » “Installing MySQL from source/binary tarball as a Linux service”

## It’s *that time* of the year

MySQL mysqlconf Opinions April 16, 2012

Even without attending the Percona Live conference in Santa Clara, you could tell something big was going on.

One way of measuring it was by looking at the flow of announcements. Here’s a brief list, and apologies if I’ve missed anyone:

- Monty Program Announcing MariaDB 5.3. (and later Announcing MariaDB 5.5.23 GA)
- Tokutek Announcing TokuDB v6.0
- Twitter releasing its own MySQL fork
- Oracle announcing so many new features I can’t list them all. But they made a TOC for it, and then announced a dozen more, not covered in the TOC.
- Continuent Announcing Tungsten Enterprise 1.5
- Zmanda announcing Recovery Manager 3.4
- And plenty of new partnerships between the major consulting companies

All within the first days of the conference.

#### What this means, over the surface

I read a post by someone who was ranting about Oracle making so many announcements just as the conference began. He obviously suspected there was no coincidence. I got the impression he was looking at it the wrong way: as if Oracle’s announcements came to discourage the relevance of the conference.

I beg the opposite.

Continue reading » “It’s *that time* of the year”

## common_schema talk at Percona Live

MySQL common_schema mysqlconf Speaking April 8, 2012April 8, 2012

Are you attending PerconaLive?

Allow me to suggest you attend the Common Schema: a framework for MySQL server administration session on April 12, 14:00 – 14:50 @ Ballroom F.

This talk is by none other than Roland Bouman. Roland co-authored parts of common_schema, and is a great speaker.

I have a personal interest, of course, being the author of most of the components in common_schema. I would like to convert you to a supporter of this project. I know a few very smart people who think this project is an important tool. I would like more people to get to know it. Eventually, I would like developers and DBAs alike to consider it an inseparable part of any MySQL installation.

Then I shall have world domination, Bwa ha ha!

PS,

Have fun, I will unfortunately not attend myself this year. Having been on the program committee, I can tell it’s going to be a great conference!

## Thoughts on using MySQL plugins

MySQL Opinions plugin Sphinx April 3, 2012

I’m giving thoughts on the viability of MySQL plugins. This is due to a particular experience I’ve had, which is thankfully solved. However, it left some bitter taste in my mouth.

MySQL plugins are a tricky business. To create a plugin, you must compile it against the MySQL version you wish the users to use it with. Theoretically, you should compile it against any existing MySQL version, minors as well (I’m not sure whether it may sometimes or most times work across minor versions).

But, most important, you must adapt your plugin to major versions.

Another option for plugin makers, is to actually not recompile it, but rather provide with the source code, and let the end user compile it with her own MySQL version. But here, too, the code must be compatible with whatever changes the new MySQL version may have.

#### And if it doesn’t compile with the new MySQL version?

That’s what happened to me. The particular case at hand was SphinxSE, a plugin which serves as a bridge between MySQL and a Sphinx Search server. I’ve been using it for years and was happy about it. But, as it happened, it took well over a year for sphinx to compile with MySQL 5,5. This meant I was unable to upgrade my 5.1 installation to 5.5, a thing I was aiming to do for quite a while. Continue reading » “Thoughts on using MySQL plugins”

## sphinx, sphinx_snippets() & MySQL 5.5

MySQL Development Open Source Sphinx March 21, 2012March 21, 2012

I’ve written a patch which completes Sphinx’s integration with MySQL 5.5.

Up until a couple months ago, Sphinx would not compile with MySQL 5.5 at all. This is, thankfully, resolved as of Sphinx 2.0.3.

However, to my disdain, I’ve found out that it only partially work: the sphinx_snippets() user defined function is not included within the plugin library. After some quick poking I discovered that it was not added to the build, and when added, would not compile.

I rely on sphinx_snippets() quite a lot, and like it. Eventually I wrote the fix to the snippets_udf.cc which allows it to run in a MySQL 5.5 server.

Here are the changes for the 2.0.4 version of Sphinx:

- snippets_udf.cc
- CMakeLists.txt

Replace your 2.0.4 files with these two and get on compiling your MySQL server.

#### Compilation guide

For completeness, here’s how to compile Percona Server 5.5 with Sphinx 2.0.4 including the above patches: Continue reading » “sphinx, sphinx_snippets() & MySQL 5.5”