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MongoDB Through a MySQL Lens

This blog post looks at MongoDB and MySQL, and covers high-level MongoDB strengths, weaknesses, features, and uses from the perspective of an SQL user.

Delving into NoSQL coming from an exclusively SQL background can seem like a daunting task. I have worked with SQL in both small MySQL environments and large Oracle SQL environments. 

When is it a good choice?

MongoDB is an incredibly robust, scalable, and operator-friendly database solution. MongoDB is a good choice when your developers will also be responsible for the database environment. In small shops and startups, this might be the case. MongoDB stores information in BSON (binary JSON). BSON is the native JSON (JavaScript Object Notation) language used by MongoDB to retrieve information stored in BSON on the back end. JSON is easily relatable to other programming languages, and many developers will already have experience with it.

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Wrong binlog size reported after moving binlogs on MySQL version 5.6

Some days ago we found very interesting bug report with my friend: http://bugs.mysql.com/bug.php?id=71879

Problem is that, if a client moves binlog files to the other folder, stops MySQL server, update in the config file bin_log and bin_log_index values for a new path and starts server again, then the result of SHOW BINARY LOG command will be wrong:

mysql> show binary logs;
+-------------------------+-----------+
| Log_name                | File_size |
+-------------------------+-----------+
| cluster-repo-bin.000001 |       120 |
| cluster-repo-bin.000002 |         0 |
| cluster-repo-bin.000003 |         0 |
| cluster-repo-bin.000004 |         0 |
| cluster-repo-bin.000005 |         0 |
| cluster-repo-bin.000006 |       120 |
+-------------------------+-----------+

As you see, the size of some binlog files are zero. So why?

If you …

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MySQL Connector/Node.js 1.0.5 m4 development has been released

MySQL Connector/Node.js is a new Node.js driver for use with the X DevAPI. This release, v1.0.5 M4, is the fourth development release of the MySQL Connector/Node.js 1.0 series.

The X DevAPI enables application developers to write code that combines the strengths of the relational and document models using a modern, NoSQL-like syntax that does not assume previous experience writing traditional SQL.

To learn more about how to write applications using the X DevAPI, see
http://dev.mysql.com/doc/x-devapi-userguide/en/index.html. For more information about how the X DevAPI is implemented in MySQL Connector/Node.js, and its usage, see http://dev.mysql.com/doc/dev/connector-nodejs.

Note

Please note that the X DevAPI …

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Bug fix – Error -1 from storage engine while creating table with non-existing datadir on MySQL version 5.6

According to this bug report http://bugs.mysql.com/bug.php?id=79151

when client tries to create a new table with not existing (or write permission denied) datadir, then he gets an error message like this: ERROR 1030 (HY000): Got error -1 from storage engine

As you see it is not informative error message, we don’t know what’s wrong at the backend. But in the version 5.7 it seems this problem was resolved already.

I discovered that at the low level of innodb storage engine, program tries to create subfolders, when occurs a problem it checks system error partly in the file storage/innobase/fil/fil0fil.cc:

success = os_file_create_subdirs_if_needed(path);

os_file_create_subdirs_if_needed function in the storage/innobase/os/os0file.cc file checks system errors but returns only boolean value depending on the …

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MySQL doesn’t show informative error message for read-only filesystem in Linux on version 5.6

My first bug fix was about a year ago. You can see bug report here: http://bugs.mysql.com/bug.php?id=72259

If to shortly explain, so problem was that when you make file system read only, and try to start the server, it fails. And there is no exact error message in the log files which explains the problem correctly.

 

I added to the sql/mysqld.cc file a simple function whitch checks the given path’s permission and returns the state code:

// checks if file system is read-only

int is_filesystem_read_only(char const* name) {

    if (access(name, W_OK) == -1) {

        if (access(name, R_OK) == 0) {

            return R_OK; // read only

        } else if (access(name, F_OK) == 0) {

      …

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MySQL Support Engineer's Chronicles, Issue #3

The original idea of this series was to publish one post per week, but it seems every other week I have some special topic that well deserves a dedicated post. Last week I had no time to complete my writing because of long (and, I hope, useful) Howto post on replacing corrupted partition using non-corrupted one from other server in replication setup. But I had links and notes collected in a draft that I am going to complete now.

First of all, during the previous week I had time to submit two more talks for the  …

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Testing MySQL 8

MySQL 8MySQL 8 is now available for testing. The list of features is impressive and I am sure many PHP developers will be interested in having a true data dictionary, invisible indexes, and more. Right now it is in a milestone release. I am still doing a lot of work with 5.7 and found a great way to have 8 around when I needed by using Docker. Follow the Docker install guide for your platform of choice and then run

docker run --name mysql8 -e MYSQL_ROOT_PASSWORD=hidave -d mysql:8.0

Checking to See if it Really Works


docker ps -a
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
ae1824fc01b5 mysql:8.0 "docker-entrypoint.sh" About a minute ago Up About a minute 3306/tcp mysql8

Note that our instance of MySQL 8 has the container id of ae1824fc01b5. And odds …

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Is Your Query Cache Really Disabled?

This blog post was motivated by an internal discussion about how to fully disable query cache in MySQL.

According to the manual, we should be able to disable “Query Cache” on the fly by changing

query_cache_type

 to 0, but as we will show this is not fully true. This blog will show you how to properly disable “query cache,” and how common practices might not be as good as we think.

Can we just disable it by changing variables, or does it requires a restart to avoid the global mutex? Let’s see how it works.

Some Query Cache context

The query cache stores the text of a “Select” statement together with the corresponding result that was sent to the client. If an identical statement is received later, the server retrieves the results …

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Log Buffer #494: A Carnival of the Vanities for DBAs

This Log Buffer Edition covers Oracle, SQL Server and MySQL.

Oracle:

Declarative REST Client Feign with Spring Boot

Sometimes it is necessary to put on your uber-geek hat and start using cryptic bits of code to retrieve information from an Oracle database.

Deployment of Oracle Applications and Database with OpenStack Murano

A current question on the OTN database forum asks: “What’s the difference between object and tablespace reorganization?” Here’s an analogy to address the question.

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My first patch for MySQL world; fixing BUG #76852

Hi dear MySQL Community.
I have reported a bunch of bugs and wrote some articles, but today is a special for me
Because today I send my first PR for MySQL source code.

So I have logged a bug -> #76852 and now fixed it after year.
Okey, I know I am lazy.

Let’s reproduce the bug.
Simulating full disk condition is easy as:

mkdir /filesystems
dd if=/dev/zero of=/filesystems/tmp_fs bs=1024 count=0 seek=$[1024*10]
mkfs.ext4 /filesystems/tmp_fs
mkdir small_mounted_dir
mount -o loop /filesystems/tmp_fs /home/sh/small_mounted_dir/

Then obtain MySQL source and compile it with DEBUG. If you don’t know how to do it just refer to -> installing-mysql-from-source-cmake-issues

From now you can use MySQL Sandbox to …

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