Mermaids have the same probability of fixing your permission problems, but people continue believing in the FLUSH PRIVILEGES myth.I see suggesting the usage of FLUSH PRIVILEGES every time someone writes a tutorial or a solution to a problem regarding creating a new account or providing different privileges. For example, the top post on /r/mysql as of the writing of these lines, “MySQL:The user specified as a definer does not exist (error 1449)-Solutions” has multiple guilty cases of this (Update: the user has corrected those lines after I posted this article).

It is not my intention to bash that post, but I have seen …

Introduction

If you, like me, are building or thinking of implementing a MySQL-powered application that has any need for prioritizing selecting certain data over other data, this article is for you.

Example

As a real world example, consider a queue-like video processing system. Your application receives new videos and processes them. The volume of incoming videos can at times be higher than the processing rate because the process is CPU bound, so occasionally a pretty long queue may form. You will try to process them as fast as you can but…

Note that I am using a queue here, so the the next item to be processed is a result of sorting by some sort of field in a ascending order, for example ORDER BY id or ORDER BY upload_date. Iâ€ll pick the id sort here.

…suddenly, you need to process a video somewhere in the middle of the queue or an important video enters and …

Last few days one of our customers (one of the largest Ruby on Rails sites on the Net) was struggling to solve some really strange problem - once upon a time they were getting an error from ActiveRecord on their site:

They have innodb_lock_wait_timeout set to 20 seconds. After a few hours of looking for strange transactions we were decided to create s script to dump SHOW INNODB STATUS and SHOW FULL PROCESSLIST commands output to a file every 10 seconds to catch one of those moments when this error occurred.

Today we’ve got next error and started digging in our logs…

After a couple of hours of looking in logs, parsing binary logs from MySQL …