[sadmysqluser]

I've never recommended MySQL for anything where the data is important, but I've had to help several businesses deal with existing installations. Here's a few teachable moments:

One customer who ran MySQL on windows experienced data loss after a historic MySQL crash left them with a corrupted MyISAM table, which caused mysqldump to SILENTLY exit before the dump was complete. Their dumps only had the tables up to the corrupted one. They had been saving those dumps for months, but never testing them. When the server eventually gave up the ghost they found that none of their dumps were good. Lesson: 1. ALWAYS test your dumps. 2. NEVER trust MyISAM.

Another customer, smart enough to use InnoDB but not smart enough to change the MySQL defaults let one database get too large and found they couldn't shrink any of their databases because unless you use innodb_file_per_table InnoDB won't ever relinquish space even if you delete all the rows in your tables even after "optimize table". Lesson: 1. ALWAYS use innodb_file_per_table if you plan to keep the database for a long time. 2. Periodically run mysqlcheck optimize or optimize table to actually reclaim deleted space.

I've also helped customers rewrite their applications to avoid the "Total number of locks exceeds lock table size" problem. What is that? Well, even if you use database level locks the number of rows a transaction can alter is still limited by the server's memory buffer pool size. Lesson: 1. Know your working set. Make sure your application checks the server's buffer pool size via SHOW VARIABLES and is smart enough to avoid large transaction updates. 2. Use a large buffer pool size whenever you can.

Coming from a PostgreSQL background, these came as total surprises to me. I've never been completely happy with PostgreSQL's transaction id wraparound issues but I have yet to experience anything like the problems I've observed with MySQL.