Ouch. Thanks for the explicit heads-up. I generally stay with PostgreSql, but I honestly thought most of these things were "fixed" in recent versions of MySQL (and assumed my subconcious dislike of MySql was at least partly irrational/rooted in Ancient and Outdated Lore). I guess not:
It took me a long term to learn this one. I suppose if I'd read the MySQL docs from cover to cover I would've found it earlier.
One other problem that popped up was ignoring indexes on tables with TEXT fields during joins, which was a planner weakness. I understand it was fixed in 5.6; I'm waiting for the Percona version to stabilise before I upgrade.
I tried MariaDB about a year ago and it had the same problem. It's possible it's been fixed since. I personally prefer the Percona fork of MySQL, which has some performance tweaks yet is basically a 100% drop-in replacement.
http://dev.mysql.com/doc/refman/5.7/en/internal-temporary-ta...
I'm not sure what the status of TEXT-fields are in mariadb:
https://mariadb.com/kb/en/optimizing-string-and-character-fi...