SQL Common Errors: confusing WHERE and ON

You probably know the ON clause of SQL JOINs. It is a condition that determines which rows from a table are related to a row from another table.

Conceptually, this is extremely different from the WHERE clause, that determines which row combinations will be returned to the client. However, sometimes a condition can be moved from the WHERE clause to the ON clause, because it will still prevent rows from being excluded. But please keep on reading, because the TL;DR of this post is: don’t do that.

Actually, why would one move conditions from WHERE to ON? Because many developers think that this is an optimization. They believe that the ON clause is executed before the WHERE clause, so moving the conditions will cause them to be evaluated at an earlier stage, avoiding some useless work.

Now, one may actually show some examples where moving a condition actually speeds up a query. This can happen in some uncommon cases, and the root cause is that the optimizer is not able to find the optimal execution plan. Don’t focus on the ON clause. Just focus on building proper indexes or finding out why the optimizer is not using them. But this is a wider topic that will not be covered here. Continue reading

Revisiting my 2018 Database Wishlist

It is December and 2018 is going to end. In January, when it just started, I wrote my 2018 Database Wishlist. Probably next January I’ll write another list. But first, it makes sense to review the last one. If some of my wishes actually happen, I really should know that – I don’t want to miss something nice, or forget topics that I considered interesting less than one year ago. Well, let’ s do some copy/paste and some googling… Continue reading

On Percona Community Blog

I liked Percona Community Blog from the beginning. First of all, the idea is great. There is no other community blog for the MySQL ecosystem.

Well, Oracle has its own planet.mysql.com – and I have to say, they are correct: as far as I know, they never censored posts about MariaDB and Percona Server, nor opinions that they don’t like. I wrote heavy criticism about Oracle, sometimes using strong terms (“another dirty trick”), but they never censored me. I like to be fair regardless who/what I’m talking about, so this is a good time to spend some good words about them. Not the first time, anyway. That said, their blogroll only advertises a very small number of blogs. Very good ones of course (except for mine?), but this has the inevitable side effect of obfuscating the rest of the world. If John Doe writes an enlightening post about MySQL, I’ll never read it, because everything I need to know appears on Planet MySQL. Continue reading