Query Cache is deterministic

I was reading some points on a Blog post today here. I didn’t see it via Planet MySQL but via a Google Alert.

The post has several good beginner points on MySQL Performance, but included the point “Literal current date is better than using the CURRENT_DATE() function because literals will use the query cache but functions won’t.”

How true, I must admit I’d not considered this in reviewing code for sites that use the Query Cache heavily. Non deterministic functions invalid use of the Query Cache (Except for recent Bug #29053 which is also an interesting read. Note to self, read the new bugs more often when free time permits). Back on point.

So if you have a query like “Select the news for today” and use CURDATE(), it will not be stored in the Query Cache, yet if your application code determines the current date and passes “Select the news for 2007-06-26″ for example it will.

Sometimes the simple things are easy to forget, or not have thought about in this case for me.