Should I COUNT(*) or not?

Question

I know it\'s generally a bad idea to do queries like this:

SELECT * FROM `group_relations`

But when I just want the count, should I go for this

Answer 1

The advice I got from MySQL about things like this is that, in general, trying to optimize a query based on tricks like this can be a curse in the long run. There are examples over MySQL's history where somebody's high-performance technique that relies on how the optimizer works ends up being the bottleneck in the next release.

Write the query that answers the question you're asking -- if you want a count of all rows, use COUNT(*). If you want a count of non-null columns, use COUNT(col) WHERE col IS NOT NULL. Index appropriately, and leave the optimization to the optimizer. Trying to make your own query-level optimizations can sometimes make the built-in optimizer less effective.

That said, there are things you can do in a query to make it easier for the optimizer to speed it up, but I don't believe COUNT is one of them.

Edit: The statistics in the answer above are interesting, though. I'm not sure whether there is actually something at work in the optimizer in this case. I'm just talking about query-level optimizations in general.