Active Record: COUNT WHERE?

[eluser]Michael Wales[/eluser]
I think the performance hit between any of the options would be minimal at best.

I believe, from best performer to worst, would be:
1. Perform a COUNT() query on the database.
2. Perform a SELECT query and use the num_rows() function to return the query metadata.
3. Perform a SELECT query, then iterate those results, to perform counting with a variable.

A general rule of thumb - perform as many operations on the MySQL server as possible - it's designed to work with data in this manner and can do so very quickly.