Right... but PostgreSQL does that as well, so it really isn't relevant to a PostgreSQL vs. MySQL comment, which is what I was responding to. ;P PostgreSQL and InnoDB have the exact same constraints and tradeoffs with respect to count() performance, AFAIK.

(That said, an index doesn't make this magically faster... the fast performance that people credit MyISAM with is that asking for the total number of rows in a table was O(1), as its lack of intricate transaction concurrency allowed it to just keep a global counter; one which of course was difficult and costly to repair after an unclean outage. In comparison, that indexes can optimize a count() over a where is obvious and true of any sane database engine, whether it be PostgreSQL or any of the common MySQL backends.)