mysql count performance

Question

select count(*) from mytable;
select count(table_id) from mytable; //table_id is the primary_key

both query were running slow on a table with 10 million rows.

Answer 1

As cherouvim pointed out in the comments, it depends on the storage engine.

MyISAM does keep a count of the table rows, and can keep it accurate since the only locks MyISAM supports is a table lock.

InnoDB however supports transactions, and needs to do a table scan to count the rows.

http://www.mysqlperformanceblog.com/2006/12/01/count-for-innodb-tables/