is SELECT COUNT(*) expensive?

Question

Do you think it\'s a good idea to count entries from a really big table (like 50K rows) on each page load?

SELECT COUNT(*) FROM table

Right

Answer 1

In MyISAM the count(*) is optimized away WHEN THERE ISN'T ANY 'WHERE' CONDITION, so the query is very fast even with billions of lines.

In the case of partitioned tables, we could think it would behave the same way if there is a simple condition on the column that defines the partition (ex: count all the lines on a few physical tables of the logical table). But this is not the case : it loops on all the lines of the physical tables considered, even if we want to count them all. For instance, here, on a 98-million-line table partitioned into 40 tables, it takes over 5 minutes to count the number of lines in the last 32 physical tables.