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

The MyISAM engine stores the row count internally, so when issuing a query like SELECT COUNT(*) FROM table, then it will be fast. With InnoDB, on the other hand, it will take some time because it counts the actual rows. Which means - more rows - the slower it gets. But there's a trick by which you use a small covering index to count all the rows in the table - then it's fast. Another trick is to simply store the row count in a corresponding summary table.