Mysql-Understand your engines
MySQL has two primary storange engines: MyISAM and InnoDB. Each has its own performance characteristics and considerations. In the broadest sense MyISAM is good for read-heavy data and InnoDB is good for write-heavy data, though there are cases where the opposite is true. The biggest gotcha is how the two differ with respect to the COUNT function.
MyISAM keeps an internal cache of table meta-data like the number of rows. This means that, generally, COUNT(*) incurs no additional cost for a well-structured query. InnoDB, however, has no such cache. For a concrete example, let’s say we’re trying to paginate a query. If you have a query SELECT * FROM users LIMIT 5,10, let’s say, running SELECT COUNT(*) FROM users LIMIT 5,10 is essentially free with MyISAM but takes the same amount of time as the first query with InnoDB. MySQL has a SQL_CALC_FOUND_ROWS option which tells InnoDB to calculate the number of rows as it runs the query, which can then be retreived by executing SELECT FOUND_ROWS(). This is very MySQL-specific, but can be necessary in certain situations, particularly if you use InnoDB for its other features (e.g., row-level locking, stored procedures, etc.).
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