I have been reading articles on how to optimise MySQL databases and queries and here are a few tips I have learnt and would like to share with you:

1. Proper use of indexes improve performance
2. Do not perform calculations on an index (eg: if you have an index for a column called salary, do not perform calculation such as salary * 2 10000)
3. Do not perform calculations on an index (eg: if you have an index for a column called salary, do not perform calculation such as salary * 2 >
“LOAD DATA INFILE” is the fastest way to insert data into MySQL database (20 times faster than normal inserts)
Use INSERT LOW PRIORITY or INSERT DELAYED if you want to delay inserts from happening until the table is free
Use TRUNCATE TABLE rather than DELETE FROM if you are deleting an entire table (DELETE FROM delete row by row, whereas TRUNCATE TABLE deletes all at once)
Always use EXPLAIN to examine if your select query is efficient
Use OPTIMIZE TABLE to reclaim unused space (Note: Table will be locked during optimisation, so only do it during low traffic time)
Better to have 10 quick queries than 1 slow one
Use caching to reduce database load
Normalize tables to ensure data consistency
Use persistent connections
Don’t query columns you don’t need, avoid using SELECT * FROM
MySQL can search on prefix of indexes (ie: If you have index INDEX (a,b), you don’t need an index on (a))
Don’t use HAVING when you can use WHERE
Use numeric values (rather than alphabetical values) when performing a join

1. Real World Web: Performance & Scalability, MySQL Edition
2. A Few Common Performance Mistakes
3. MySQL Conference Keynote 2008 - Scalability Panel
4. Multiple column index vs multiple indexes