Syntax

• Don’t use DISTINCT and GROUP BY in the same SELECT.
• Don’t paginate via OFFSET, “remember where you left off”.
• WHERE (a,b) = (22,33) does not optimize at all.
• Explicitly say ALL or DISTINCT after UNION – it reminds you pick between the faster ALL or the slower DISTINCT.
• Don’t use SELECT *, especially if you have TEXT or BLOB columns that you don’t need. There is overhead in tmp tables and transmission.
• It is faster when the GROUP BY and ORDER BY can have exactly the same list.
• Don’t use FORCE INDEX; it may help today, but will probably hurt tomorrow.

Remarks

See also discussions about ORDER BY, LIKE, REGEXP, etc. Note: this needs editing with links and more Topics.

Cookbook on building optimal indexes.