Just to note, the easiest heuristic I've been using to figure out when optimization might be useful is when you have an OR clause on more than one table. Column

Just to note, the easiest heuristic I've been using to figure out when optimization might be useful is when you have an OR clause on more than one table. Column A or B is fine, but Table A column A or Table B column A is gonna cause interesting problems. Also: union = distinct values union all = no distinct