ORDER BY

This is a quick note on the importance of ORDER BY for the order of the result set produced by a SELECT. The mantra is very simple: Without an order_by_clause, no guarantee exists that the same query executed more than once will retrieve rows in the same order. Repeat it as necessary many times everywhere, [...]

NL join (ordered)

Some time ago there was a thread on the SQL.ru forum where user has asked the never-ending question “why CBO is doing this?”. The problem was a simple count(*) of parent-child tables join with no FK constraint was executed in very strange way: via NESTED LOOPS using child as a driving table.