Colin McGuigan <cmcguigan@xxxxxxxxxxxxxxx> writes: > I know I can do it by adjusting cost parameters, but I was really > curious as to why adding a "LIMIT 5000" onto a SELECT from a table with > only 530 rows in it would affect matters at all. The LIMIT prevents the sub-select from being flattened into the main query. In the current code this has a side-effect of preventing any statistical information from being used to estimate the selectivity of the filter conditions --- so you get a default rowcount estimate that's way too small, and that changes the shape of the join plan. It's giving you the "right" answer for entirely the wrong reason. regards, tom lane
