Re: 7.4 - FK constraint performance

On Friday 13 February 2004 04:25, Tom Lane wrote:
> Rod Taylor <rbt(at)rbt(dot)ca> writes:
> > Statistics say there are 10 values. Statistics list the 10 most common
> > values (all of them). Given this, would it not be reasonable to assume
> > that 239 is a recent addition (if there at all) to the table and not
> > very common?
>
> We don't know that it's 239 when we make the plan. In order to know
> that, we'd have to abandon caching of RI check query plans and re-plan
> for each row. That strikes me as inevitably a losing proposition.

In this precise example, could you not:
1. Check index for value
2. If found, seq-scan

Of course that's only going to be a sensible thing to do if you're expecting
one of two results:
1. Value not there
2. Lengthy seq-scan if it is there
--
Richard Huxton
Archonet Ltd