Re: 7.4 - FK constraint performance

On Thu, 12 Feb 2004, 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.

One thing is that IIRC we're going to ask for only one row when we do the
SPI_execp_current. However, unless I misremember, the behavior of for
update and limit means that saying limit 1 is potentially unsafe (if you
block on a row that goes away). Is there anyway for us to let the planner
know this?