From: Richard Huxton <dev@archonet.com>
To: Tom Lane <tgl@sss.pgh.pa.us>, Rod Taylor <rbt@rbt.ca>
Subject: Re: 7.4 - FK constraint performance
Date: Fri, 13 Feb 2004 07:22:17 +0000
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On Friday 13 February 2004 04:25, Tom Lane wrote:
> Rod Taylor <rbt@rbt.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