Re: Different execution plan between PostgreSQL 8.4 and 12.11

On Tue, Oct 11, 2022 at 09:59:43AM -0400, Tom Lane wrote:
> David Rowley <dgrowleyml@gmail.com> writes:
> > It feels like something is a bit lacking in our cost model here. I'm
> > just not sure what that is.
> 
> The example you show is the same old problem that we've understood for
> decades: for cost-estimation purposes, we assume that matching rows
> are more or less evenly distributed in the table.  Their actual
> location doesn't matter that much if you're scanning the whole table;
> but if you're hoping that a LIMIT will be able to stop after scanning
> just a few rows, it does matter.

We do have a correlation statistics value for each column but I am
unclear if that would help here.

-- 
  Bruce Momjian  <bruce@momjian.us>        https://momjian.us
  EDB                                      https://enterprisedb.com

  Indecision is a decision.  Inaction is an action.  Mark Batterson