Re: Patch to support SEMI and ANTI join removal

Robert Haas <robertmhaas@gmail.com> writes:
> On Thu, Sep 11, 2014 at 7:14 AM, David Rowley <dgrowleyml@gmail.com> wrote:
>> 5. I've added a flag to pg_class called relhasfkey. Currently this gets set
>> to true when a foreign key is added, though I've added nothing to set it
>> back to false again. I notice that relhasindex gets set back to false during
>> vacuum, if vacuum happens to find there to not be any indexes on the rel. I
>> didn't put my logic here as I wasn't too sure if scanning pg_constraint
>> during a vacuum seemed very correct, so I just left out the "setting it to
>> false" logic based on the the fact that I noticed that relhaspkey gets away
>> with quite lazy setting back to false logic (only when there's no indexes of
>> any kind left on the relation at all).

> The alternative to resetting the flag somehow is not having it in the
> first place.  Would that be terribly expensive?

The behavior of relhaspkey is a legacy thing that we've tolerated only
because nothing whatsoever in the backend depends on it at all.  I'm not
eager to add more equally-ill-defined pg_class columns.
        regards, tom lane