Re: What is an 'unused item pointer'

"Jim C. Nasby" <jnasby@pervasive.com> writes:
> On Sat, Sep 24, 2005 at 07:19:10PM -0400, Tom Lane wrote:
>> Currently, when a tuple is reclaimed by VACUUM, we just mark its item
>> pointer as unused (and hence recyclable).  I think it might be safe to
>> decrease pd_lower if there are unused pointers at the end of the page's
>> pointer array, but we don't currently do that.

> Sounds like a good newbie TODO?

Uh, no, because the $64 question is whether it actually *is* safe, or
perhaps would be safe with more locking than we do now.  I'm not sure of
the answer myself, and would have zero confidence in a newbie's answer.

Decreasing pd_lower would definitely be a win if we can do it free or
cheaply.  If it requires significant additional locking overhead, then
maybe not.

            regards, tom lane