| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | "Jim C(dot) Nasby" <jnasby(at)pervasive(dot)com> |
| Cc: | pgsql-general(at)postgresql(dot)org, pgman(at)candle(dot)pha(dot)pa(dot)us |
| Subject: | Re: What is an 'unused item pointer' |
| Date: | 2005-09-25 04:09:24 |
| Message-ID: | 26606.1127621364@sss.pgh.pa.us |
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| Lists: | pgsql-general |
"Jim C. Nasby" <jnasby(at)pervasive(dot)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
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