Re: Much Ado About COUNT(*)

On Sun, 16 Jan 2005 20:49:45 +0100, Manfred Koizar wrote:
> On Thu, 13 Jan 2005 00:39:56 -0500, Tom Lane wrote:
>> A would-be deleter of a tuple would have to go and clear the "known
>> good" bits on all the tuple's index entries before it could commit.
>> This would bring the tuple back into the "uncertain status" condition
>> where backends would have to visit the heap to find out what's up.
>> Eventually the state would become certain again (either dead to
>> everyone or live to everyone) and one or the other hint bit could be
>> set again.
>
> Last time we discussed this, didn't we come to the conclusion, that
> resetting status bits is not a good idea because of possible race
> conditions?

http://archives.postgresql.org/pgsql-performance/2004-05/msg00004.php

> In a previous post you wrote:
> | I think we still have one free bit in index tuple headers...
>
> AFAICS we'd need two new bits: "visible to all" and "maybe dead".
>
> Writing the three status bits in the order "visible to all", "maybe
> dead", "known dead", a normal index tuple lifecycle would be
>
> 000 -> 100 -> 110 -> 111
>
> In states 000 and 110 the heap tuple has to be read to determine
> visibility.
>
> The transitions 000 -> 100 and 110 -> 111 happen as side effects of
> index scans. 100 -> 110 has to be done by the deleting transaction.
> This is the operation where the additional run time cost lies.
>
> One weakness of this approach is that once the index tuple state is
> 110 but the deleting transaction is aborted there is no easy way to
> reset the "maybe deleted" bit. So we'd have to consult the heap for
> the rest of the tuple's lifetime.

How bad is that really with a typical workload?

Jochem