Re: vacuum, performance, and MVCC

> On 6/24/06, Mark Woodward <pgsql(at)mohawksoft(dot)com> wrote:
>> Currently it looks like this:
>>
>> ver001->ver002->ver003->...-verN
>>
>> That's what t_ctid does now, right? Well, that's sort of stupid. Why not
>> have it do this:
>>
>> ver001->verN->...->ver003->ver002->|
>
> Heh, because that's crazy. The first time you insert a key into the
> index it will point to v1 of a tuple... say after 5 updates you have
> v2,v3,v4,v5... your c_tid pointer chain looks like v1
> (original)->v2->v3->v4-v5 (newest). However, your whole idea is based
> on not having to do another index insert for unchanged keys, so the
> index still points to v1... which means you have to follow the c_tid
> chain to get to the newest version just like a sequential scan. I
> don't see how you think you can reverse pointer it.

In the scenario, as previously outlined:

ver001->verN->...->ver003->ver2->|
^-----------------------------/

The index points to version 1 (ver001) which points to the latest version
(verN).

>
>> This will speed up almost *all* queries when there are more than two
>> version of rows.
>
> Nope.

Of course it will.

>
>> When you vacuum, simply make the latest version (verN) the key row
>> (ver001).
>
> How are you going to do this without a ton of locking... remember, the
> index is pointing to v1 with a tid... so you'll have to physically
> move the newest version v5 to v1's tid from wherever it was... like a
> vacuum full on steroids. Unless of course, you rebuild the index...
> but that's not a solution either.

I don't understand how you can assume this. In fact, it wil proably reduce
locking and disk IO by not having to modify indexes.
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