Re: Performance Improvement by reducing WAL for Update Operation

On 2013-07-19 10:40:01 +0530, Hari Babu wrote:
>
> On Friday, July 19, 2013 4:11 AM Greg Smith wrote:
> >On 7/9/13 12:09 AM, Amit Kapila wrote:
> >> I think the first thing to verify is whether the results posted can be validated in some other environment setup by another person.
> >> The testcase used is posted at below link:
> >> http://www.postgresql.org/message-id/51366323.8070606@vmware.com
>
> >That seems easy enough to do here, Heikki's test script is excellent.
> >The latest patch Hari posted on July 2 has one hunk that doesn't apply
> >anymore now.
>
> The Head code change from Heikki is correct.
> During the patch rebase to latest PG LZ optimization code, the above code change is missed.
>
> Apart from the above changed some more changes are done in the patch, those are.

FWIW I don't like this approach very much:

* I'd be very surprised if this doesn't make WAL replay of update heavy
workloads slower by at least factor of 2.

* It makes data recovery from WAL *noticeably* harder since data
corruption now is carried forwards and you need the old data to decode
new data

* It makes changeset extraction either more expensive or it would have
to be disabled there.

I think my primary issue is that philosophically/architecturally I am of
the opinion that a wal record should make sense of it's own without
depending on heap data. And this patch looses that.

Greetings,

Andres

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Andres Freund http://www.2ndQuadrant.com/
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