Re: [WIP] Performance Improvement by reducing WAL for Update Operation

On 09.08.2012 14:11, Simon Riggs wrote:
> Given the marginal gain because of a low percentage of cross-block
> updates, I'm not keen. Low percentage because HOT tries hard to keep
> things on same block - even for non-HOT updates (which is the case,
> even though it sounds weird).

That depends entirely on the workload. If you do a bulk update that 
updates every row on the table, most are going to be cross-block 
updates, and the WAL size does matter.

>> But then again, full-page writes cover that too. There
>> will be a full-page image of the old block in the WAL anyway.
>
> Right, but we're planning to remove that, so its not a safe assumption
> to use when building new code.

I don't think we're going to get rid of full-page images any time soon. 
I guess you could easily check if full-page writes are enabled, though, 
and only do it for cross-page updates if it is.

--   Heikki Linnakangas  EnterpriseDB   http://www.enterprisedb.com